SNAP Program - HFO-1234yf Acceptable for Heavy-Duty Vehicle AC Retrofit

Content

ACTION:

Supplemental proposed rule.

SUMMARY:

Pursuant to the U.S. Environmental Protection Agency's Significant New Alternatives Policy program, this action proposes to
list the refrigerant 2,3,3,3-tetrafluoropropene, also known as HFO-1234yf, as acceptable, subject to use conditions, in the
motor vehicle air conditioning end-use for retrofit of heavy-duty pickup trucks and complete heavy-duty vans. This action
supplements the Agency's November 10, 2025, proposal with respect to the proposed listings in the motor vehicle air conditioning
end-use for retrofit of heavy-duty pickup trucks and heavy-duty vans (both complete and incomplete vans). The EPA is also
supplementing that proposal to clarify the intended scope of that proposed rule. The EPA is providing an opportunity for public
comment on the additional listing and the clarification. The EPA is not reopening the comment period for any portions of the
November 10, 2025, proposal which are not explicitly addressed in this supplemental proposal.

DATES:

Comments on this supplemental proposal must be received on or before May 6, 2026 unless a public hearing is held. If a public
hearing is held, comments on this supplemental proposal must be received on or before 30 days after the date of the public
hearing. Public hearing: Any party requesting a public hearing must notify the contact listed in the
FOR FURTHER INFORMATION CONTACT
section, which is Emily Maruyama at email address: maruyama.emily@epa.gov by 5 p.m. Eastern Daylight Time on or before April 13, 2026. If a public hearing is held, it will take place on or around
April 21, 2026. Please refer to the
SUPPLEMENTARY INFORMATION
section for additional information on the public hearing.

ADDRESSES:

You may send comments, identified by Docket ID No. EPA-HQ-OAR-2024-0503 by any of the following methods:

• Federal Rulemaking Portal: https://www.regulations.gov (our preferred method). Follow the online instructions for submitting comments.

• Email: a-and-r-Docket@epa.gov. Include Docket ID No. EPA-HQ-OAR-2024-0503 in the subject line of the message.

• Mail: U.S. Environmental Protection Agency, EPA Docket Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania Avenue
NW, Washington, DC 20460.

• Hand Delivery or Courier: EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. The Docket Center's hours
of operations are 8:30 a.m. to 4:30 p.m., Monday-Friday (except Federal Holidays).

Instructions: All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change
to https://www.regulations.gov, including personal information provided. For detailed instructions on sending comments and additional information on the rulemaking
process, see the “Public Participation” heading of the
SUPPLEMENTARY INFORMATION
section of this document. For information on EPA Docket Center services, please visit us online at https://www.epa.gov/dockets.

If a public hearing is requested on or before April 13, 2026, the EPA will post an update at https://www.epa.gov/snap. The EPA does not intend to publish a document in the
Federal Register
announcing updates. The public hearing will be held on or around April 21, 2026. Information on the hearing including the
time and URL will be posted on the EPA's Stratospheric Ozone website at https://www.epa.gov/snap. Refer to the section titled, Public Participation for additional information.

FOR FURTHER INFORMATION CONTACT:

For information about this proposed rule, contact Emily Maruyama, Chemicals, Coatings, and Products Division, Office of Clean
Air Programs (Mail Code 6205A), Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC 20460; telephone

     number: (202) 564-2809; email address: *maruyama.emily@epa.gov.* Notices and rulemakings under the EPA's Significant New Alternatives Policy program are available on the EPA's website at *https://www.epa.gov/snap/snap-regulations.*

SUPPLEMENTARY INFORMATION:

Preamble acronyms and abbreviations. Throughout this preamble the use of “we,” “us,” or “our” is intended to refer to the EPA. We use multiple acronyms and terms
in this preamble. While this list may not be exhaustive, to ease the reading of this preamble and for reference purposes,
the EPA defines the following terms and acronyms here:

AC Air Conditioning

ANSI American National Standards Institute

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers

CAA Clean Air Act

CAS Reg. No. Chemical Abstracts Service Registry Identification Number

CBI Confidential Business Information

CFC Chlorofluorocarbon

CFR Code of Federal Regulations

CO 2 Carbon Dioxide

CRP Cooperative Research Program

DIY Do it yourself

EEAP Environmental Effects Assessment Panel

EPA United States Environmental Protection Agency

FR Federal Register

GVWRv Gross Vehicle Weight Rating

HCFC Hydrochlorofluorocarbon

HD Heavy-duty

HFC Hydrofluorocarbon

HFO Hydrofluoroolefin

ICF ICF International, Inc.

LD Light-duty

LFL Lower Flammability Limit

LMDV Light- and medium-Duty Vehicle

MD Medium-duty

mJ Millijoules

MVAC Motor Vehicle Air Conditioning or Motor Vehicle Air Conditioner

MY Model Year

NAAQS National Ambient Air Quality Standard

NAICS North American Industrial Classification System

NPRM Notice of Proposed Rulemaking

ODS Ozone-Depleting Substances

OEL Occupational Exposure Limit

OEM Original Equipment Manufacturer

OMB United States Office of Management and Budget

PBI Proprietary Business Information

PMN Pre-Manufacture Notice

ppm Parts Per Million

PRA Paperwork Reduction Act

RFA Regulatory Flexibility Act

SAE SAE International, previously known as the Society of Automotive Engineers

SDS Safety Data Sheet

SIP State Implementation Plan

SNAP Significant New Alternatives Policy

SNUR Significant New Use Rule

TFA Trifluoroacetic Acid

TLV Threshold Limit Value

TWA Time Weighted Average

UMRA Unfunded Mandates Reform Act

VOC Volatile Organic Compounds

Table of Contents

I. Executive Summary

A. Purpose of the Regulatory Action

B. Summary of the Major Provisions of the Regulatory Action

II. Public Participation

A. Written Comments

B. Participation in Virtual Public Hearing

III. General Information

A. Does this action apply to me?

B. What action is the Agency proposing to take?

C. What is the Agency's authority for taking this action?

D. What are the guiding principles of the SNAP program and what are the SNAP criteria for evaluating substitutes?

E. Children's Environmental Health

IV. Proposed Listing for Motor Vehicle Air Conditioning

A. What is the EPA proposing in this action?

B. Background on MVACs in HD Pickup Trucks and HD Vans

C. What are the ASHRAE classifications for refrigerant flammability and toxicity?

D. What is refrigerant HFO-1234yf and how does it compare to other refrigerants in this end-use?

E. What use conditions is the EPA proposing for HFO-1234yf for retrofit of MVACs in this end-use and what existing requirements
apply to this refrigerant?

V. Clarification of Intended Scope of the 2025 NPRM and Other Clarifications

VI. On which topics is the EPA specifically requesting comment?

VII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

C. Paperwork Reduction Act (PRA)

D. Regulatory Flexibility Act (RFA)

E. Unfunded Mandates Reform Act (UMRA)

F. Executive Order 13132: Federalism

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution, or Use

J. National Technology Transfer and Advancement Act

VIII. References

I. Executive Summary

A. Purpose of the Regulatory Action

The EPA is proposing a new listing after our evaluation of human health and environmental information for one substitute under
Clean Air Act (CAA) section 612, Significant New Alternatives Policy (SNAP) program. The Agency is proposing this new listing
in the refrigeration and air conditioning (AC) sector based on the information the EPA included in the docket. This supplemental
notice of proposed rulemaking, hereafter referred to as the “supplemental proposal” or “supplemental action,” would provide
a new refrigerant option in specific uses, thereby increasing flexibility for industry. The EPA is also supplementing the
Agency's November 10, 2025, Notice of Proposed Rulemaking (NPRM), hereafter referred to as the “2025 NPRM,” to clarify the
intended scope of that proposal. (1)

