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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 82

[EPA-HQ-OAR-2004-0488; FRL-XXXX-X]

[RIN 2060-AM54] 

Protection of Stratospheric Ozone: Alternative for the Motor Vehicle Air Conditioning Sector 

under the Significant New Alternatives Policy (SNAP) Program

AGENCY: Environmental Protection Agency (EPA)

ACTION: Final Rule

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SUMMARY: Pursuant to the U.S. Environmental Protection Agency (EPA)’s Significant New 

Alternatives Policy (SNAP) program, this action lists carbon dioxide (CO2) or R-744, as 

acceptable substitute, subject to use conditions, in the motor vehicle air conditioning (MVAC) 

end-use for motor vehicles (i.e., passenger cars, light-duty and heavy-duty vehicles) within the

refrigeration and air-conditioning sector. This final rule only concerns the use of CO2 in MVAC 

systems designed specifically for the use of CO2 refrigerant. The substitute is non-ozonedepleting and therefore does not contribute to stratospheric ozone depletion.

DATES: This final rule is effective on [INSERT 60 DAYS FROM DATE OF PUBLICATION 

IN THE FEDERAL REGISTER]. 

ADDRESSES: EPA has established a docket for this action under Docket ID No. EPA-HQOAR-2004-0488. All documents in the docket are listed on the www.regulations.gov web site.

Although listed in the index, some information is not publicly available, e.g., confidential 

business information (CBI) or other information whose disclosure is restricted by statute. Certain 

other material, such as copyrighted material, is not placed on the Internet and will be publicly
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available only in hard copy form. Publicly available docket materials are available either 

electronically through www.regulations.gov or in hard copy from the EPA Air and Radiation

Docket, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC. This 

Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding 

legal holidays. The telephone number for the Public Reading Room is (202) 566-1744, and the 

telephone number for the Air and Radiation Docket is (202) 566-1742. 

FOR FURTHER INFORMATION CONTACT: Yaidi Cancel, Stratospheric Protection 

Division, Office of Air and Radiation, MC 6205J, Environmental Protection Agency, 1200 

Pennsylvania Ave., NW, Washington, DC 20460; telephone number: (202) 343-9512; fax 

number: (202) 343-2338; email address: cancel.yaidi@epa.gov. 

SUPPLEMENTARY INFORMATION:

This final action provides motor vehicle manufacturers and their suppliers with a

refrigerant option subject to use conditions for motor vehicle air conditioning systems for use in

new vehicles. The refrigerant discussed in this action, carbon dioxide (R-744, CO2) is nonozone-depleting and has a global warming potential (GWP)1

of 1.

TABLE OF CONTENTS

I. Does this action apply to me?

II. What abbreviations and acronyms are used in this action?

III. How does the SNAP program work?

 A. What are the statutory requirements and authority for the SNAP program? 

B. What are EPA's regulations implementing section 612 of the Clean Air Act?

 

1 GWP, is defined as the ratio of heat trapped by one unit mass of the greenhouse gas to that of one unit mass of CO2

over a over a specified period of time. Consistent with the international standards under the United Nations 

Framework Convention on Climate Change (UNFCCC), all GWPs in this rule are given using a 100-year period

(IPCC, 1996).
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C. How do the regulations for the SNAP program work?

D. Where can I get additional information about the SNAP program?

IV. What is EPA's final decision for CO2 as an alternative for MVAC?

V. Why is EPA establishing these final use conditions for the use of CO2 in MVAC?, 

VI. Why is EPA listing CO2 acceptable subject to use conditions? 

VII. What is the relationship between this SNAP rule and other EPA rules? 

VIII. What is EPA's response to public comments on the proposal?

A. Use conditions 

B. Risk mitigation strategies

C. Industry standards

D. Servicing

IX. Statutory and executive order reviews

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

Improving Regulation and Regulatory Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Constitution and Coordination with Indian Tribal Governments

G. Executive Order 13045: Protection of Children from Environmental Health Risks and 

Safety Risks

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution, or 

Use

I. National Technology Transfer Advancement Act
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K. Congressional Review Act

X. References

I. Does This Action Apply to Me?

This final rule lists carbon dioxide (CO2)

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, also known as R-744, as an acceptable 

substitute subject to use conditions for use as a refrigerant in new motor vehicle air conditioning

(MVAC) systems designed specifically for the use of CO2 refrigerant in motor vehicles3. 

Businesses in this end-use that may want to use CO2 in MVAC systems include:

 motor vehicle manufacturers

 motor vehicle air conditioning service and repair shops

Regulated entities may include:

Table 1–Potentially Regulated Entities, 

by North American Industrial Classification System (NAICS) Code

Category NAICS code Description of regulated entities

Industry 336111, 336112, 336120 Motor Vehicle Manufacturing

Services 811198 Vehicle Air Conditioning Repair 

This table is not intended to be exhaustive, but rather a guide regarding entities likely to be 

regulated by this action. If you have any questions about whether this action applies to a 

particular entity, consult the person listed in the preceding section, “FOR FURTHER 

INFORMATION CONTACT.”

 

2 Chemical Abstracts Service [CAS] Registry: No. 124-38-9

3 This final action applies only to air conditioning systems in motor vehicles consistent with the definition of light 

duty vehicles and heavy-duty vehicles under 40 CFR §86.1803-01, with the exception of passenger busses.
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II. What abbreviations and acronyms are used in this action?

ACGIH-American Conference of Governmental Industrial Hygienists 

ASE-National Institute for Automotive Service Excellence

CAA-Clean Air Act

CAS-Chemical Abstracts Service

CBI-confidential business information

CFC-chlorofluorocarbon

CFC-12-the chemical dichlorodifluoromethane, CAS Reg. No. 75-71-8

CFD-computational fluid dynamics

CFR-Code of Federal Regulations

CNS – central nervous system

CO2-carbon dioxide, CAS Reg. No. 124-38-9, also known as R-744

CRP-Cooperative Research Program

EPA-the United States Environmental Protection Agency

EO-Executive Order

FMEA- Failure Mode and Effect Analysis

FR-Federal Register

FTA-fault-tree analysis

GWP-Global warming potential

HCFC-22 – the chemical chlorodifluoromethane, CAS Reg No. 75-45-6

HCFC-142b – the chemical 1-chloro-1,1-difluoroethane, CAS Reg No. 75-68-3

HFC-hydrofluorocarbon

HFC-134a-the chemical 1,1,1,2-tetrafluoroethane, CAS Reg. No. 811-97-2
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HFC-152a-the chemical 1,1-difluoroethane, CAS Reg. No. 75-37-6

HFO-hydrofluoroolefin

HFO-1234yf-the chemical 2,3,3,3-tetrafluoroprop-1-ene, CAS Reg. No. 754-12-1

IDLH-Immediately Dangerous to Life and Health 

MVAC-motor vehicle air conditioning

NIOSH-National Institute for Occupational Safety and Health

NODA-Announcement of Data Availability, formerly known as Notice of Data Availability

NPRM-Notice of Proposed Rulemaking

NTTAA-National Technology Transfer and Advancement Act

ODP-ozone depletion potential

ODS-ozone-depleting substance

OEM-original equipment manufacturer

OMB-the United States Office of Management and Budget

OSHA-the United States Occupational Safety and Health Administration

PEL-Permissible Exposure Level

ppm-parts per million

RDECOM-U.S. Army Research, Development and Engineering Command 

REL-Recommended Exposure Level

RFA-Regulatory Flexibility Act

SAE-SAE International, formerly the Society of Automotive Engineers

SAE CRP-SAE Cooperative Research Program

SBREFA-Small Business Regulatory Enforcement Fairness Act

SNAP-Significant New Alternatives Policy
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STEL-Short Term Exposure Limit

TWA-Time Weighted Average

UMRA-Unfunded Mandates Reform Act

III. How does the SNAP program work?

A. What are the statutory requirements and authority for the SNAP program?

Section 612 of the Clean Air Act (CAA) requires U.S. Environmental Protection Agency

(EPA) to develop a program for evaluating alternatives to ozone-depleting substances (ODS).

EPA refers to this program as the Significant New Alternatives Policy (SNAP) program. The 

major provisions of section 612 are: 

1. Rulemaking

Section 612(c) requires EPA to promulgate rules making it unlawful to replace any class I 

(i.e., chlorofluorocarbon, halon, carbon tetrachloride, methyl chloroform, methyl bromide, and 

hydrobromofluorocarbon) or class II (i.e., hydrochlorofluorocarbon) substance with any

substitute that the Administrator determines may present adverse effects to human health or the 

environment where the Administrator has identified an alternative that (1) reduces the overall 

risk to human health and the environment, and (2) is currently or potentially available.

2. Listing of Unacceptable/Acceptable Substitutes

Section 612(c) requires EPA to publish a list of the substitutes unacceptable for specific 

uses and to publish a corresponding list of acceptable alternatives for specific uses. The list of 

acceptable substitutes is found at http://www.epa.gov/ozone/snap/lists/index.html and the lists of 

“unacceptable,” “acceptable subject to use conditions,” and “acceptable subject to narrowed use 

limits” substitutes are found in the appendices to 40 CFR part 82 subpart G. 
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3. Petition Process

Section 612(d) grants the right to any person to petition EPA to add a substance, add or 

delete use restrictions, or delete a substance from the lists published in accordance with section 

612(c). The Agency has 90 days to grant or deny a petition. Where the Agency grants the 

petition, EPA must publish the revised lists within an additional six months.

4. 90-day Notification

Section 612(e) directs EPA to require any person who produces a chemical substitute for 

a class I substance to notify the Agency not less than 90 days before new or existing chemicals 

are introduced into interstate commerce for significant new uses as substitutes for a class I 

substance. The producer must also provide the Agency with the producer's unpublished health 

and safety studies on such substitutes.

5. Outreach

Section 612(b)(1) states that the Administrator shall seek to maximize the use of federal

research facilities and resources to assist users of class I and II substances in identifying and 

developing alternatives to the use of such substances in key commercial applications.

6. Clearinghouse

Section 612(b)(4) requires the Agency to set up a public clearinghouse of alternative 

chemicals, product substitutes, and alternative manufacturing processes that are available for 

products and manufacturing processes which use class I and II substances.

B. What are EPA’s regulations implementing section 612?

On March 18, 1994, EPA published the original rulemaking (59 FR 13044) which 

established the process for administering the SNAP program and issued EPA's first lists 
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identifying acceptable and unacceptable substitutes in the major industrial use sectors (40 CFR 

part 82, subpart G). These sectors include: refrigeration and air conditioning; foam blowing; 

solvents cleaning; fire suppression and explosion protection; sterilants; aerosols; adhesives, 

coatings and inks; and tobacco expansion. These sectors comprise the principal industrial sectors 

that historically consumed the largest volumes of ODS.

Section 612 of the CAA requires EPA to list as acceptable those substitutes that do not 

present a significantly greater risk to human health and the environment as compared with other 

substitutes that are currently or potentially available.

C. How do the regulations for the SNAP program work?

Under the SNAP regulations, anyone who produces a substitute to replace a class I or II 

ODS in one of the eight major industrial use sectors must provide notice to the Agency, 

including health and safety information on the substitute at least 90 days before introducing it 

into interstate commerce for significant new use as an alternative. 40 CFR 82.176(a). This 

requirement applies to the person planning to introduce the substitute into interstate commerce,4

typically chemical manufacturers, but may also include importers, formulators, equipment 

manufacturers, or end-users

5 when they are responsible for introducing a substitute into 

commerce. The 90-day SNAP review process begins once EPA receives the submission and 

determines that the submission includes complete and adequate data. 40 CFR 82.180(a). The 

CAA and the SNAP regulations, 40 CFR 82.174(a), prohibit use of a substitute earlier than 90-

 

4 As defined at 40 CFR 82.104 “interstate commerce” means the distribution or transportation of any product 

between one state, territory, possession or the District of Columbia, and another state, territory, possession or the 

District of Columbia, or the sale, use or manufacture of any product in more than one state, territory, possession or 

District of Columbia. The entry points for which a product is introduced into interstate commerce are the release of 

a product from the facility in which the product was manufactured, the entry into a warehouse from which the 

domestic manufacturer releases the product for sale or distribution, and at the site of United States Customs 

clearance.

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As defined at 40 CFR 82.172 “end-use” means processes or classes of specific applications within major industrial 

sectors where a substitute is used to replace an ozone-depleting substance. 
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days after notice has been provided to the Agency.

The Agency has identified four possible decision categories for substitutes: acceptable; 

acceptable subject to use conditions; acceptable subject to narrowed use limits; and 

unacceptable.640 CFR 82.180(b).Use conditions and narrowed use limits are both considered 

“use restrictions” and are explained below. Substitutes that are deemed acceptable with no use 

restrictions (no use conditions or narrowed use limits) can be used for all applications within the 

relevant end-uses within the sector. Substitutes that are acceptable subject to use restrictions may

be used only in accordance with those restrictions. 

After reviewing a substitute, the Agency may determine that a substitute is acceptable 

only if certain conditions in the way that the substitute is used are met to minimize risks to 

human health and the environment. EPA describes such substitutes as "acceptable subject to use 

conditions." Entities that use these substitutes without meeting the associated use conditions are 

in violation of section 612 of the Clean Air Act and EPA’s SNAP regulations. 40 CF 82.174(c).

For some substitutes, the Agency may permit a narrow range of use within an end-use or 

sector. For example, the Agency may limit the use of a substitute to certain end-uses or specific 

applications within an industry sector. The Agency requires a user of a narrowed use substitute to 

demonstrate that no other acceptable substitutes are available for their specific application by 

conducting comprehensive studies. EPA describes these substitutes as “acceptable subject to 

narrowed use limits.” A person using a substitute that is acceptable subject to narrowed use 

limits in applications and end-uses that are not consistent with the narrowed use limit is using 

these substitutes in an unacceptable manner and is in violation of section 612 of the CAA and 

EPA’s SNAP regulations. 40 CFR 82.174(c).

 

6 The SNAP regulations also include “pending,” referring to submissions for which EPA has not reached a 

determination, under this provision.
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The Agency publishes its SNAP program decisions in the Federal Register (FR). EPA 

publishes decisions concerning substitutes that are deemed acceptable subject to use restrictions 

(use conditions and/or narrowed use limits), or for substitutes deemed unacceptable, as proposed 

rulemakings to allow the public opportunity to comment, before publishing final decisions. 

In contrast, EPA publishes substitutes that are deemed acceptable with no restrictions in 

“notices of acceptability,” rather than as proposed and final rules. As described in the preamble 

to the rule initially implementing the SNAP program (59 FR 13044; March 18, 1994)), EPA does 

not believe that rulemaking procedures are necessary to list alternatives that are acceptable 

without restrictions because such listings neither impose any sanction nor prevent anyone from 

using a substitute.

Many SNAP listings include “comments” or “further information” to provide additional 

information on substitutes. Since this additional information is not part of the regulatory

decision, these statements are not binding for use of the substitute under the SNAP program. 

However, regulatory requirements so listed are binding under other regulatory programs. The 

“further information” classification does not necessarily include all other legal obligations 

pertaining to the use of the substitute. While the items listed are not legally binding under the 

SNAP program, EPA encourages users of substitutes to apply all statements in the “further 

information” column in their use of these substitutes. In many instances, the information simply 

refers to sound operating practices that have already been identified in existing industry and/or 

building-codes or standards. Thus, many of the statements, if adopted, would not require the 

affected user to make significant changes in existing operating practices.

D. Where can I get additional information about the SNAP program?

For copies of the comprehensive SNAP lists of substitutes or additional information on 
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SNAP, refer to EPA’s Ozone Depletion web site at www.epa.gov/ozone/snap/index.html. For 

more information on the Agency's process for administering the SNAP program or criteria for 

evaluation of substitutes, refer to the SNAP final rulemaking published March 18, 1994 (59 FR 

13044), codified at 40 CFR part 82, subpart G. A complete chronology of SNAP decisions and 

the appropriate citations are found at http://www.epa.gov/ozone/snap/chron.html.

IV. What is EPA's final decision for CO2 as an alternative for MVAC?

In this final rule, EPA is modifying its previous determination that listed CO2 as an

acceptable substitute for CFC-12 in new MVAC systems (59 FR 13044; March 18, 1994) and is 

listing CO2 acceptable, subject to use conditions, as a substitute for CFC-12 in new MVAC 

systems. This final action does not apply to the use of CO2 as a conversion or retrofit for existing 

MVAC systems. In addition, it does not apply to the use of CO2 in the air conditioning or 

refrigeration systems of buses, trains, rail or subway cars, or appliances such as refrigerated 

transport. This refrigerant may be used only in equipment designed specifically and clearly 

identified for this refrigerant (i.e., it may not be used as a conversion or “retrofit” refrigerant for 

existing equipment).EPA is not mandating the use of CO2 or any other alternative to ODS in

MVAC systems. Vehicle manufacturers have the option of using any refrigerant listed as 

acceptable for this end-use, so long as they meet the applicable use conditions. This action 

removes CO2 from the list of acceptable substitutes for MVAC systems and instead lists it as 

acceptable subject to the following use conditions: 

1. Engineering strategies and/or mitigation devices shall be incorporated such that in 

the event of refrigerant leaks the resulting CO2 concentrations do not exceed:

 The short term exposure level (STEL) of 3% or 30,000 ppm averaged 

over 15 minutes in the passenger free space; and 
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 The ceiling limit of 4% or 40,000 ppm in the passenger breathing 

zone.

2. Vehicle manufacturers (i.e., original equipment manufacturers [OEMs]) must keep 

records of the tests performed for a minimum period of three years demonstrating

that CO2 refrigerant levels do not exceed the STEL of 3% averaged over 15 minutes 

in the passenger free space, and the ceiling limit of 4% in the breathing zone.

3. The use of CO2 in MVAC systems must adhere to the standard conditions identified 

in SAE

7 Standard J639 (2011 version) including:

 Installation of a high pressure system warning label;

 Installation of a compressor cut-off switch

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; and

 Use of unique fittings with:

i. Outside diameter of 16.6 +0/-0.2 mm (0.6535 +0/-0.0078 inches) 

for the MVAC low-side service port;

ii. Outside diameter of 18.1 +0/-0.2 mm (0.7126 +0/-0.0078 inches) 

for the MVAC high-side service port; and 

iii. Outside diameter of 20.955 +0/-0.127 mm (0.825 +0/-0.005 

inches) and right-hand thread direction for CO2 refrigerant service 

containers

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.

To help ensure that the first use condition is met, we are including several 

recommendations in the listing decision. First, OEMs should conduct and keep on file Failure 

 

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SAE International, formerly the Society of Automotive Engineers.

8 A compressor cut-off switch causes a device to stop compressor operation before activation of any pressure relief 

device. 

9 The refrigerant service containers fitting requirement applies only to refrigerant service containers used during 

servicing of the MVAC, in accordance with the provisions established for MVAC servicing under 40 CFR part 82, 

subpart B.
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Mode and Effect Analysis (FMEA) on the MVAC as stated in SAE J1739 (Potential Failure 

Mode and Effects Analysis in Design [Design FMEA], Potential Failure Mode and Effect 

Analysis in Manufacturing and Assembly Process [Process FMEA]), or equivalent. Second, 

OEMs should factor in background CO2 concentrations that come about from normal respiration 

by the maximum number of vehicle occupants.10 Third, EPA recommends the use of the 

following industry standards as additional references when locating the driver’s and passengers’ 

breathing zone consistent with the head and seating position, measuring refrigerant 

concentrations at different locations inside the passenger compartment including the breathing 

zone, and addressing risks associated with MVAC use:

 SAE J1052-Motor Vehicle Driver and Passenger Head Position;

 SAE J2772-Measurement of Passenger Compartment Refrigerant Concentrations 

under System Refrigerant Leakage Conditions; and 

 SAE J2773-Standard for Refrigerant Risk Analysis for Mobile Air Conditioning 

Systems.

Fourth, EPA recommends additional training for MVAC service technicians that will 

service MVAC systems using CO2 as the refrigerant. 