B. Summary of the Major Provisions of the Regulatory Action

This action proposes to list 2,3,3,3-tetrafluoropropene, (2) also known as hydrofluoroolefin (HFO)-1234yf or R-1234yf, hereafter referred to as “HFO-1234yf,” as acceptable, subject to
use conditions, in the motor vehicle air conditioning (MVAC) end-use for retrofit of heavy-duty (HD) pickup trucks and complete
HD vans. This proposal supplements the 2025 NPRM with respect to the proposed listings in the MVAC end-use for retrofit of
HD pickup trucks and HD vans (both complete and incomplete vans). In the 2025 NPRM, the EPA proposed to list three other substitutes
as acceptable, subject to use conditions, for similar end-uses. The EPA is supplementing the 2025 NPRM to clarify the intended
scope of that proposal for R-444A as acceptable, subject to use conditions, in the MVAC end-use. In the 2025 NPRM, the EPA
proposed a listing for R-444A in the MVAC end-use for retrofit of HD pickup trucks and HD vans (both complete and incomplete).
The Agency included incomplete HD vans in error and is clarifying that we intended for this proposed listing to apply to HD
pickup trucks and complete HD vans only. The scope of the proposals in the 2025 NPRM for the two other substitutes proposed
in this end-use, R-456A and R-480A, are not affected by this supplemental proposal and thus remain unchanged.

The proposed new listing for HFO-1234yf would appear as a change to appendix B of 40 Code of Federal Regulations (CFR) part
82, subpart G, within row 12 in the table titled “Refrigerants—Acceptable Subject to Use Conditions.” The clarification of
the intended scope of the 2025 NPRM listing for R-444A in the MVAC end-use

  discussed in section V of this supplemental proposal would appear as a change to appendix B of 40 CFR part 82, subpart G,
  within row 10 in the same table titled “Refrigerants—Acceptable Subject to Use Conditions.” In this supplemental proposal,
  the EPA is not proposing changes beyond these. The specific proposed regulatory changes to appendix B are available in a document
  in the docket under the title “Proposed Changes to Appendix B for SNAP 27 Supplemental Proposal.” [(3)]()

II. Public Participation

A. Written Comments

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2024-0503 at https://www.regulations.gov (our preferred method), or the other methods identified in the
ADDRESSES
section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received in
the public docket. Do not submit to the EPA's docket at https://www.regulations.gov any information you consider to be Confidential Business Information (CBI), Proprietary Business Information (PBI), or other
information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by
a written comment. The written comment is considered the official comment and should include discussion of all points you
wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (i.e., on the web, cloud, or other file sharing system). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions;
and general guidance on making effective comments.

B. Participation in Virtual Public Hearing

The EPA may hold a virtual public hearing if the Agency receives a request to hold one. Any party requesting a public hearing
must notify the contact listed in the
FOR FURTHER INFORMATION CONTACT
section, which is Emily Maruyama at email address: maruyama.emily@epa.gov by 5 p.m. Eastern Daylight Time on or before April 13, 2026. If a virtual public hearing is held, it will take place on or
around April 21, 2026 and further information will be provided on the EPA's Stratospheric Ozone website at https://www.epa.gov/snap.

The EPA will make every effort to follow the schedule as closely as possible on the day of the hearing; however, please plan
for the hearings to run either ahead of schedule or behind schedule. Each commenter will have three to five minutes to provide
oral testimony. The EPA encourages commenters to provide a copy of their oral testimony electronically by emailing it to maruyama.emily@epa.gov. The EPA also recommends submitting the text of your oral comments as written comments to the rulemaking docket EPA-HQ-OAR-2024-0503.
Written statements and supporting information submitted during the comment period will be considered with the same weight
as oral comments and supporting information presented at the public hearing. The EPA may ask clarifying questions during the
oral presentations but will not respond to the presentations at that time.

Please note that any updates made to any aspect of the hearing will be posted online at https://www.epa.gov/snap. While the EPA expects the hearing to go forward as set forth above, please monitor our website or contact Emily Maruyama,
202-564-2809, maruyama.emily@epa.gov to determine if there are any updates. The EPA does not intend to publish a document in the
Federal Register
(FR) announcing updates.

III. General Information

A. Does this action apply to me?

The following list identifies regulated entities that may be affected by this rulemaking and their respective North American
Industrial Classification System (NAICS) codes:

• All Other Basic Organic Chemical Manufacturing (325199).
• Motor Vehicle Manufacturing (3361).
• Motor Vehicle Parts Manufacturing (3363).
• Recyclable Material Merchant Wholesalers (423930).
• General Automotive Repair (811111). This list is not intended to be exhaustive, but rather to provide a guide for readers regarding entities likely to be affected by this action. To determine whether your facility, company, business, or organization could be affected by this action, you should carefully examine the regulations at 40 CFR part 82, subpart G, and the proposed revisions. If you have questions regarding the applicability of this action to a particular entity, consult the person listed in the FOR FURTHER INFORMATION CONTACT section.

B. What action is the Agency proposing to take?

The EPA is proposing to list HFO-1234yf as acceptable, subject to use conditions, in the MVAC end-use for retrofit of HD pickup
trucks and complete HD vans. The EPA is also supplementing the 2025 NPRM to clarify the intended scope of that proposal. Specifically,
in the 2025 NPRM, the EPA proposed to list R-444A as acceptable, subject to use conditions, in the MVAC end-use for retrofit
of complete and incomplete HD vans. This supplemental action is clarifying that the EPA intended for this proposed listing
to apply only to complete HD vans.

C. What is the Agency's authority for taking this action?

This proposal supplements the 2025 NPRM. See the 2025 NPRM for a full discussion of the Agency's authority for taking this
action.

D. What are the guiding principles of the SNAP program and what are the SNAP criteria for evaluating substitutes?

This proposal supplements the 2025 NPRM. See the 2025 NPRM for a full discussion of the guiding principles of the SNAP program
and the SNAP criteria for evaluating substitutes.

E. Children's Environmental Health

This action is subject to the EPA's Policy on Children's Health (https://www.epa.gov/children/childrens-health-policy-and-plan) because the rule has considerations for human health. Accordingly, we have evaluated the environmental health effects of
HFO-1234yf to general population exposure.

In summary, the risk screen for the use of HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and complete HD
vans found that this substitute is not expected to cause a significant risk to human health in the general population when
manufactured for use and used as a refrigerant in HD pickup truck and complete HD van AC systems. Additionally, the EPA found
that the toxicity risks of using HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans are
comparable to or lower than that of other available substitutes in the same end-use. The risk screen found that HFO-1234yf
can be used without exceeding the recommended

  occupational exposure limit (OEL) of 500 ppm (8-hr OEL); thus, the toxicity risks of this refrigerant are comparable to those
  of other acceptable substitutes in MVACs, which also are used without exceeding their OELs. The risk screen also found that
  HFO-1234yf in HD pickup trucks and complete HD vans does not pose a significant risk of end-use exposure, provided systems
  are installed in appropriate spaces with proper engineering controls, emergency response plans, and according to guidelines
  from the manufacturer, standards, and the safety data sheet. While the EPA has not conducted a separate analysis of risks
  to infants and children associated with this rule, the rule does contain use conditions that would reduce exposure risks to
  the general population, with the reduction of exposure being most important to the most sensitive individuals.

The results of this evaluation are contained in section IV.D. of this preamble and in the risk screen titled “Risk Screen
on Substitutes in Motor Vehicle Air Conditioning (Heavy-Duty Pickup Trucks and Complete Heavy-Duty Vans) (Retrofit Equipment);
Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf).” A copy of this document is available in the public docket for
this action at Docket ID No. EPA-HQ-OAR-2024-0503.

This action is consistent with the EPA's Policy on Children Health because it provides an additional retrofit option and would
not pose additional adverse effects to human health when used in accordance with existing and proposed requirements and as
intended by the submitter.