V. Why is EPA establishing these final use conditions for the use of CO2 in new MVAC? 

Summary of SNAP actions on the use of CO2 as a refrigerant in MVAC 

In the initial SNAP rulemaking issued on March 18, 1994 (59 FR 13044), EPA found

CO2 acceptable as a substitute for CFC-12 in new MVAC systems. In that final rule, EPA also 

found other substitutes (i.e., HFC-134a and R-401C, evaporative cooling and stirring cycle)

 

10 Maximum number of vehicle occupants includes the maximum number of passengers in a normal seating position 

inside the passenger compartment. This may vary between vehicle types.
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acceptable for use in new MVAC systems. On June 13, 1995 (60 FR 31092) and October 16, 

1996 (61 FR 54040) EPA took two separate actions requiring the use of unique fittings for

several refrigerants then currently listed as acceptable for use in new MVAC systems (60 FR 

31092) and for refrigerants subsequently found acceptable for use in MVAC (61 FR 54040). The 

use conditions requiring unique fittings were codified at 40 CFR Part 82, Subpart G, Appendix 

D. None of these actions applied to CO2. However, in the preamble to the October 16, 1996 

SNAP rule, EPA stated that for any decision made under SNAP, the Agency may, on its own, 

determine that additional conditions or restrictions should be added or removed through future 

rulemaking (61 FR 54032). Also, EPA stated in the October 16, 1996 SNAP rule that due 

concerns about potential cross-contamination as a result of the large number of MVAC 

refrigerants, the Agency may choose to list a substitute as acceptable subject to use the 

conditions listed (in that rule, i.e., use of unique fittings) while proceeding with notice-andcomment rulemaking to impose other restrictions (61 FR 54034).

Although the initial SNAP rulemaking listed CO2 as acceptable for use in new MVAC 

systems, at that time, EPA was not aware of any interest in using CO2 in MVAC systems and did 

not receive any submission for unique fittings to be used on CO2 MVAC systems or any

information specified in 40 CFR Part 82, Subpart G, Appendix D. EPA was subsequently made 

aware through risk screens of concerns regarding health risks to exposure of CO2 from 

refrigerant leaks into the passenger compartment (EPA-HQ-OAR-2004-0488-0025.2). EPA was 

also made aware of potential interest in using CO2 as a refrigerant for MVAC systems and of 

technology being developed (71 FR 55141; September 21, 2006). On September 21, 2006, we 

issued a Notice of Proposed Rulemaking (referred to hereinafter as “the proposal” or NPRM) 

proposing to find CO2 acceptable as a substitute for CFC-12 in new MVAC systems, subject to 
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the use conditions specified at 40 CFR Part 82, Subpart G, Appendix D (71 FR 55140). In 

addition, due to concerns regarding the possibility of driver performance decrement and adverse 

effects on passengers if exposed to concentrations of CO2 above 3% during a short period of time 

(e.g., 15 minutes), we proposed use conditions restricting CO2 refrigerant concentrations to a 

STEL of 3% averaged over 15 minutes in the passenger free space caused by leaks from the 

MVAC.11 Subsequently, on September 17, 2009 (74 FR 47774), EPA issued a notice of data 

availability (NODA) making available to the public additional information received supporting a 

ceiling limit of 4% CO2 as a level that should not be exceeded for any period of time due to 

possible adverse health effects. We also requested public comment on whether EPA should 

include in a final rule, listing CO2 as acceptable subject to use conditions for new MVAC 

systems, a ceiling limit of 4% CO2 in addition to the proposed STEL of 3% averaged over 15 

minutes inside the passenger compartment, and whether the proposed use conditions should 

apply when the ignition is off. 

Basis for use conditions included in this final rule

EPA proposed three use conditions in the NPRM. One use condition required that 

systems be designed to avoid occupant exposure to CO2 concentrations above a STEL of 3% 

CO2 averaged over 15 minutes in the passenger free space, during the event of a leak. The

passenger free space is the space inside the passenger compartment excluding the space enclosed 

by the ducting in the HVAC module (71 FR 55149). The proposal also stated that a breathing 

zone ceiling limit may provide additional assurance regarding vehicle driver alertness and 

requested comment on whether a maximum limit should be applied in the driver and passenger 

breathing zone, in addition to the 3% CO2 free space limit averaged over 15 minutes. In the 

 

11 In the same NPRM, EPA also proposed to find HFC-152a acceptable subject to use conditions. On June 12, 2008, 

EPA published a final rule listing HFC-152a as an acceptable substitute, subject to use conditions, for new MVAC 

(73 FR 33304), but deferred final ruling on the use of CO2 in new MVAC systems. 
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NODA, we defined the breathing zone as the space where people breathe (74 FR 47775), and 

data received during the public comment period defined this zone as the area inside the 

passenger compartment where the driver’s and passengers’ heads are located during a normal 

sitting position.12

The other proposed use conditions required OEMs to: 1) keep records of the test 

performed to ensure that MVAC systems are safe and designed with sufficient safety mitigation 

devices so that occupants are not exposed to levels above the CO2 STEL; and 2) adhere to all the 

safety requirements listed in the SAE Standard J639, in addition to the use conditions already 

established under Appendix D to Subpart G of 40 CFR Part 82, for MVAC substitutes: unique 

fittings13, label, and a compressor cut-off switch. 

We received a number of public comments on the proposed use conditions and 

subsequent data announced in the NODA regarding the 4% CO2 ceiling limit. Some commenters 

claimed that the proposed STEL of 3% CO2 averaged over 15 minutes was enough to protect 

passengers and ensure driver alertness (EPA-HQ-OAR-2004-0448-0025.1, -0032, -0044). Other 

commenters stated that there are sufficient arguments for choosing percent concentration limits 

higher than the proposed STEL of 3% CO2 averaged over 15 minutes (EPA-HQ-OAR-2004-

0448-0043, -0049). Alternatively, some commenters requested a maximum CO2 ceiling limit in 

the passenger breathing zone (EPA-HQ-OAR-2004-0448-0030, -0035, -0047.1) and one 

commenter considered appropriate the 4% CO2 ceiling limit as an additional use condition (EPAHQ-OAR-2004-0448-0047.1).

After considering the information in the docket at the time of proposal, comments 

 

12 This was the location considered in the U.S. Army risk assessment, in addition to the rest of the vehicle occupant 

compartment (EPA-HQ-OAR-2004-0488-0025.2)

13 The unique fittings provision applies for MVAC service ports and containers intended for servicing of the MVAC 

(Appendix D to Subpart G of 40 CFR Part 82, 61 FR 54040; October 16, 2006).
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received on the proposed rule, and additional information we have received in response to the 

NODA, we have decided to finalize the use conditions as proposed in the September 21, 2006, 

NPRM, and to add a ceiling limit of 4% CO2, which would apply in addition to the 3% averaged 

over 15-minute CO2 STEL. We believe that requiring a CO2 ceiling limit is necessary because it 

is possible for a time-weighted average concentration, such as the STEL, to be under 3%, while 

peak concentrations could reach higher limits resulting in possible hearing and vision effects that 

could distract and endanger a driver, or cause other, potentially more severe adverse health 

effects (EPA-HQ-OAR-2004-0488-0041). Thus, the proposed use condition requiring mitigation 

strategies for MVAC systems, to prevent leaks of CO2 refrigerant reaching concentrations above 

3% averaged over 15 minutes inside the passenger compartment free space, may not be sufficient 

on its own to protect drivers and passengers. This further protective limit is necessary to ensure 

that overall risks to human health and the environment from CO2 will be similar to or less than 

those of other available refrigerants that EPA has already listed as acceptable for MVAC. 

In the final rule, we also revised the proposed use condition on recordkeeping to refer to 

the 4% ceiling limit. The September 21, 2006 NPRM proposed requiring OEMs to keep records 

demonstrating they have met the use condition requiring safety mitigation devices to avoid 

occupant exposure above the 3% CO2 STEL in the passenger compartment. The final use 

condition addressing recordkeeping requires OEMs to keep records of the tests performed for a 

minimum period of three years demonstrating that MVAC systems are designed incorporating

engineering devices or mitigation strategies so that in the event of refrigerant leak, the resulting 

concentrations of CO2 in the passenger free space do not exceed the STEL of 3% averaged over 

15 minutes and do not exceed the ceiling limit of 4% in the passenger breathing zone. Keeping 

records of tests performed evaluating system safety is a customary practice for OEMs while 
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vehicles are in production and for several years afterward (EPA-HQ-OAR-2004-0488-0057).

For purposes of the final rule, we are referencing to the 2011 version of SAE J639 instead 

of the 2005 version referenced in the proposed rule. The SAE J639 2011 version added new 

provisions designed specifically to address use of another refrigerant, HFO-1234yf (EPA-HQOAR-2004-0488-0059). The provisions under the 2011 version of SAE J639 for high pressure 

system warning label, compressor cut-off switch, and unique fittings, remain unchanged.

Consistent with the proposed rule, the criteria for uniqueness of fittings under Appendix H to 

Subpart G of 40 CFR Part 82, and the provisions of Appendix D to Subpart G of 40 CFR Part 82 

and SAE J639 (2011 version), in this final rule we specify that the CO2 refrigerant fittings must 

have: 1) an outside diameter of 16.6 +0/-0.2 mm (0.6535 +0/-0.0078 inches) for the MVAC lowside, 2) an outside diameter of 18.1 +0/-0.2 mm (0.7126 +0/-0.0078 inches) for the MVAC highside, and 3) an outside diameter of 20.955 +0/-0.127 mm (0.825 +0/-0.005 inches) and right-hand 

thread direction for refrigerant service containers.

14

Why is EPA listing CO2 acceptable subject to use conditions?

EPA is listing CO2 acceptable subject to use conditions because the use conditions are 

necessary to ensure that use of CO2 will not present greater risk to human health and the 

environment than other available substitutes acceptable for use in new MVAC systems.

Examples of other substitutes that EPA has already found acceptable subject to use conditions 

for use in new MVAC systems include HFC-152a and HFO-1234yf.15 A list of acceptable 

substitutes subject to use conditions for use in new MVAC systems can be found at Appendix B 

 

14 The SAE J639 standard specifies unique fittings for high-side and low-side service ports and makes reference to 

SAE J2683 “Refrigerant Purity and Container requirements for Carbon Dioxide (CO2 R-744) Used in Mobile Air 

Conditioning Systems” which specifies that the unique fitting for CO2 refrigerant service containers must be 

consistent with the Cylinder Gas Association’s fitting CGA 320 (for 0-3000 psi) which has an outside diameter of 

0.825 +0/-0.005 inches (20.955 +0/-0.127 mm) and right-hand thread direction (EPA-HQ-OAR-2004-0488-0059, -

0060).

15 HFO-1234yf was found acceptable only for MVAC systems in new passenger cars and light duty trucks (76 FR 

17488, March 29, 2011).
20

to Subpart G of 40 CFR, Part 82 and 

http://www.epa.gov/ozone/snap/refrigerants/lists/mvacs.html. 

EPA is requiring the use of unique fittings for CO2 refrigerant consistent with Appendix 

D to Subpart G of 40 CFR Part 82 (61 FR 54040; October 16, 1996). All acceptable substitutes for 

use in MVAC systems are subject to those use conditions (and thus are identified as acceptable 

subject to use conditions). For CO2, the unique fittings that must be used for MVAC systems are 

those identified in the industry standard SAE J639 (2011 version).

In addition to the use conditions regarding unique fittings, EPA is requiring OEMs to 

adhere to all the safety requirements of SAE J639 (2011 version) for the safe design of new 

MVAC systems using CO2. We are establishing this as a use condition to ensure that new MVAC

systems that use CO2 are specifically designed to minimize release of the refrigerant into the 

passenger cabin. Adherence to the standard will minimize the risks that CO2 refrigerant levels in 

the passenger compartment and breathing zone would exceed the CO2 limits of 3% averaged over 

15 minutes in the passenger cabin free space and the 4% ceiling limit in the passenger breathing 

zone. 

Environmental impacts

EPA finds that CO2 does not pose greater risk to the environment than other substitutes 

that are currently available in the end-use being evaluated in this rulemaking. In at least one aspect, 

CO2 is significantly better for the environment than most alternatives currently listed as acceptable 

subject to use conditions in the MVAC end-use. CO2 has a hundred-year time horizon (100-yr) 

global warming potential (GWP) of one, compared with a GWP of four for HFO-1234yf, 124 for 

HFC-152a, and 1,430 for HFC-134a. Further, CO2 has an ozone depletion potential (ODP) of zero, 

comparable to HFO-1234yf, HFC-152a, and HFC-134a. Other SNAP-approved refrigerant blends 
21

containing HCFCs have ODPs ranging from 0.065 to 0.022. Additionally, CO2 is excluded from 

the definition of volatile organic compound (VOC) under CAA regulations (see 40 CFR 

51.100(s)).

Human health and safety impacts 

Carbon dioxide is not flammable, similar to HFC-134a and most other acceptable 

alternatives for MVACs. Therefore, it does not add risks of fire in a vehicle when used. For the 

MVAC end-use, the EPA has listed two flammable alternatives (HFC-152a and HFO-1234yf) 

acceptable, subject to use conditions to mitigate flammability risks.

CO2 is an asphyxiant that obstructs the oxygen flow into the body (OSHA, 1996; as cited 

in EPA-HQ-OAR-2004-0488-0041). However, it is not the only gas that may cause asphyxia. 

Releasing almost any gas16 into an unventilated or poorly ventilated space can lower the oxygen 

concentration to a level that poses significant health risks (EPA-HQ-OAR-2004-0488-0041).

Health risks could occur to drivers or vehicle occupants during release of CO2 refrigerant into the 

passenger compartment. Additionally, occupational risks could occur during the manufacture of 

the refrigerant, initial installation of the refrigerant into the MVAC system at the vehicle 

assembly plant, servicing of the MVAC system, or final disposition of the MVAC system (i.e., 

recycling or disposal). 

We evaluated potential human health and safety impacts, including the short- and longterm toxicity of CO2 and risk of injury to service personnel from high-pressure CO2 MVAC 

systems, and considered detailed risk assessments with fault-tree analysis (FTA), (EPA-HQOAR-2004-0488-0017, -0022, and -0025.2), scientific data provided in public comments (EPAHQ-OAR-2004-0488-0037.1) and other information obtained during the notice of data 

 

16 Any refrigerant can act as an asphyxiant by limiting available oxygen in a space. When oxygen levels in air are 

reduced to 12-14% by displacement, symptoms of asphyxiation, loss of concentration, increased pulse rate and 

deeper respiration will occur.
22

availability (EPA-HQ-OAR-2004-0488-0041). We also reviewed a risk assessment with faulttree analysis from the SAE Corporate Research Program (CRP) for HFO-1234yf and CO2, 

submitted during the public comment period for another SNAP rulemaking17 (EPA-HQ-OAR2004-0488-0051, EPA-HQ-OAR-2008-0664-0008, and -0056). We also evaluated and provided 

additional information on the health effects and risks to CO2 exposure through a contractorauthored report “Review of Health Impacts from Short-Term Carbon Dioxide Inhalation 

Exposures” (EPA-HQ-OAR-2004-0488-0041). This report revealed that exposures over 4% 

(40,000 ppm) CO2 are likely to cause discomfort and signs of intoxication that could impair the 

driver’s response to road and driving conditions, and could create safety and health risks to the 

passengers. In addition to this report, a revised risk analysis performed by the U.S. Army 

Research, Development and Engineering Command (herein referred as U.S. Army risk 

analysis)

18, submitted during the public comment period, indicated that limiting passenger 

exposure to 4% CO2 is sufficiently protective to avoid serious or irreversible health effects in 

potentially sensitive subpopulations (EPA-HQ-OAR-2004-0488-0025.2). Also, the U.S. Army

risk analysis selected the 4% CO2 level based on the lowest level at which performance 

decrements were observed in studies by Wong, 1992 (EPA-HQ-OAR-2004-0488-0025.2).

Vehicle driver and passenger risks

EPA’s review of vehicle driver and passenger risks from CO2 refrigerant exposure 

indicated that a potential refrigerant leak into the vehicle passenger compartment is not expected 

to present an unreasonable exposure risk if engineering strategies or mitigation strategies are 

applied (EPA-HQ-OAR-2004-0488-0025, -0037.1). The U.S. Army risk assessment indicated a 

 

17 SNAP rule listing as acceptable subject to use conditions HFO-1234yf for MVACs in new passenger cars and 

light-duty vehicles (76 FR 17488, March 29, 2011).

18 Blackwell et. al 2006; Risk Analysis for Alternative Refrigerant in Motor Vehicle Air Conditioning (revised risk 

analysis made in collaboration with EPA and several stakeholders, EPA-HQ-OAR-2004-0488-0025.2). The original 

risk screen referred in the NRPM (71 FR 55140) contained technical errors (EPA-HQ-OAR-2004-0488-0017). This 

final rule relies on the results of the revised U.S. Army risk analysis.
23

possible strategy to limit refrigerant leakage into the passenger compartment by installing a 

device referred as a “3-second squib valve” to discharge refrigerant to a location outside the 

passenger compartment three seconds after a major leak is detected.19 The assessment showed 

that for CO2 MVAC systems, using a squib valve to evacuate the charge in three seconds after a 

leak is detected kept passenger exposure to below levels of concern (i.e., 3% over 15 minutes in 

the passenger compartment, as a whole, and 4% in the breathing zone).We listed in the proposal 

additional possible mitigation strategies that may reduce the likelihood of exceeding refrigerant 

levels of concern inside the passenger compartment, including within the breathing zone. We 

also received information from commenters on additional engineering strategies and mitigation 

strategies (EPA-HQ-OAR-2004-0488-0037.1, -0025.2, -0030, -0050). In this final rule, we are 

not establishing a use condition requiring a specific mitigation strategy, but instead leaving to 

vehicle manufacturers the choice of which mitigation strategy to use in order to ensure that in the 

event of refrigerant leak, the resulting concentrations of CO2 in the passenger free space above

3% or 30,000 ppm averaged over 15 minutes are avoided and the resulting concentrations of CO2

in the passenger breathing zone do not exceed the ceiling limit of 4% or 40,000 ppm at any time.

Occupational risks

EPA evaluated risks of injury and refrigerant exposure to workers by examining risk 

screens, published research information and data made available during the public comment 

period (EPA-HQ-OAR-2004-0017, -0025.2, -0041, 0022, -0015, -0051). We compared longterm occupational exposures to CO2 to a workplace exposure limit of 5,000 ppm (or 0.5%) time 

weighted average CO2 concentration over a period of eight hours, consistent with the 

Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit–Time 

 

19 Refers to the assessment by Blackwell, et. al, 2006 (EPA-HQ-OAR-2004-0488-0025).
24

Weighted Average (PEL-TWA)20, the Centers for Disease Control and Prevention’s (CDC’s) 

National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure LimitTime Weighted Average (REL-TWA)21, and the American Conference of Governmental 

Industrial Hygienists (ACGIH) Threshold Limit Value-Time Weighted Average (TLVTWA).22,23 Additionally, we compared short-term occupational worker exposures to CO2 to a 

workplace short-term exposure limit of 30,000 ppm (or 3%) time weighted average CO2

concentration over a 15-minute period during a workday, consistent with NIOSH’s 

Recommended Exposure Limit-Short term Exposure Limit (REL-STEL)24 and ACGIH’s 

Threshold Limit Value-Short Term Exposure Limit (TLV-STEL).25,26

The U.S. Army risk assessment (EPA-HQ-OAR-2004-0488-0025.2) evaluated 

occupational risks for the MVAC service sector using FTA. The FTA found probabilities of 

refrigerant exposure while servicing CO2 MVAC systems of approximately 10-5cases per year

(i.e., approximately 5 annual cases per 100,000 technicians) (EPA-HQ-OAR-2004-0488-0025). 