Furthermore, Executive Order 13045 (“ Protection of Children from Environmental Health Risks and Safety Risks”) applies to this action. Information on how this action is subject to this Executive Order is available in a section of this
preamble under the same name.

IV. Proposed Listing for Motor Vehicle Air Conditioning

A. What is the EPA proposing in this action?

The EPA is proposing to list HFO-1234yf as acceptable, subject to use conditions, in the MVAC end-use for retrofit of HD pickup
trucks and complete HD vans. In the 2025 NPRM, the EPA proposed to list three substitutes, R-444A, R-456A, and R-480A as acceptable,
subject to use conditions, in the MVAC end-use for retrofit of HD pickup trucks and complete and incomplete HD vans. During
the public comment period, the EPA received comments and feedback from stakeholders highlighting that the proposal did not
include a listing of HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and HD vans. The EPA reviewed these comments
and agrees that the Agency should have included all four refrigerants for this end-use consistent with other MVAC listings
in the 2025 NPRM and is issuing this supplemental proposal to add this listing. To support this listing, the Agency has provided
in the docket the risk screen for HFO-1234yf for this end-use and proposes to find HFO-1234yf acceptable, subject to use conditions,
in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans.

The 2025 NPRM proposed to list HFO-1234yf as acceptable, subject to use conditions, in the MVAC end-use for retrofit of light-
and medium-duty vehicles (LMDVs). HFO-1234yf has not otherwise been listed as acceptable for other MVAC retrofit applications.
The proposed listing for HFO-1234yf in this supplemental action would allow for retrofits of chlorofluorocarbon (CFC)-12 MVACs
as well as for retrofits of MVACs using any of the refrigerants the SNAP program lists as acceptable or acceptable, subject
to use conditions, in this end-use.

B. Background on MVACs in HD Pickup Trucks and HD Vans

The SNAP program uses the term MVAC broadly to describe a wide variety of non-stationary AC systems that provide passenger
comfort cooling for LMDVs, HD vehicles, nonroad vehicles, buses, and trains. The SNAP MVAC end-use includes systems that may
also be subject to other CAA regulatory programs, including for example, where those systems fit within the regulatory definition
of “MVAC” under 40 CFR 82.32, (4) or the definition of an “MVAC-like appliance” (5) or “appliance” under 40 CFR 82.152, or both.

To appropriately evaluate human health and environmental risks, the SNAP program considers the type of vehicle in which the
proposed alternative would be used. The vehicle types within the MVAC end-use addressed in this supplemental proposal to list
HFO-1234yf as acceptable, subject to use conditions, include limited types of vehicles, specifically, HD pickup trucks and
complete HD vans (e.g., large passenger vehicles such as large pickup trucks or vans). In this supplemental action, the EPA is not making any changes
to the proposed MVAC acceptability listings for HFO-1234yf included in the 2025 NPRM (e.g., retrofit LMDVs, new HD on-highway vehicles, and new buses) and is not reopening the comment period for those proposed listings.

HD vehicles are often subdivided by vehicle weight classifications, as defined by the vehicle's gross vehicle weight rating
(GVWR), which is a measure of the combined curb (empty) weight and cargo carrying capacity of the truck. HD vehicles have
GVWRs above 8,500 pounds. HD pickup trucks and HD vans are Class 2b and 3 vehicles with GVWRs between 8,501 and 14,000 pounds.

The types of vehicles for which the EPA is proposing to list HFO-1234yf for retrofit use as acceptable, subject to use conditions,
in this supplemental action, are in many ways more similar to light-duty (LD) or medium-duty (MD) vehicles than they are to
the HD vehicles with a higher GVWR classification. These vehicle types are similar to LD vehicles technologically and most
are manufactured by companies with major LD markets in the United States and in a similar manner to LD vehicles. (6) In many cases, these types of HD vehicles are versions of their LD counterparts. (7) The primary difference between HD pickup trucks and HD vans and their LD counterpart vehicles is that HD pickup trucks and
HD vans are occupational or work vehicles that are designed for much higher towing and payload capabilities than are LD pickup
trucks and LD vans. HD pickup trucks and HD vans share many design similarities with their

  lighter counterparts. For example, MVAC systems in HD pickup trucks and HD vans generally have a similar configuration and
  use similar components as their lighter counterparts. Differences may exist in terms of cooling capacity (*e.g.,* based on cabin volume), system layout (*e.g.,* the number of evaporators), and the durability requirements due to longer truck life. [(8)]()

All types of HD vehicles can be sold as “complete” or “incomplete” vehicles. (9) Approximately 90 percent of HD pickup trucks and HD vans are
3/4
-ton and 1-ton pickup trucks, 15-passenger vans, and large work vans that are sold by vehicle manufacturers as complete vehicles. (10) Complete vehicles are sold by vehicle manufacturers to end users with no secondary manufacturer making substantial modifications
prior to registration and use. Incomplete vehicles are sold by vehicle manufacturers to secondary manufacturers without the
primary load-carrying device or container attached. (11)

Examples of modifications by secondary manufacturers to HD pickup trucks are installing a flatbed platform or tool storage
bins. The EPA is not aware of any equipment added by a secondary manufacturer to an incomplete HD pickup truck that would
result in a secondary manufacturer modifying or adjusting the already installed MVAC system to provide cooling capacity.

Incomplete HD vans are typically sold with no enclosed cabin area behind the driver's seat, and secondary manufacturer modifications
could include applications such as conversion to ambulances, shuttle vans, and motor homes. Incomplete HD vans may include
original equipment manufacturer (OEM) MVACs that are identical to those installed in the complete HD van on which the incomplete
model is based. In some cases, these systems are designed solely for cooling the front driver area, while other systems are
manufactured by the OEM with additional capability to provide cooling behind the driver area to the cabin after modification.

MVACs across all vehicle types are typically charged during vehicle manufacture. Incomplete HD vehicles are modified by secondary
manufacturers and that modification may or may not involve the installation of additional AC or refrigeration equipment. (12) While some secondary manufacturers use the OEM MVAC system with no modification to the contained refrigerant system (hoses,
connections, heat exchangers, compressor, etc.), this is not a uniform practice. At the time of this supplemental action,
the EPA does not have sufficient information on the potential for modifications to OEM-installed MVAC systems of incomplete
HD vans by secondary manufacturers and the impact of those modifications on the safe use of HFO-1234yf. For this reason, the
EPA is not proposing to find HFO-1234yf acceptable, subject to use conditions, in the MVAC end-use for retrofit of incomplete
HD vans in this supplemental proposal.

Historically, the class I ozone-depleting substances (ODS) refrigerant CFC-12 was the primary refrigerant used in MVACs for
passenger vehicles and trucks. In the initial 1994 SNAP rulemaking, hydrofluorocarbon (HFC)-134a, amongst other substitutes,
was listed as acceptable for use in new and retrofit MVACs, including HD pickup trucks and HD vans. (13) Since then, the EPA has listed additional alternatives for MVACs as acceptable, subject to use conditions, for use in new
HD pickup trucks and complete HD vans, including HFO-1234yf, HFC-152a, and carbon dioxide (R-744).

The EPA previously listed HFO-1234yf as acceptable, subject to use conditions, in newly manufactured HD pickup trucks and
complete HD vans. (14) As of model year (MY) 2026, both HFO-1234yf and HFC-134a are used in new HD pickup trucks that are manufactured and imported
in the United States. New complete HD vans continue to primarily use HFC-134a. (15)

The EPA considers other relevant regulatory programs when developing listing decisions and use conditions. For example, CAA
section 609 and implementing regulations in 40 CFR part 82, subpart B address the repair and servicing of MVACs as well as
technician training and certification. CAA section 608 and implementing regulations in 40 CFR part 82, subpart F restrict
the sale of refrigerant and address disposal and other activities involving MVACs that are not regulated under CAA section
609.