This figure is significantly lower when compared to the general injury and illness rate for auto 

repair technicians, which is approximately 4 annual cases per 100 full time technicians (BLS, 

2003; EPA-HQ-OAR-2004-0488-0025); thus risks from CO2 exposures in the MVAC service 

field are expected to be significantly less than the risks of injury already present in shops (EPAHQ-OAR-2004-0488-0025). The U.S. Army risk assessment additionally found that the chances 

 

20 PELs are based on an eight hour TWA exposure (OSHA, 1988a).

21 REL-TWAs are concentrations for up to 10-hour workday during a 40-hour workweek (NIOSH, 2005).

22 TLV-TWAs are concentrations for an eight hour workday and a 40-hour workweek, to which is believed that 

nearly all workers may be repeatedly exposed, day after day, for a working lifetime without adverse effect (ACGIH, 

2005).

23 OSHA’s PEL-TWA, NIOSH’s REL-TWA, and ACGIH’S TLV-TWA are all the same, 5,000 ppm (0.5%), for 

CO2 (EPA-HQ-OAR-2004-0488-0041).

24 REL-STELs are 15-minute TWA exposure limits that should not be exceeded at any time during a workday

(NIOSH, 2005).

25 TLV-STELs are 15-minute exposure that should not be exceeded at any time during a workday, even if the eight 

hour TWA is within the TLV-TWA (ACGIH, 2005).

26 NIOSH’s REL-STEL, and ACGIH’S TLV-STEL for CO2 are all 30,000 ppm (3%) 15-minute TWA (EPA-HQOAR-2004-0488-0041).
25

of refrigerant exposure for persons servicing an MVAC system do not vary considerably by the 

type of refrigerant used and found similar results for end-of-life (i.e., recycling or disposal) 

activities (EPA-HQ-OAR-2004-0488-0025).

EPA notes that occupational risks could occur during the manufacture of the refrigerant 

and initial installation of the refrigerant into the MVAC system at the vehicle assembly plant. 

Although we did not analyze the risk of exposure during refrigerant manufacture and initial 

installation of CO2 refrigerant into the MVAC system at the vehicle assembly plant, we expect 

risks at the vehicle assembly plant, and at other workplaces were CO2 refrigerant handling will 

occur (e.g., service shops, and recycling or disposal facilities), to be similar to or lower than the 

risks from other refrigerants used for these purposes due to occupational safety practices (e.g., 

proper ventilation, use of personal protective equipment) established for these type of facilities

and subject to occupational safety and health standards under 29 CFR Part 1910, which are 

intended to address risk to such workers. 

Overall conclusion 

EPA finds that the overall environmental and human health risks posed by the use of CO2

in new MVAC systems, subject to the use conditions being adopted in this final rule, is lower 

than or comparable to the risks posed by other substitutes found acceptable subject to use 

conditions in the same end-use.

VI. What is the relationship between this SNAP rule and other EPA rules?

Rules under Sections 609 and 608 of the Clean Air Act

This final SNAP rule addresses the conditions for safe use of CO2 in new MVAC 

systems. Thus, the requirements in this rule apply primarily to OEMs, except for specific 

requirements for unique fittings required under Appendix D to Subpart G of 40 CFR Part 82
26

which also applies for servicing of the MVAC. Section 609 of the CAA establishes standards and 

requirements regarding servicing of MVAC systems. These requirements include training and 

certification of any person that services MVAC systems for consideration27, as well as standards 

for certification of equipment for refrigerant recovery and recycling. EPA has issued regulations 

implementing these statutory requirements and those regulations are codified at subpart B of 40 

CFR part 82. MVAC end-of-life disposal and recycling specifications are covered under section 

608 of the CAA and our regulations are codified at subpart F of 40 CFR part 82. The statutory

and regulatory provisions regarding MVAC servicing, refrigerant recovery, and refrigerant 

venting prohibition apply to any refrigerant alternative and are not limited to refrigerants that are 

also ODS. CO2 is exempted from the refrigerant venting prohibition provisions promulgated 

under CAA 608 (40 CFR §82.154 and 70 FR 19278; April 13, 2005).

VII. What is EPA’s response to public comments on the proposal? 

This section summarizes EPA’s response to significant comments received during the 

public comment periods for the NPRM and the NODA. EPA’s response to all comments 

received can also be found in a response to comments document in docket EPA-HQ-OAR-2004-

0488.

A. Use Conditions 

Comment: Some commenters claimed that the proposed STEL of 3% CO2 averaged over 

15 minutes in the cabin free space is enough to protect passengers and ensure driver alertness. 

One commenter suggested to consider a 3% CO2 concentration limit averaged over 15 minutes in 

the breath level (i.e., breathing zone) instead of 3% in the cabin free space. The commenter 

considered the breathing zone to be a relevant point for measurement and claimed that high 

 

27 Service for consideration means receiving something of worth or value to perform service, whether in money, 

credit, goods, or services (see 40 CFR §82.32 (g)). 
27

refrigerant concentrations lower in the vehicle would not impair vehicle operation. Other 

commenters supported higher CO2 concentration limits but over a shorter period of time (e.g., 

5.5% CO2 for 5 minutes and 9% CO2 as a ceiling limit). Other commenters requested that EPA 

include a CO2 ceiling limit in the passenger breathing zone and one commenter considered 

appropriate a 4% CO2 ceiling limit (i.e., a limit not to be exceeded at any time) as an additional 

use condition. Another commenter stated that use conditions requiring mitigation strategies are 

not necessary for low probability events (i.e., exceeding 4% CO2 for any duration) and that 

requiring such conditions would prevent the use of this refrigerant.

Response: After evaluating the comments and risk screens (EPA-HQ-OAR-2004-0488-

0025.2, -0041, 0051), EPA is revising the proposed use conditions to add a ceiling limit of 4% 

CO2, in addition to the CO2 STEL of 3% averaged over 15 minutes. We believe that the original 

proposed use condition requiring mitigation strategies for MVAC systems, to prevent leaks of 

CO2 refrigerant reaching concentrations above 3% averaged over 15 minutes inside the 

passenger compartment free space, may not be sufficient on its own to protect drivers and 

passengers. We also believe that requiring a CO2 ceiling limit of 4% is necessary because it is 

possible for a time-weighted average concentration, such as the STEL, to be under 3%, while 

peak concentrations could reach higher limits for a few minutes. As shown in published data, 

CO2 concentration peaks above 4% could result in effects on hearing and vision that could 

distract and endanger a driver, or other, potentially more severe adverse health effects (EPA-HQOAR-2004-0488-0041).

CFD modeling showed that during unmitigated refrigerant leak scenarios, CO2 refrigerant 

concentrations in the passenger breathing zone can reach up to 10.2% in 50 seconds (0.83 

minutes) and 8.0% in 200 seconds (3.33 minutes) (EPA-HQ-OAR-2004-0488-0025.2). The U.S. 
28

Army risk analysis’s FTA showed that unmitigated leak scenario occurrences for CO2 systems 

(reaching concentrations above 4% CO2) could reach 4,300 per year for the vehicle fleet (EPAHQ-OAR-2004-0488-0025.2). These occurrences are about 10,000 higher than the expected 

occurrences associated with leaks of a fluorinated refrigerant (e.g., HFC-134a, HFC-152a, or 

HFO-1234yf) breakdown product (i.e., hydrogen fluoride) exposure above health based limits.

28

Several studies reported that exposure ranging from 7% to 10% CO2 for few minutes (e.g., 2-3 

minutes) resulted in loss of consciousness (EPA-HQ-OAR-2004-0488-0441).29

EPA disagrees with the commenter stating that use conditions requiring mitigation 

strategies are not necessary for low probability events and that requiring such conditions would 

prevent the use of this refrigerant. Consistent with the information above, we believe that 

unmitigated exposure occurrences are not rare. We believe the use conditions required in this 

final rule are necessary to ensure that overall risks to human health and the environment from 

CO2 will be similar to or less than those of other available refrigerants that EPA has already 

listed as acceptable for MVAC. We also believe that requiring the use conditions listed in this 

final rule would not make the refrigerant use less practicable. Use conditions imposed on other 

acceptable alternatives for MVACs (e.g., adherence to all safety requirements under SAE 

standard J639, use of unique fittings and labels, use of pressure relief devices) has not prevented

use of such alternatives.30 

 

28 The risk due to exposure to HF when using HFO-1234yf is approximately the same as that with the current use of 

HFC-134a (on order of 10-12 occurrences per operating hour, or one in one trillion). (76 FR 17488; March 29, 2011). 

When this factor is multiplied by the approximate vehicle fleet and annual vehicle operating hours (250 million and 

500hr/yr, respectively) the occurrences per year are in the order of 10-1.

29 Unconsciousness caused by short term exposure (e.g., 2-3 minutes) of CO2 concentration ranging from 7 to 10% 

was reported in studies by Aero Medical Association (1953), Flury and Zernik (1931), Hunter (1975), Schaefer 

(1951), and NIOSH (1996), as cited in Review of Health Impacts for Short-Term Carbon Dioxide Inhalation 

Exposures (2009). EPA-HQ-OAR-2004-0488-0041.

30 On March 29, 2011, EPA issued a final rule listing HFO-1234yf as acceptable subject to use conditions for 

MVACs in new passenger car and light duty trucks. One of the use conditions in that rule require OEMs to perform 

and keep on file an FMEA. In an FMEA vehicle designers analyze all the ways in which parts of the MVAC system 

could fail and identify how they will address those risks in design of the system. (76 FR 17488). If the FMEA 
29

Comment: Several commenters indicated that concentration measurements of CO2 inside 

the passenger compartment should consider passenger respiration in addition to a refrigerant leak 

from the A/C system. Another commenter indicated that the MVAC recirculation mode operates 

with at least 1% of fresh air. One commenter suggested changing the text of the proposed use 

condition indicating the STEL for CO2 refrigerant inside the passenger compartment to state that 

the STEL is “3% v/v fully-occupied-volume, time averaged over 15 minutes” and to clarify that 

the calculation of such value is based on a double average consisting of the average CO2

concentration over the air volume of a fully occupied car and a time-average of volume-average

over 15 minutes.31 Another commenter suggested alternative language for the use condition 

specifying a ceiling limit of 4% CO2 applicable in any part of the free space inside the passenger 

compartment for a time period of 60 seconds when the car ignition is on. The suggested language 

reads:

“Engineering strategies and/or devices shall be incorporated into the system such that 

foreseeable leaks into the passenger compartment do not result in R744 concentrations of 4.0% 

v/v or above in any part of the free space inside the passenger compartment for more than 60 

seconds when the car ignition is on.”

Response: EPA notes that the U.S. Army risk analysis assumed that a maximum number 

of passengers were in the vehicle before the release of refrigerant into the passenger 

compartment, allowing for some build-up of respiratory CO2 (EPA-HQ-OAR-2004-0488-

0025.2). Thus, that analysis recognized that CO2 concentrations can occur from human 

respiration in a space with limited exchange of outside air and may consequently build up in the 

 

reports that mitigation strategies are necessary in the MVAC for safety reasons, manufacturers are required to design 

safety components (e.g., mitigation strategies) to comply with the use condition of that rule. In the U.S. an OEM 

publicly announced that it will be using HFO-1234yf in some vehicles starting 2013 model year (EPA-HQ-OAR2004-0488-0062).

31 Fully occupied is defined as the maximum design occupancy determined by the number of sets of seat belts (EPAHQ-OAR-2004-0488-0025.1 – 0025.2).
30

passenger cabin. For that reason, in the proposal, we indicated that OEMs should account for 

background CO2 concentrations in the passenger compartment that can result from human 

respiration when designing their systems and mitigation devices (71 FR 55140; September 21, 

2006). However, we did not specify whether the vehicle should be fully occupied to account for 

CO2 background concentrations. We believe that CO2 refrigerant concentrations may reach 

levels of concern (i.e., above 4% CO2) during an unmitigated event of refrigerant leak either 

when the vehicle is fully occupied or when not fully occupied (e.g., the vehicle is occupied by 

the driver only). Thus we do not agree with the commenter’s suggestion to state that the STEL is 

“3% v/v fully-occupied-volume, time averaged over 15 minutes”. In this final rule, we 

recommend but do not require, consistent with the NPRM, to account for background CO2

concentrations from human respiration, in addition to refrigerant leaks when designing the 

MVAC.

EPA notes that the proposal (79 FR 55140; September 21, 2006) specifies the CO2 STEL

as a concentration limit averaged over 15 minutes, in the event of a refrigerant leak. The STEL is 

determined from the sum of concentration and exposure time products (e.g., concentration 1 

times exposure time 1 plus concentration 2 times exposure time 2), divided by the total exposure 

time which shall not exceed 15 minutes (EPA-HQ-OAR-2004-0448-0041). Thus the STEL is a 

time-weighted average concentration and not necessarily a time-average of a volume-average as 

indicated by the commenter since STEL refers to a total exposure time (i.e., 15 minutes) and not 

an average time. For this reason, we do not agree with the commenter’s suggestion to clarify that 

the calculation of the 3% STEL is based on a double average consisting of the average CO2

concentration over the air volume of a fully occupied car and a time-average of volume-average

over 15 minutes since the approach does not provide further clarity of the use condition. In this 
31

final rule, the CO2 STEL of 3% averaged over 15 minutes considers the average CO2

concentration in a passenger cabin over a total time period of 15 minutes during the event of 

refrigerant leak; and the ceiling limit of 4% CO2 considers the total CO2 in the passenger 

breathing zone at any one moment in a passenger compartment during the event of a leak.

Regarding the alternative language suggested by the other commenter specifying a 

ceiling limit of 4% CO2 applicable in any part of the free space inside the passenger 

compartment for a time period of 60 seconds when the car ignition is on, we note that the 

commenter did not provided information supporting his suggestion that the ceiling limit apply in 

areas other than the passenger breathing zone for the specified 60 second time period. 

Comment: Two commenters indicated the need for clarity on whether the use conditions 

apply when the ignition is off as well as when the ignition is on. Other commenters suggested 

considering the results of a risk assessment performed by SAE’s CRP indicating a significantly

low probability for a leak when the ignition is off, and several other commenters stated that the 

use conditions should only apply when the ignition is on. 

Response: The NODA provided data and requested additional comment on whether the 

use conditions should apply when the engine is off. In December, 2009, after the public comment 

period closed on the NODA, SAE issued a report, “Risk Assessment for HFO-1234yf and R-744 

(CO2) Phase III” (referred herein after as SAE CRP report), that evaluated toxicity effects and 

quantitative risks of CO2, similarly to the U.S. Army risk analysis. This report was submitted to 

EPA during the public comment period for another SNAP rulemaking.

32 The report evaluates 

CO2 exposure estimations due to leaks into the passenger compartment during different modeled 

scenarios such as different MVAC operation mode, system failure, and during a collision (EPA-

 

32 We refer here to the SNAP rule listing HFO-1234yf as acceptable subject to use conditions for MVACs in new 

passenger cars and light duty vehicles (EPA-HQ-OAR-2008-0644, 74 FR 17488; March 29, 2011)
32

HQ-OAR-2008-0664-0056.2, EPA-HQ-OAR-2004-0488-0025.2, -0051). The SAE CRP report 

also evaluated refrigerant release into the passenger compartment during a scenario where the 

engine is expected to be off (EPA-HQ-OAR-2004-0488-0051, EPA-HQ-OAR-2008-0664-

0056.2). For this scenario, which involves passengers sleeping inside a vehicle with the windows 

closed while refrigerant leaks occur, the SAE CRP report showed a probability for occurrences 

of CO2 refrigerant exposure above 6% (a threshold limit used by the CRP for this scenario) to be 

in the order of 10-12per vehicle/hour/occupant (EPA-HQ-OAR-2004-0488-0051, EPA-HQOAR-2008-0664-0056.2). We believe that exposures of concern inside the passenger 

compartment are more likely to result from a large, sudden release of refrigerant inside the 

passenger compartment and that such a situation is most likely during a collision while the 

ignition is on, as described on the U.S. Army risk analysis (EPA-HQ-OAR-2004-0488-0025.2) 

and consistent with the SAE CRP report (EPA-HQ-OAR-2004-0488-0051, EPA-HQ-OAR2008-0664-0056.2). In addition, even if a rupture on the evaporator line is large, the overall leak 

rate is limited to the maximum flow rate of refrigerant through the fixed orifice tube opening of 

the MVAC (EPA-HQ-OAR-2004-0488-0025.2). The maximum flow rate is determined by the 

differential compressor discharge pressure, which is only available when the vehicle ignition is 

on and MVAC system is running. Therefore, EPA finds that the overall risks to human health 

and the environment from CO2 will be similar to or less than those of other available refrigerants 

that EPA has already listed as acceptable for MVAC when the ignition is off. Thus, consistent 

with a SNAP rule issued in June 12, 2008 (73 FR 33304) listing HFC-152a33 as acceptable 

subject to use conditions for use in new MVAC systems, the use conditions in this final rule 

apply only when the ignition is on. 

 

33 HFC-152a poses risks comparable to CO2 and other available refrigerants found acceptable subject to use 

conditions under SNAP (73 FR 33304; June 12, 2008).
33

Comment: One commenter suggested that the regulation should precisely define the area 

in the vehicle being regulated and indicated that SAE is working on a standard to establish 

standards for passenger compartment refrigerant concentration measurement. Another 

commenter indicated that a CO2 concentration limit should focus on the driver breathing zone 

rather than the cabin free space. 

Response: EPA has clarified the regulatory text of the use condition to define the 

passenger free space as the space inside the passenger compartment, excluding the space 

enclosed by the ducting in the HVAC module. The passenger breathing zone, where the ceiling 

limit of 4% must be met, is defined as the area inside the passenger compartment where the 

driver’s and passengers’ heads are located during normal sitting position (i.e., space where 

people breathe, as defined in the NODA (71 FR 47775; September 17, 2009)). Additionally, we 

note that the passenger breathing zone is defined in SAE J2772 and the driver’s head position in 

SAE J1052. Since the automotive industry often relies on standards for designs and assessments, 

we recommend the use of the SAE J1052 and SAE J2772 standards as references for further 

specifications regarding the driver’s and passengers’ head and seating position and to establish 

the passenger breathing zone consistent with our explanation provided in Section V of the 

preamble (i.e., the area inside the passenger compartment where the driver’s and passengers’ 

heads are located during a normal sitting position). 

EPA disagrees with the comment indicating that a CO2 concentration limit should only

focus on the driver breathing zone rather than the passenger cabin free space. Based on the risk 

analyses and available data, we include in this final rule a 4% ceiling limit that must not be 

surpassed at any time in the passenger (and driver) breathing zone (EPA-HQ-OAR-2004-0488-

0044-0025.2). We also include, as proposed, a 3% CO2 STEL averaged over 15 minutes in the 
34

passenger cabin free space as an additional protective measure for passenger exposure to CO2. 

As indicated by the U.S. Army risk assessment, sensitive subpopulations (e.g., elderly and 

children) may be affected from exposures to high concentrations of CO2 (EPA-HQ-OAR-2004-

0488-0025.2), thus we believe it is necessary to set a limit that would address risk to all people in 

the passenger compartment and not solely the driver. We also take into consideration that 

passengers may not be in a normal sitting position all the time (e.g., passenger may rest in a 

reclined position) and note CO2 is heavier than air, thus higher concentrations may be found at 

lower points of the passenger cabin (EPA-HQ-OAR-2004-0488-0025.2, -0041, -0051). As 

indicated previously, the STEL is the concentration limit that people can be exposed 

continuously for a short period of time (i.e., 15 minutes) without suffering adverse health risks. 

For these reasons we include both limits (i.e., 4% CO2 ceiling limit in the passenger breathing 

zone and 3% CO2 averaged over 15 minutes in the passenger cabin free space) in this final rule.

Comment: Several commenters suggested that a CO2 ceiling limit should rely on

exposure time since potential effects of CO2 vary with both concentration and duration of the 

exposure. One commenter stated that if the ceiling limit is exceeded, it is likely due to collision 

events.

Response: EPA agrees the health effects of CO2 are functions of exposures over time.