By considering the regulatory requirements that already exist, consistent with the SNAP program's guiding principles, the
EPA has been able to limit the use conditions the Agency would have otherwise considered, particularly for retrofits. See
the 2025 NPRM for a full discussion of the EPA's regulatory approach under CAA sections 609 and 612 regarding the repair and
servicing of MVACs and recovery, recycling, and recharging equipment.

C. What are the ASHRAE classifications for refrigerant flammability and toxicity?

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 34-2024 assigns a safety group
for each refrigerant, which consists of two to three alphanumeric characters (e.g., A2L or B1). (16) The initial character indicates the toxicity, and the numeral, with or without suffix letter, denotes the flammability. HFO-1234yf
is in the A2L Safety Group. ASHRAE classifies Class A refrigerants as refrigerants for which toxicity has not been identified
at concentrations less than or equal to 400 parts per million (ppm) by volume, based on data used to determine threshold limit
value-time-weighted average (TLV-TWA) or consistent indices. Throughout this document, refrigerants in the flammability class
of “2L” are referred to as lower flammability refrigerants. See the 2025 NPRM for a full discussion of the ASHRAE classifications
for refrigerant flammability and toxicity.

D. What is refrigerant HFO-1234yf and how does it compare to other refrigerants in this end-use?

The EPA is proposing to list HFO-1234yf as acceptable, subject to use conditions, in the MVAC end-use for retrofit of HD pickup
trucks and complete HD vans.

The redacted submission and supporting documentation for this proposed refrigerant are in the docket. (17) The EPA performed a risk screening assessment to examine the human health and environmental risks of this substitute, also
available in the docket. 18 The EPA notes that the environmental, flammability, and toxicity information in this section is similar to the information
provided in the 2025 NPRM related to the proposed listing of HFO-1234yf in the MVAC end-use for retrofit of LMDVs. As mentioned
in the 2025 NPRM, the environmental, flammability, and toxicity information about this proposed substitute does not differ
between LMDV MVACs and HD pickup truck and complete HD van MVACs.

Environmental information: The specific atmospheric effects values can be found in the risk screen developed for HFO-1234yf for the proposed listing
in this supplemental action. These were determined consistent with the source information noted in section III.D. of the 2025
NPRM.

HFO-1234yf is excluded from the EPA's regulatory definition of volatile organic compounds (VOCs), which is used for addressing
the development of state implementation plans (SIPs) to attain and maintain the National Ambient Air Quality Standard (NAAQS). (19)

HFO-1234yf can break down into trifluoroacetic acid (TFA) in the atmosphere. HFO-1234yf is almost completely transformed into
TFA. (20) For more information on TFA, see the response to comments section of SNAP Rule 26. (21)

Flammability information: HFO-1234yf is a lower flammability refrigerant (ASHRAE flammability classification 2L). HFO-1234yf may pose a greater flammability
risk than nonflammable substitutes in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans. The flammability
risk, determined by the likelihood of exceeding the lower flammability limit (LFL), is evaluated in the risk screen referenced
in this section. The EPA is proposing to determine that HFO-1234yf may be used safely since flammability risk can be mitigated
by use consistent with the proposed labeling requirements in appendix D of 40 CFR part 82, subpart G, recommendations in the
manufacturers' safety data sheet (SDS), and other safety precautions common in the refrigeration and AC industry. The flammability
characteristics of HFO-1234yf make the risk of ignition low. HFO-1234yf requires an open flame to ignite, such as a match
or lighter, because of its relatively high minimum ignition energy of greater than 5,000 millijoules (mJ). (22) HFO-1234yf has an LFL of 62,000 ppm, (23) and has a low burning velocity (24) compared to refrigerants with flammability classification of 2 such as HFC-152a (25) or with flammability classification of 3 such as hydrocarbon refrigerants. (26) As a result of these flammability characteristics, HFO-1234yf is difficult to ignite, and is generally unable to propagate
a flame once ignited (i.e., flames resulting from HFO-1234yf put themselves out).

Consistent with the other proposed listings in the 2025 NPRM, under this supplemental action, HFO-1234yf could be used to
retrofit MVACs originally designed for an A1 refrigerant. The EPA considered if this could create additional flammability
risk distinct from its use in a new MVAC that is specifically designed with mitigation measures to use a flammable refrigerant.
The original submission for HFO-1234yf in new vehicles included analyses that evaluated the flammability and toxicity risks
of HFO-1234yf in MVACs that were originally designed for HFC-134a. These analyses consist of reports published in 2008, 2009,
and 2013 from the SAE International, previously known as the Society of Automotive Engineers (SAE), Cooperative Research Program
(CRP). The vehicles in these analyses did not feature any design changes to address potential flammability. In this way, MVACs
used in the original analyses were analogous to vehicles that would be retrofit under this supplemental proposal, because
they could be originally designed for an A1 refrigerant. (27) The 2008 report found that the increased flammability risk of HFO-1234yf in a vehicle designed for use with HFC-134a is well
below those commonly accepted by the general public. (28) A revised 2009 report found that the risks of HFO-1234yf were low overall, and somewhat less than the toxicity risks posed
by R-744. (29) The submitter of HFO-1234yf provided these analyses to the EPA to support the Agency's original consideration of HFO-1234yf
in new vehicles, and the Agency based its listing of acceptability in part on the findings of these analyses. The EPA concluded
that the risks of HFO-1234yf are comparable to or less than the risks from other available or potentially available alternatives
in this end-use that the Agency had already listed or proposed as acceptable (e.g., HFC-152a, HFC-134a, and R-744). (30)

SAE revised its assessment of HFO-1234yf and released a supplemental report in 2013 that contained two new fault tree analyses
that included additional “worst-case scenarios.” (31) The report revised the probability of a vehicle fire due to ignition of HFO-1234yf in a system featuring no design changes
compared to an HFC-134a system to about 3 × 10 −12 events per hour of vehicle operation.

The submitter of HFO-1234yf in the MVAC end-use for retrofit of LMDVs and HD pickup trucks and complete HD vans provided an
updated fault tree analysis that evaluated the additional risk associated with use of HFO-1234yf specifically in retrofit
applications and the EPA considered this new analysis in our review of HFO-1234yf. (32) The analysis only considered scenarios that increased the flammability risk in a retrofit (such as increased risk of mechanical
fan failure and electrical fires and less consistent presence and deployment of airbags) and did not

  consider scenarios that reduced the flammability risk in a retrofit (such as the larger cabin size in older vehicles that
  would be retrofit). The overall estimated risk was about 8 × 10 −12 events per operating hour, which is similar to the risk of vehicle fire due to HFO-1234yf ignition in new MVAC equipment (5
  × 10 −12 events per operating hour). [(33)]() The actual increased risk is likely lower than this, as the evaluation only considered circumstances that would increase the
  probability of a vehicle fire and did not consider circumstances that would reduce the probability.