The commenter appears to misunderstand what a ceiling limit is. A ceiling limit is a limit that 

shall not be exceeded for any period of time, thus it is not consistent with the concept of a ceiling 

limit to also include a period of time during which it cannot be exceeded. As explained 

previously, we believe that both a ceiling limit and a STEL are necessary to ensure that risks 

posed from CO2 MVAC systems are not greater than risks posed by other available MVAC 

systems. 
35

While EPA agrees with the commenter that collision events are the most likely cause of 

a refrigerant leak that could cause CO2 levels to the exceed the ceiling limit established in the 

use conditions, there may be other system failures that could cause the ceiling limit to be 

exceeded. OEMs should consider risks from all possible events in designing MVACs for use 

with CO2. 

Comment: Several commenters suggested considering ceiling limits of CO2 above 4% 

(e.g., 6%, 9%) based on studies showing that visual disturbances occur at concentrations of 6%

CO2. They stated that the SAE CRP report’s rationale suggested a 9% CO2 concentration ceiling 

limit, based on studies showing central nervous system (CNS) effects at CO2 exposure 

concentrations of 10% (100,000 ppm). 

Response: Studies report that human exposures to 6% CO2 for periods as short as 2 

minutes can lead to hearing and visual disturbances, and that exposures to 7.5% for 5 minutes 

lead to significant reasoning and performance decrements (Gellhorn, 1936; Sayers, 1987 as cited 

in EPA-HQ-OAR-2004-0488-0041). To provide a margin of safety, EPA considers it necessary

to require a ceiling limit of 4% CO2 in the passenger breathing zone as indicated in the NODA 

and suggested by some commenters, to avoid driver performance decrement and other adverse 

health effects on passengers. 

Comment: Several commenters said that the ceiling limit should rely on NIOSH’s 

Immediately Dangerous to Life and Health (IDLH) value of 4% CO2 based on a 30-minute 

exposure. 

B. Response: EPA disagrees with the commenters to the extent they are suggesting that 

the 4% limit be based on a 30 minute exposure. The NIOSH IDLH value is a 

worker’s exposure limit based on the effects that might occur as a consequence of a 
36

30-minute exposure (NIOSH 2005; EPA-HQ-OAR-2004-0488-0041). The OSHA 

regulation (1910.134(b)) defines the term as "an atmosphere that poses an immediate 

threat to life, would cause irreversible adverse health effects, or would impair an 

individual's ability to escape from a dangerous atmosphere." We believe NIOSH’s 

IDLH is inappropriate as a ceiling limit for the use of CO2 as a refrigerant in MVACs 

because, as indicated above, a ceiling limit is a limit that shall not be exceeded for 

any period of time. Also, a 4% limit over 30 minutes would not protect drivers and 

passengers from the effects of CO2 exposure at concentrations equal or higher than 

4%. CO2 is an asphyxiant that obstructs the oxygen flow into the body and we believe 

that 30 minute duration, in particular, where the person affected by such a 

concentration may be operating a vehicle and thus posing risk to others, creates a 

significant risk. Risk Mitigation Strategies

Comment: The U.S. Army noted a CFD parameter error in their 2005 risk analysis which 

used an incorrect refrigerant leak angle in their 2005 risk analysis (EPA-HQ-OAR-2004-0488-

0017). The U.S. Army corrected this error for purposes of their 2006 risk analysis by using a 

perpendicular leak angle to the rupture cross-sectional area (EPA-HQ-OAR-2004-0488-0025.1).

The 2006 analysis finds that an unmitigated discharge of CO2, in full recirculation mode, results 

in CO2 concentration above 3% for more than 60 minutes. The U.S. Army also indicated that a 3-

second, rather than a 10-second squib valve, as originally determined, would be needed to ensure 

that CO2 concentrations remain below the 3% on a 15-minute average inside the passenger 

compartment. 

Response: EPA notes that the U.S. Army is commenting on its own risk assessment 

performed in collaboration with EPA and several stakeholders, and referenced in the NPRM (71 
37

FR 55140). We also note that the 2005 U.S. Army risk analysis referred in the NRPM (71 FR 

55140) contained technical errors (EPA-HQ-OAR-2004-0488-0017). This final rule relies on the 

results of the revised (2006) U.S. Army risk analysis submitted during the public comment 

period.

Based on the U.S. Army revised assessment, we understand that, in order for a squib 

valve to be an effective mitigation device, the activation time of such device should be 3 seconds 

rather than the 10 seconds indicated in the original risk assessment. Since we are not specifying 

in this final rule what mitigation strategies must be used, we believe the 2006 revised risk 

analysis does not affect the use conditions addressed in this final rule, but may affect the 

potential risk mitigation strategies OEMs might apply for use with CO2 refrigerant.

Comment: One commenter stated that secondary loop technology is not a viable risk 

mitigation strategy for CO2 because of reduced system performance and reduced fuel efficiency. 

Response: This final rule does not specify design options. EPA does not intend to limit 

engineering innovation by requiring any specific risk mitigation strategy; however, EPA notes

that secondary loop technology could potentially reduce the risks of exceeding the ceiling limit 

of CO2 in the passenger compartment because the refrigerant charge stays separate from the 

passenger compartment. OEMs may choose to investigate secondary loops as a risk mitigation 

strategy, and would have to weigh the pros and cons, including any potential effect on fuel 

efficiency. However, even if secondary loop technology were not an attractive option, other 

feasible mitigation technologies could be applied to meet the use conditions of this final rule, 

such as a squib valve with a 3-second response time.

Comment: One commenter indicated that squib valves with activation time of less than 

10 seconds (e.g., few milliseconds) are available and such devices have been tested. Another 
38

commenter stated that a 10-second squib valve is not technically feasible given CO2 sensor 

performance. Additionally, the commenter stated that during sharp increases in CO2

concentration in the passenger compartment, a short activation time for a squib-valve would 

increase the possibility of purging the refrigerant from the air conditioning system to outside the 

vehicle when no leak in fact exists. 

Response: EPA agrees with the first commenter regarding the availability of squib valves 

and disagrees with the second commenter’s statement regarding feasibility of a squib valve. The 

2006 U.S. Army risk analysis indicated that a squib valve is one effective strategy and viable 

engineering option to reduce the amount of charge that could potentially leak into the passenger 

compartment (EPA-HQ-OAR-2004-0488-0025.2). EPA notes that in the proposal, we intended 

for the squib valve activation time to include: 1) the time the sensor takes to detect a significant 

leak that would cause CO2 refrigerant to enter into the passenger compartment, and 2) the time it 

takes for the squib valve to open (71 FR 55140; September 21, 2006). The 2006 U.S. Army risk 

assessment evaluated different activation times (i.e., 30, 10 and 3 seconds) of squib valve during 

modeled scenarios of CO2 refrigerant leak. The results showed higher effectiveness of the valve 

preventing high refrigerant concentration reaching the passenger compartment during the shorter 

activation time. 

EPA believes that sharp increases in CO2 concentration in the passenger compartment 

will likely occur only when a significant amount of CO2 refrigerant leaks into the passenger 

compartment. Risk assessments showed that CO2 buildup due to passenger respiration occurs 

slowly (e.g., 60 minutes) to levels up to 2.4% in a fully-occupied 100 cubic feet sealed passenger 

compartment of a vehicle with no introduction of outside air (EPA-HQ-OAR-2004-0488-0025.2, 

-0041). EPA notes that a passenger compartment in a vehicle is not confined space and 
39

infiltration/exfiltration rates of air changes within the passenger compartment and outside air are 

at least 0.3 air changes per hour (NREL, 2003 as cited in EPA-HQ-OAR-2004-0488-0025.2).

Therefore, we do not agree that refrigerant purging from the air conditioning system to outside 

the vehicle will occur when no leak in fact exists.

Comment: A commenter stated that odorants that alert drivers to a leak should be another 

option for compliance with the rule. 

Response: EPA did not propose the use of odorants, and this final action neither requires 

nor prohibits the use of odorants in new CO2 MVAC systems. Odorized CO2 may be an effective 

means to alert the driver and passengers to a refrigerant leak into the passenger compartment.

However, EPA does not believe odorants used alone provide sufficient risk mitigation as it may

take vehicle occupants a period of time to recognize what the odor signifies. Documentation has 

not been provided to show how long and how much odorized CO2 drivers must be exposed to 

before they recognize that the smell indicates a health and safety risk. 

Comment: One commenter suggested that EPA consider use of sensors to allow 

continuous monitoring of refrigerant concentration inside the passenger cabin as a mitigation 

strategy. Another commenter mentioned that an alarm system or other technical solutions should 

allow for air renewal and lowering concentration levels below the limits indicated in the use 

conditions within a reasonable time period

Response: As noted previously, EPA is not specifying the risk mitigation strategies that 

must be used to ensure CO2 levels do not exceed the levels established in the use conditions. We 

do not believe that a sensor alone would be sufficient to provide effective protection to the 

passengers and to ensure that concentrations inside the passenger compartment and passenger 

breathing zone do not exceed the established CO2 concentration limits of this final rule. In 
40

response to the commenter stating that an alarm system or other technical solutions should allow 

for air renewal, EPA believes the use of such tool might be effective but that such strategy would 

need to rely on an automatic supply of air, rather than a driver’s response, to ensure CO2

concentrations do not exceed the exposure limits established in the use conditions. Thus, an 

additional mitigation device would need to be used in addition to any alarm system. 

Comment: One commenter said that evaporator isolation valves are not realistic as 

mitigation devices because of cost. The commenter stated that close-coupled and hermetically 

sealed systems are technically feasible and noted that an automatic increase in air exchange is a 

possible strategy that is technically feasible. Another commenter suggested that switching the 

MVAC blower to operate on outside air mode on high, rapidly after CO2 refrigerant is released, 

could reduce the overall refrigerant concentration in the compartment to a peak lower than 4%. 