The MVAC systems, vehicle designs, and the potential for exposure for the HD vehicle types for which the EPA is proposing
HFO-1234yf for retrofit use as acceptable, subject to use conditions, in this supplemental action are identical or very similar
to those of LD vehicles. (34) In 2016, the EPA evaluated how the risks of using HFO-1234yf in new HD pickup trucks and complete HD vans compare to the risks
of using HFO-1234yf in new LD vehicles. (35) The EPA presented information on the highest refrigerant charge to passenger compartment volume ratios for different vehicle
types and the highest ratio for all HD pickup truck and HD van vehicle types was 410 g/m 3 in HD pickup trucks, which was less than the maximum ratio identified in the analysis for HFO-1234yf in LD vehicles in two
seaters, which was 641 g/m 3. (36) The EPA concluded that the available assessments on the use of HFO-1234yf in LD vehicles were sufficiently conservative to
account for all possible flammability risks from the use of HFO-1234yf in HD pickup trucks and complete HD vans. (37) Consistent with the approach taken in SNAP Rule 21 when the EPA listed HFO-1234yf in new HD pickup trucks and complete HD
vans and relied on analysis for the use of HFO-1234yf in LD vehicles, the EPA proposes to find it appropriate to rely on the
same analysis included in this section related to the use of HFO-1234yf in LD vehicle types. (38)

The EPA conducted a risk screen in 2026 for HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and complete HD
vans to support the proposed listing in this supplemental action. The risk screen found that concentrations of HFO-1234yf
did exceed the LFL in the passenger compartment of a HD pickup truck under the modeled worst-case scenario. For the HD pickup
truck and HD van vehicle types evaluated in this risk screen, the highest refrigerant charge to passenger compartment ratio
was 390 g/m 3 in HD pickup trucks. (39) This ratio is substantially less than the maximum ratio identified for many vehicle types mentioned in the risk screen for
HFO-1234yf use in retrofit LMDVs including two seaters (640.75 g/m 3), small pickup trucks (633.27 g/m 3), sport utility vehicles (414.96 g/m 3), and standard pickup trucks (397.84 g/m 3). (40) The lower ratio indicates a relatively lower risk.

As discussed in the 2025 NPRM, the risk screen for HFO-1234yf use for retrofit of LMDVs found that concentrations of HFO-1234yf
did exceed the LFL in the passenger compartment under certain worst-case scenarios but remained well below the LFL in more
realistic industry consortium field testing. For example, using a simple box model, combining the highest ratio of refrigerant
charge to observed passenger compartment size with a catastrophic release of 60 percent of the charge in 60 seconds, resulted
in a maximum instantaneous charge of 172,000 ppm, compared to an LFL of 62,000 ppm. However, analysis using the more accurate
technique of computational fluid dynamics modeling found the instantaneous concentration of HFO-1234yf to vary from 65,000
ppm to 34,000 ppm. The industry consortium field testing found a maximum instantaneous concentration of HFO-1234yf of 29,774
ppm when a vehicle's full charge was released. (41)

The EPA's original 2009 risk analysis of HFO-1234yf for use in new LMDVs also identified scenarios in which concentrations
exceeded the LFL. (42) The EPA listed HFO-1234yf as acceptable, subject to use conditions, in new LMDVs leveraging this risk analysis. In the EPA's
original listing, the Agency stated that it found that the use of HFO-1234yf in the MVAC end-use for new passenger vehicle
and LD trucks, subject to the use conditions adopted in that listing, does not present a greater overall risk to human health
and the environment compared to the currently approved MVAC alternatives or as compared to R-744. (43) The EPA has also subsequently listed R-744 as acceptable, subject to use conditions, in new LMDVs. Finally, HFO-1234yf in
new LMDVs has been widely adopted since being listed in 2012. In MY2023, the share of new LMDVs sold in the United States
with HFO-1234yf reached 97 percent. (44) HFO-1234yf has also been adopted for use in new HD pickup trucks and complete HD vans. Even with its broad use, the EPA is
not aware of any real-world instances in which HFO-1234yf has ignited and caused a vehicle fire, which further augments the
record for this refrigerant.

SAE J1660 currently provides guidance on how to retrofit a vehicle originally charged with CFC-12 to HFC-134a. The EPA anticipates
that SAE would develop an analogous standard or revise this standard for retrofitting vehicles using newer refrigerants, including
HFO-1234yf. Following such standards may further reduce the flammability risk associated with retrofitting MVACs, which is
already expected to be extremely small in magnitude.

Given the findings of the evaluation materials available in the docket, that the environmental, flammability, and toxicity
information about HFO-1234yf does not differ between MVAC end-use for LMDVs and HD pickup trucks and complete HD vans, that
the MVACs used in the original analysis for HFO-1234yf in new vehicles were analogous to vehicles that would be retrofit,
and the widespread adoption of HFO-1234yf without documented flammability issues, the EPA is proposing that HFO-1234yf may
be safely used in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans.

Toxicity information: Toxicity risk, determined by the likelihood of exceeding the exposure limits in these end-uses, are evaluated in the previously
referenced risk screen. HFO-1234yf is a lower toxicity (ASHRAE toxicity group A) refrigerant. ASHRAE has adopted an OEL for
this refrigerant of 500 ppm. The toxicity risks of using HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and
complete HD vans are comparable to or lower than that of other available substitutes in the same end-use,

  including HFC-134a. [(45)]() Toxicity risks of the proposed refrigerant can be mitigated by use consistent with applicable industry safety standards, recommendations
  in the manufacturers' SDS, and other safety precautions common in the refrigeration and AC industry.

HFO-1234yf is subject to a significant new use rule (SNUR) under 40 CFR 721.10182(a). Significant new uses under this requirement
include: use other than as a refrigerant: in MVAC systems in new passenger cars and vehicles (as defined in 40 CFR 82.32(c)
and (d)), in stationary and transport refrigeration, or in stationary AC, commercial use other than in passenger cars and
vehicles in which the original charging of MVAC systems with the pre-manufacture notice (PMN) substance was done by the motor
vehicle original equipment manufacturer (OEM), in stationary and transport refrigeration, or in stationary AC., and use in
consumer products other than products used to recharge the MVAC systems in passenger cars and vehicles in which the original
charging of MVAC systems with the PMN substance was done by the motor vehicle OEM.

Use in all MVAC end-uses, except for when originally charged with HFO-1234yf, would fall under (B) or (C) as commercial or
consumer use to recharge an MVAC in which the original charging of the MVAC was with a substance other than HFO-1234yf. The
EPA considers retrofitting a vehicle to use HFO-1234yf that was not originally charged by the OEM with HFO-1234yf to be a
significant new use of HFO-1234yf under this SNUR. Significant new uses require the chemical producer to submit a significant
new use notice to the EPA for review of a substance before introducing the substance into interstate commerce in the significant
new use.

Comparison to other substitutes in these end-uses: The Agency understands that this substitute will be marketed as a retrofit option for different refrigerants, including HFC-134a.
HFC-134a is the only available refrigerant listed as acceptable for retrofit of MVACs in HD pickup trucks and complete HD
vans.

The specific atmospheric effects values can be found in the individual risk screen for HFO-1234yf. These were determined consistent
with the source information noted in section III.D. of the 2025 NPRM. The atmospheric effects for HFO-1234yf are overall better
than or comparable to many of the substitutes currently listed as acceptable in this end-use, such as HFC-134a. The EPA acknowledges
that the atmospheric effects of HFO-1234yf are relatively lower than the three blends, R-444A, R-456A, and R-480A, that were
proposed as acceptable, subject use conditions, in the MVAC end-use for retrofit of HD pickup trucks and HD vans in the 2025
NPRM. The EPA's analysis found that the effects on human health and the environment associated with retrofitting HD pickup
trucks and complete HD vans with HFO-1234yf are comparable to the other alternatives proposed for this use in the 2025 NPRM,
and lower than that of HFC-134a. (46)

The EPA's risk screen for HFO-1234yf in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans found that
this substitute can be used without exceeding the recommended OEL of 500 ppm (8-hr OEL); thus, the toxicity risks of this
refrigerant are comparable to those of other acceptable substitutes in MVACs, which also are used without exceeding their
OELs. (47)

The flammability of HFO-1234yf may be greater than that of other available substitutes in the same end-use that have an ASHRAE
flammability classification of 1. The EPA's analysis of the flammability risks of HFO-1234yf found that when used in accordance
with the proposed use conditions, this A2L refrigerant may be safely used in this end-use without presenting additional adverse
effects to human health and the environment than other alternatives. HFO-1234yf was listed as acceptable, subject to use conditions,
in MVAC end-use for new HD pickup trucks and complete HD vans in 2016. Since then, no reports of harm or incidences of fire
were recorded. We note that flammability risk can be minimized by use consistent with applicable industry safety standards
as well as recommendations in the manufacturers' SDS and other safety precautions common in the MVAC industry and any difference
in flammability can be addressed by the existing labeling requirements in appendix D of 40 CFR part 82, subpart G. (48)

This proposed refrigerant provides an additional retrofit option and would not pose additional adverse effects to human health
or the environment when used in accordance with existing and proposed requirements and as intended by the submitter. To provide
additional options for the full range of MVACs, the EPA is proposing this listing for HFO-1234yf as acceptable, subject to
use conditions, in the MVAC end-use for retrofit of HD pickup trucks and complete HD vans.

E. What use conditions is the EPA proposing for HFO-1234yf for retrofit of MVACs in this end-use and what existing requirements

apply to this refrigerant?

The EPA is proposing the use condition that unique service port fittings specific to HFO-1234yf must be used in retrofit applications
for this end-use. Service port fittings for HFO-1234yf were previously established and are identified in appendix B of 40
CFR part 82, subpart G.

Appendix D of 40 CFR part 82, subpart G specifies requirements for unique fittings for new and retrofit MVAC listings and
specifies information that must appear on a new label when a retrofit is performed, and outlines requirements for how the
retrofit is completed including specifications for how unique fittings must be applied when performing a retrofit. The requirements
for labeling, unique fittings, and the performance of the retrofit would apply to this proposed acceptability listing for
MVAC retrofits. In the case of HFO-1234yf, the requirement to include a label would mitigate risk by ensuring that technicians
are aware that the MVAC refrigerant is flammable. In the 2025 NPRM, the EPA proposed minor adjustments to these retrofit specifications
and labeling requirements that would also apply to the proposed listing of HFO-1234yf in this supplemental action. The existing
requirements and proposed amendments are described fully in section VIII.G. of the 2025 NPRM. While the proposed changes to
section VIII.G. of the 2025 NPRM are relevant to the proposed listing of HFO-1234yf in this supplemental action, the EPA is
not reopening comment on the proposed changes since the changes are broadly applicable to all MVAC retrofits, is not specific
to this end-use, and is not specific to the proposed listing of HFO-1234yf in this supplemental proposal.

In the 2025 NRPM, the EPA also proposed to amend appendix B of 40 CFR part 82, subpart G. This provision currently states
that flammable refrigerants in MVACs, both new and retrofit are unacceptable, except for HFO-1234yf and HFC-152a when used
in new MVAC equipment. The 2025 NPRM proposed to amend this provision so that unacceptability also

  would not apply to HFO-1234yf used in retrofit MVACs. This proposed amendment is described fully in section VIII.F. of the
  2025 NPRM and this supplemental proposal does not adjust or change those proposed revisions. However, the proposed revisions
  are relevant to this listing in that they would allow for use of HFO-1234yf in retrofits not only in LMDVs but also in HD
  pickup trucks and complete HD vans.

The EPA's SNAP program has a longstanding approach of requiring unique fittings for use with each refrigerant in MVACs. Appendix
D of 40 CFR part 82, subpart G requires that each refrigerant be used with a set of fittings that is unique to that refrigerant.
This is intended to prevent cross contamination of different refrigerants, preserve the purity of recycled refrigerants, and
ultimately to avoid venting of refrigerant consistent with requirements under CAA section 608(c). (49) In the 1996 SNAP Rule requiring the use of unique fittings on all refrigerants submitted for use in MVACs, the EPA urged industry
to develop mechanisms to ensure that the venting prohibition under CAA section 608(c) and the implementing regulations at
40 CFR 82.154 are observed. (50) The EPA has issued multiple SNAP rules requiring the use of fittings unique to a refrigerant for use on “containers of the
refrigerant, on can taps, on recover, recycle, and recharge equipment, and on all [motor vehicle] air conditioning system
service ports.” (51)

The EPA expects that the companies selling refrigerants intended to be used as retrofits would make appropriate unique fittings
and refrigerant labels available to certified technicians and do-it-yourselfers (DIYers) to allow them to conduct a retrofit
in a manner that meets requirements under the CAA.

V. Clarification of Intended Scope of the 2025 NPRM and Other Clarifications

The EPA is supplementing the 2025 NPRM to clarify the intended scope of that proposal. In the 2025 NPRM, the EPA proposed
a listing for R-444A as acceptable, subject to use conditions, in the MVAC end-use for retrofit of HD pickup trucks and HD
vans (both complete and incomplete). The Agency included incomplete HD vans in error and is clarifying that we intended for
this proposed listing to apply to HD pickup trucks and complete HD vans only.

See sections VIII.B., VIII.D., and VIII.E. of the 2025 NPRM for a full discussion of the EPA's basis for proposing to list
R-444A in this end-use. This discussion only provided a basis for the proposed listing of R-444A in the MVAC end-use for retrofit
of HD pickup trucks and complete HD vans. The EPA's risk screen of R-444A was for HD pickup trucks and complete HD vans. The
risk screen did not include a scenario for reviewing risk of R-444A in incomplete HD vans. The SNAP program has thus far not
listed any A2L refrigerant as acceptable for either new or retrofit use in incomplete HD vans due to lack of sufficient information
on the appropriate risk scenarios for use of flammable refrigerants in incomplete HD vans.

In 2016, when the EPA listed HFO-1234yf as acceptable, subject to use conditions, in the MVAC end-use for new HD pickup trucks
and complete HD vans, the EPA did not finalize a listing for HFO-1234yf in any incomplete HD vans, stating that we did not
have sufficient information on the potential for modifications to OEM-installed MVAC systems of incomplete HD vans by secondary
manufacturers and the impact of those modifications on the safe use of HFO-1234yf. At the time of the 2025 NPRM, the same
limitation applied and the EPA did not have sufficient information on the potential for modifications to OEM-installed MVAC
systems of incomplete HD vans by secondary manufacturers and the impact of those modifications on the safe use of R-444A.
For this reason and to remain consistent with our previous approach in SNAP Rule 21, the EPA is clarifying that the intended
scope of the proposed listing for R-444A does not include incomplete HD vans and is requesting comment on the proposed listing
as clarified. (52) The Agency will consider the comments received during the comment period for the 2025 NPRM on this proposed listing and will
review any additional comments regarding the clarification to the scope of the listing made in this supplemental action.

The clarification in this supplemental action does not impact any of the other MVAC end-uses in the 2025 NPRM where R-444A
was proposed as acceptable, subject to use conditions, including as a retrofit in LMDVs, HD pickup trucks, and complete HD
vans.

In this supplemental action, the EPA is also clarifying language in the 2025 NPRM and associated proposed regulatory text
document related to HD pickup trucks. In the 2025 NPRM, the EPA proposed listings for R-444A, R-456A, and R-480A in the MVAC
end-use for retrofit of HD pickup trucks (complete and incomplete). The Agency differentiated between complete and incomplete
HD pickup trucks in error and is clarifying that we intended for the proposed listings for R-444A, R-456A, and R-480A to apply
to HD pickup trucks generally. In contrast to HD vans, the EPA does not see complete and incomplete HD pickup trucks as two
separate applications with unique risk profiles and has previously treated complete and incomplete HD pickup trucks the same.
For purposes of consistency, the EPA is clarifying the regulatory language for the proposed listings of R-444A, R-456A, and
R-480A to use the term “HD pickup trucks” rather than HD pickup trucks (complete and incomplete). From a practical perspective,
this clarification does not change the types of vehicles that the proposed listings apply to; it is simply a clarification
of terminology. The proposed regulatory text related to the clarifications addressed in this section may be found in the docket
for this rulemaking. (53)

VI. On which topics is the EPA specifically requesting comment?

1. The EPA is requesting comment on the proposed listing of HFO-1234yf as acceptable, subject to use conditions, in the MVAC
   end-use for retrofit of HD pickup trucks and complete HD vans. Specifically, retrofitting MVACs designed for a nonflammable
   refrigerant such as HFC-134a to use a flammable refrigerant may present new risks. The EPA seeks comment on whether additional
   strategies to mitigate the flammability risk of A2L refrigerants beyond those required by appendix D to part 82, subpart G
   are necessary and suggestions of what those strategies may be.

2. The EPA is requesting comment on the clarifications described in section V. of this supplemental action, including the
   clarification of the intended scope of the 2025 NPRM as it relates to the proposed listing of R-444A in the MVAC end-use for
   retrofit of complete HD vans and the clarification of terminology related to HD pickup trucks.

VII. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be

  found at *https://www.epa.gov/laws-regulations/laws-and-executive-orders.*

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

This action is not a significant regulatory action and was therefore not submitted to the Office of Management and Budget
(OMB) for review.

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

This action is expected to be an Executive Order 14192 deregulatory action. This proposed rule is expected to provide burden
reduction by proposing to list more alternatives that would be available for use by industry.

C. Paperwork Reduction Act (PRA)

This action does not impose any new information collection burden under the PRA. OMB has previously approved the information
collection activities contained in the existing regulations and has assigned OMB control number 2060-0226. This rule contains
no new requirements for reporting or recordkeeping.

D. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of small entities under the
RFA. In making this determination, the EPA concludes that the impact of concern for this rule is any significant adverse economic
impact on small entities and that the Agency is certifying that this rule will not have a significant economic impact on a
substantial number of small entities because the rule has no net burden on the small entities subject to the rule. This action
proposes to add the additional options under SNAP of HFO-1234yf in the specified end-uses but does not mandate such use. Thus,
if the rule were finalized as proposed, it would not impose new costs on small entities. We have therefore concluded that
this action will have no net regulatory burden for all directly regulated small entities.

E. Unfunded Mandates Reform Act (UMRA)

This action does not contain an unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. The action imposes no enforceable duty on any state, local or Tribal governments or the private
sector.

F. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct effects on the states, on the relationship
between the national government and the states, or on the distribution of power and responsibilities among the various levels
of government.

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

This action does not have Tribal implications as specified in Executive Order 13175. It will not have substantial direct effects
on Tribal governments, on the relationship between the Federal government and Indian Tribes, or on the distribution of power
and responsibilities between the Federal government and Indian Tribes, as specified in Executive Order 13175. Thus, Executive
Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

Executive Order 13045 directs Federal agencies to include an evaluation of the health and safety effects of the planned regulation
on children in Federal health and safety standards and explains why the regulation is preferable to potentially effective
and reasonably feasible alternatives. This action is not subject to Executive Order 13045 because it is not a significant
regulatory action under section 3(f)(1) of Executive Order 12866, and because the EPA does not believe the environmental health
or safety risks addressed by this action present a disproportionate risk to children. While the EPA has not conducted a separate
analysis of risks to infants and children associated with this rule, the rule does contain use conditions that would reduce
exposure risks to the general population, with the reduction of exposure being most important to the most sensitive individuals.
This action's health and risk assessments are contained in the comparison of toxicity for the proposed substitute in section
IV.D. of this supplemental action, as well as in the risk screen for the substitute that is listed in this supplemental proposed
rule. The risk screen is in the docket under the title “Risk Screen on Substitutes in Motor Vehicle Air Conditioning (Heavy-Duty
Pickup Trucks and Complete Heavy-Duty Vans) (Retrofit Equipment); Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf),”at
Docket ID No. EPA-HQ-OAR-2024-0503. However, the EPA's Policy on Children's Health applies to this action. Information on how the Policy was applied is available under “Children's Environmental Health” in
the General Information section of this preamble.

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution, or Use

This action is not subject to Executive Order 13211, because it is not a significant regulatory action under Executive Order
12866.

J. National Technology Transfer and Advancement Act

This supplemental rulemaking does not involve technical standards.

VIII. References

Unless specified otherwise, all documents are available electronically at https://regulations.gov, docket number EPA-HQ-OAR-2024-0503.

ASHRAE. (2024). ANSI/ASHRAE Standard 34-2024: Designation and Safety Classification of Refrigerants.

EEAP. (2023). Environmental Effects of Stratospheric Ozone Depletion, UV Radiation, and Interactions with Climate Change.
2022 Assessment Report. UNEP, Environmental Effects Assessment Panel. https://ozone.unep.org/system/files/documents/EEAP-2022-Assessment-Report-May2023.pdf.

Gradient Corporation. (2008). Risk Assessment for Alternative Refrigerant HFO-1234yf. Confidential report prepared for SAE
International Cooperative Research Program 1234.

Gradient Corporation. (2008). Risk Assessment For Alternative Refrigerant HFO-1234yf. (Phase I) Prepared for the Society of
Automotive Engineers (SAE) Cooperative Research Project 150.

Gradient Corporation. (2009). Risk Assessment for Alternative Refrigerants HFO-1234yf and R-744 (CO 2). Confidential report prepared for SAE International Cooperative Research Program 1234.

Gradient Corporation. (2013a). Additional Risk Assessment of Alternative Refrigerant R-1234yf. Confidential report prepared
for SAE International Cooperative Research Program 1234-4.

Gradient Corporation. (2023a). Retrofit Analysis Letter. Prepared for Honeywell International.

ICF. (2009). Risk Screen on Substitutes for CFC-12 in Motor Vehicle Air Conditioning: Substitute: HFO-1234yf.

ICF. (2016b). Technical Support Document for Acceptability Listing of HFO-1234yf for Motor Vehicle Air Conditioning in Limited
Heavy-Duty Applications.

ICF. (2025k). Risk Screen on Substitutes in Motor Vehicle Air Conditioning (Light-Duty and Medium-Duty Vehicles) (Retrofit
Equipment); Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf).

ICF. (2026a). Technical Support Document for Motor Vehicle Air Conditioning in Limited Heavy-Duty Applications.

ICF. (2026b). Risk Screen on Substitutes in Motor Vehicle Air Conditioning (Heavy-Duty Pickup Trucks and Complete Heavy-Duty
Vans) (Retrofit Equipment); Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf).

Metghalchi, M. & Keck, J.C. (1980). Laminar Burning Velocity of Propane-Air Mixtures at High Temperature and Pressure. Combustion And Flame 38: 143-154: https://james-keck-memorial-collection.unibs.it/JCKeck-papers/MetghalchiKeck-CombustionFlame-38-143-1980.pdf.

Minor, B. et al. (2009). (111g) Flammability Characteristics of Low GWP Refrigerant HFO-1234yf. AIChE 2009 Spring Meeting & 5th Global Congress on Process Safety. https://proceedings.aiche.org/conferences/aiche-spring-meeting-and-global-congress-on-process-safety/2009/proceeding/paper/111g-flammability-characteristics-low-gwp-refrigerant-hfo-1234yf.

SAE. (2011). J1660 “Fittings and Labels for Retrofit of CFC-12 (R-12) Mobile Air-Conditioning Systems to HFC-134a (R-134a).”

Takizawa, K. et al. (2005). Burning velocity measurement of fluorinated compounds by the spherical-vessel method, Combustion and Flame, Volume 141, Issue 3, Pages 298-307: https://doi.org/10.1016/j.combustflame.2005.01.009.

U.S. Environmental Protection Agency. (2024). EPA Automotive Trends Report: Greenhouse Gas Emissions, Fuel Economy, and Technology
since 1975: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P101CUU6.pdf.

List of Subjects in 40 CFR Part 82

Environmental protection, Administrative practice and procedure, Air pollution control, Chemicals.

Lee Zeldin, Administrator. [FR Doc. 2026-06665 Filed 4-3-26; 8:45 am] BILLING CODE 6560-50-P

Footnotes

(1) See 90 FR 50766 (November 10, 2025).

(2) CAS Reg. No. 754-12-1.

(3) To see the adjustments to the proposed regulatory text discussed in this supplemental action in context with all the other
proposed changes and listings discussed in the 2025 NPRM, see the document in the docket for this rulemaking under the title
“Proposed Regulatory Text for SNAP Rule 27—Supplemental Proposal.”

(4) As defined in 40 CFR 82.32, Motor vehicle air conditioners mean mechanical vapor compression refrigeration equipment used
to cool the driver's or passenger's compartment of any motor vehicle. This definition is not intended to encompass the hermetically
sealed refrigeration systems used on motor vehicles for refrigerated cargo and the air conditioning systems on passenger buses
using hydrochlorofluorocarbon (HCFC)-22 refrigerant. See also 40 CFR 82.152 (defining MVAC to mean “any appliance that is a motor vehicle air conditioner as defined in subpart B of 40
CFR part 82”).

(5) As defined in 40 CFR 82.152 MVAC-like appliance means a mechanical vapor compression, open-drive compressor appliance with
a full charge of 20 pounds or less of refrigerant used to cool the driver's or passenger's compartment of off-road vehicles
or equipment. This includes, but is not limited to, the air-conditioning equipment found on agricultural or construction vehicles.
This definition is not intended to cover appliances using R-22 refrigerant.

(6) This is more broadly true for HD pickup trucks than HD vans because every manufacturer of HD pickup trucks also makes LD
pickup trucks, while only some HD van manufacturers also make LD vans (80 FR 40148; July 13, 2015).

(7) ICF. (2026a). Technical Support Document for Motor Vehicle Air Conditioning in Limited Heavy-Duty Applications.

(8) ICF. (2026a).

(9) See 76 FR 57259-60 (September 15, 2011).

(10) ICF. (2026a).

(11) Incomplete HD vehicles can also be sold to and modified by tertiary or subsequent manufacturers. For the purposes of this
supplemental proposal, the discussion of modifications made by secondary manufacturers also applies to modifications made
by tertiary or subsequent manufacturers.

(12) For example, AC for the rear compartment of an ambulance or shuttle van.

(13) See 59 FR 13044 (March 18, 1994).

(14) See 81 FR 86778 (December 1, 2016).

(15) ICF. (2026a).

(16) ASHRAE. (2024). ANSI/ASHRAE Standard 34-2024: Designation and Safety Classification of Refrigerants.

(17) See Docket ID No. EPA-HQ-OAR-2024-0503.

(18) ICF. (2026b). Risk Screen on Substitutes in Motor Vehicle Air Conditioning (Heavy-Duty Pickup Trucks and Complete Heavy-Duty
Vans) (Retrofit Equipment); Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf). See Docket ID No. EPA-HQ-OAR-2024-0503.

(19) 40 CFR 51.100(s).

(20) EEAP. (2023). Environmental Effects of Stratospheric Ozone Depletion, UV Radiation, and Interactions with Climate Change.
2022 Assessment Report. UNEP, Environmental Effects Assessment Panel. https://ozone.unep.org/system/files/documents/EEAP-2022-Assessment-Report-May2023.pdf.

(21) See 88 FR 50457-8 (June 13, 2024).

(22) Minor, B. et al. (2009). (111g) Flammability Characteristics of Low GWP Refrigerant HFO-1234yf. AIChE 2009 Spring Meeting & 5th Global Congress on Process Safety. https://proceedings.aiche.org/conferences/aiche-spring-meeting-and-global-congress-on-process-safety/2009/proceeding/paper/111g-flammability-characteristics-low-gwp-refrigerant-hfo-1234yf.

(23) Manufacturer's Safety Data Sheet for HFO-1234yf. Honeywell (May 23, 2019). See also Minor, B. et al. (2009).

(24) A2L refrigerants have a burning velocity of less than 0.1 meters/second (m/s), per International Standards Organization 817
and ASHRAE 34-2024. HFO-1234yf has a burning velocity of 0.015m/s, see Minor, B. et al. (2009).

(25) The burning velocity of HFC-152a is measured at approximately 0.236 m/s. Takizawa, K. et al. (2005). Burning velocity measurement
of fluorinated compounds by the spherical-vessel method, Combustion and Flame, Volume 141, Issue 3, Pages 298-307: https://doi.org/10.1016/j.combustflame.2005.01.009.

(26) The burning velocity of R-290 is at least 0.4 m/s, depending on temperature and pressure. Metghalchi, M. & Keck, J.C. (1980).
Laminar Burning Velocity of Propane-Air Mixtures at High Temperature and Pressure. Combustion And Flame 38: 143-154 https://james-keck-memorial-collection.unibs.it/JCKeck-papers/MetghalchiKeck-CombustionFlame-38-143-1980.pdf.

(27) Gradient Corporation. (2008). Risk Assessment for Alternative Refrigerant HFO-1234yf. (Phase I) Prepared for the Society
of Automotive Engineers (SAE) Cooperative Research Project 150.

(28) Gradient Corporation. (2008). Risk Assessment for Alternative Refrigerant HFO-1234yf. Confidential report prepared for SAE
International Cooperative Research Program 1234.

(29) Gradient Corporation. (2009). Risk Assessment for Alternative Refrigerants HFO-1234yf and R-744 (CO 2). Confidential report prepared for SAE International Cooperative Research Program 1234.

(30) See 76 FR 17491 (March 29, 2011).

(31) Gradient Corporation. (2013a). Additional Risk Assessment of Alternative Refrigerant R-1234yf. Confidential report prepared
for SAE International Cooperative Research Program 1234-4.

(32) Gradient Corporation. (2023a). Retrofit Analysis Letter. Prepared for Honeywell International.

(33) Gradient Corporation. (2009).

(34) See 81 FR 86778 (Dec. 1, 2016).

(35) ICF. (2016b). Technical Support Document for Acceptability Listing of HFO-1234yf for Motor Vehicle Air Conditioning in Limited
Heavy-Duty Applications.

(36) ICF. (2016b).

(37) See 81 FR 22810 (April 16, 2016).

(38) See 81 FR 86778 (December 1, 2016).

(39) ICF. (2026b).

(40) ICF. (2025k). Risk Screen on Substitutes in Motor Vehicle Air Conditioning (Light-Duty and Medium-Duty Vehicles) (Retrofit
Equipment); Substitute: HFO-1234yf (Solstice® yf or Solstice® 1234yf).

(41) ICF. (2025k).

(42) ICF. (2009). Risk Screen on Substitutes for CFC-12 in Motor Vehicle Air Conditioning: Substitute: HFO-1234yf.

(43) See SNAP Rule 16, 76 FR 17488 (March 29, 2011).

(44) U.S. Environmental Protection Agency. (2024). EPA Automotive Trends Report: Greenhouse Gas Emissions, Fuel Economy, and Technology
since 1975: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P101CUU6.pdf.

(45) See previous listing decisions for information regarding the toxicity of other available alternatives. (https://www.epa.gov/snap/substitutes-motor-vehicle-air-conditioning).

(46) The EPA is aware that the submitter of HFO-1234yf is likely to market this substitute to retrofit MVACs originally charged
with HFC-134a.

(47) ICF. (2026b).

(48) Described in section IV.E of this supplemental proposal.

(49) Codified at 40 CFR 82.154(a).

(50) See 61 FR 54032 (October 16, 1996).

(51) See appendix D of 40 CFR part 82, subpart G.

(52) See SNAP Rule 21, 81 FR 86778 (December 1, 2016).

(53) See Docket ID No. EPA-HQ-OAR-2024-0503 in documents titled “Proposed Changes to Appendix B for SNAP 27 Supplemental Proposal”
and “Proposed Regulatory Text for SNAP Rule 27—Supplemental Proposal.”

Download File

Download