Response: EPA believes the mitigation strategies mentioned by the commenters may all 

be technically feasible means to meet concentration levels specified in the use conditions. We 

note that in the proposed rule we suggested using evaporator isolation valves, close coupled or 

hermetically closed systems that would reduce refrigerant charge size, and increasing air 

exchange (with outside air) in the passenger compartment upon detection of leaks as some of 

several potential risk mitigation strategies (71 FR 55140; September 21, 2006). In this final rule 

we are not requiring a specific mitigation strategy or engineering device. We are allowing OEMs 

to choose a mitigation strategy that is consistent with the use conditions and that they will 

employ to protect the driver and passengers in a vehicle from CO2 exposure above the limits 

specified in this rule. 

Comment: One commenter stated that a vehicle crash could be so severe that the MVAC 

system evaporator could be damaged and possibly reduce a risk mitigation system’s 
41

effectiveness. The commenter proposed the inclusion of an evaporator crush resistance standard 

in this action. 

Response: EPA agrees that a vehicle crash could reduce the effectiveness of the risk 

mitigation strategy. However we believe that in such a case, the damage to the car would be so 

severe as to result in an inflow of ambient air that would negate the risks associated with 

potentially elevated CO2 concentrations. A crush-resistant evaporator could be selected as a

possible mitigation strategy but, as stated previously, in this final rule we do not specify which 

engineering device or strategies must be incorporated into the MVAC system and leave this 

choice to the OEMs. 

C. Industry Standards

Comment: Several commenters indicated that SAE is developing standards for safety and 

servicing of CO2 MVAC systems and that it is customary for OEMs to follow those standards.

Other commenter claimed that every OEM is responsible for its own safety concept and has to 

show compliance with already existing and future safety standards.

Response: EPA notes and agrees with the important role industry standards play

particularly for the MVAC sector. In addition, we note that the regulatory text references the 

relevant SAE technical standards to promote consistency with established industry practices.

Specifically, use conditions in this final rule reference SAE J639 (2011 version). Other standards 

such as SAE J1739, which addresses design, safety, and recordkeeping requirements, are 

recommended to help ensure that the use conditions are met.

We disagree with the comment stating that every OEM is responsible for its own safety 

concept because we believe that in addition to customary business standards and industry

practices outside the scope of this rule, OEMs will comply with all the use conditions specified 
42

in this rule. Comment: Commenters noted that SAE is developing SAE J2772 standard, 

“Measurement of Passenger Compartment Refrigerant Concentrations under system refrigerant 

leakage conditions” (EPA-HQ-OAR-2004-0448-0054) and SAE J2773 standard, “Standard for 

Safety and Risk Analysis for Use in Mobile Air Conditioning Systems” (EPA-HQ-OAR-2004-

0448-00553), formerly known as Refrigerant Guidelines for Safety and Risk Analysis for Use in 

Mobile Air Conditioning Systems. 

Response: We note that standards J2772 and J2773 were recently published and are 

readily available. In the comments column of our listing decision, we recommend the use of 

J2772 and J2773 standards as well as other available standards such as SAE J1052, Motor 

Vehicle Driver and Passenger Head Position (EPA-HQ-OAR-2004-0448-0055). 

D. Servicing

Comment: One commenter indicated CAA Section 609-certified, independent MVAC 

service technicians should be consulted before the rule is issued. 

Response: EPA took comments on a range of topics during the 60-day public comment 

period. In addition, EPA contacted the National Institute for Automotive Service Excellence 

(ASE), which represents a significant number of MVAC service technicians. A summary is in 

the docket for this final rule. ASE stated they did not see any servicing concerns in the proposal 

that would impact the service technicians they represent, but would be interested in any followon rulemaking that will address MVAC servicing for consideration under CAA Section 609 and 

codified at 40 CFR Part 82 Subpart B (EPA-HQ-OAR-2004-0488-0031). 

Comment: One commenter said risks associated with MVAC service should be 

considered. 

Response: EPA agrees with the commenter and notes that risk associated with service 
43

were evaluated in the published risk analyses (EPA-HQ-OAR-2004-0488-0017, -0025.2, -0041, -

0051) and discussed in the preamble to the proposed rulemaking (71 FR 55144 September 21, 

2006). Additional details regarding our evaluation of risk associated with MVAC service can be 

found in Section VI of this final rule preamble (Why is EPA listing CO2 acceptable subject to 

use conditions?). As explained in more detail in Section VI above, we do not believe it is 

necessary to establish any use conditions regarding servicing because the overall environmental 

and human health risks posed by the use of CO2 in new MVAC systems, subject to the use

conditions being adopted in this final rule, is lower than or comparable to the risks posed by 

other substitutes found acceptable subject to use conditions in the same end-use. Comment: One 

commenter requested more information on why CO2 systems are not found acceptable as a 

substitute in retrofitted systems. 

Response: In the original SNAP rulemaking (59 FR 13854; March 18, 1994), EPA listed 

CO2 as an acceptable substitute for CFC-12 only for new MVAC systems. We have never

received a SNAP submission requesting consideration of CO2 in retrofitted MVAC systems. EPA 

understands that the higher working pressure of CO2 compared to CFC-12 and other SNAPacceptable refrigerants could raise significant issues with retrofitting such systems to CO2.

Because we have not received a request to use CO2 in retrofitted systems, which would include 

the technical and other analyses necessary to determine whether such use would present more 

risk than other available substitutes, this final rule only applies to the use of CO2 as a refrigerant 

in new MVAC systems, consistent with the NPRM (71 FR 55140; September 21, 2006). When 

and if the Agency receives a submission for retrofitting to CO2, we will consider CO2 for use as a 

refrigerant to retrofit existing MVAC systems. 
44

VIII. Statutory and Executive Order Reviews

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

Improving Regulation and Regulatory Review

Under Executive Order (EO) 12866, (58 FR 51735; October 4, 1993) this action is a “significant 

regulatory action.” It raises novel legal or policy issues arising out of legal mandates, the 

President’s priorities, or the principles set forth in the Executive Order. Accordingly, EPA 

submitted this action to the Office of Management and Budget (OMB) for review under

Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011) and any changes made in 

response to OMB recommendations have been documented in the docket for this action. 

B. Paperwork Reduction Act

This action does not impose any new information collection burden under the Paperwork 

Reduction Act (44 U.S.C. 3501 et seq). This action is an Agency determination. It contains no 

new requirements for reporting. The only new recordkeeping requirement involves customary

business practice. This rule requires minimal recordkeeping of studies done for three years to 

ensure that MVAC systems using CO2 meet the requirements set forth in this rule. Because it is 

customary business practice that OEMs conduct and keep on file records of the tests they

perform, consistent with a widely recognized industry standard, SAE J1739 (Potential Failure 

Mode and Effect Analysis in Design [Design FMEA], Potential Failure Mode and Effect 

Analysis in Manufacturing and Assembly Processes [Process FMEA]), on any potentially 

hazardous part or system from the beginning of production of a vehicle model until three years or 

more after production of the model ends, we believe this requirement will not impose an 

additional paperwork burden. However, the Office of Management and Budget (OMB) has 

previously approved the information collection requirements contained in the existing 
45

regulations in subpart G of 40 CFR part 82 under the provisions of the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. and has assigned OMB control numbers 2060-0226. The OMB 

control numbers for EPA's regulations are listed in 40 CFR Part 9.

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a 

regulatory flexibility analysis of any rule subject to notice and comment rulemaking 

requirements under the Administrative Procedure Act or any other statute unless the agency

certifies that the rule will not have a significant economic impact on a substantial number of 

small entities. Small entities include small businesses, small organizations, and small 

governmental jurisdictions.

For purposes of assessing the impacts of today's rule on small entities, small entity is 

defined as: (1) a small business, as defined by the Small Business Administration’s (SBA) 

regulations at 13 CFR 121.201;” (2) a small governmental jurisdiction that is a government of a 

city, county, town, school district or special district with a population of less than 50,000; and (3) 

a small organization that is any not-for-profit enterprise which is independently owned and 

operated and is not dominant in its field.

After considering the economic impacts of this final rule on small entities, I certify that 

this action will not have a significant adverse economic impact on a substantial number of small 

entities. This final rule modifies the current listing of CO2 as an acceptable alternative refrigerant 

subject to use conditions necessary for the safe use of CO2 in MVAC in new motor vehicles. The 

use conditions will need to be met by large entities (i.e., OEMs) that manufacturer motor 

vehicles if these choose to use CO2 as a refrigerant in new MVACs. This final rule does not 

mandate the use of CO2 as a refrigerant in new MVAC systems, thus will not impose significant
46

requirements on small entities such as MVAC service shops. This final rule effectively ensures 

consistency with current practices in MVAC service shops regarding the use of unique fittings. It

is not clear that there would be any cost differential between the unique fittings required for the 

use of CO2 and those used with the current automotive refrigerant, HFC-134a, or other fittings 

that the industry could adopt instead, for other refrigerants. It is possible that the fittings 

required in the revised use condition will be equally or less expensive than those required for 

other acceptable alternative refrigerants because they are a standard shape and size and can be 

easily produced in a metal-working shop. Thus, cost impacts of this final rule on small entities 

are expected to be small. 

Although this final rule will not have a significant economic impact on a substantial 

number of small entities, EPA nonetheless has tried to reduce the impact of this rule on small 

entities. EPA has worked together with SAE International and with groups representing 

professional service technicians such as the Mobile Air Conditioning Society Worldwide, which 

conducts regular outreach with technicians and owners of small businesses such as retail 

refrigerant suppliers and automobile repair shops.

D. Unfunded Mandates Reform Act

This action contains no Federal mandates under the provisions of Title II of the Unfunded 

Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 1531-1538 for State, local, or tribal 

governments or the private sector. This action does not affect State, local, or tribal governments.

The enforceable requirements of this action related to integrating risk mitigation devices and 

documenting the safety of alternative MVAC systems affect only a small number of OEMs. The 

impact of this action on the private sector will be less than $100 million per year. Thus, this

action is not subject to the requirements of sections 202 and 205 of the UMRA. This action is 
47

also not subject to the requirements of section 203 of UMRA because it contains no regulatory

requirements that might significantly or uniquely affect small governments. This action applies 

directly to facilities that use these substances and not to governmental entities.

E. 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, as 

specified in EO 13132 (64 FR 43255, August 10, 1999). This regulation applies directly to 

facilities that use these substances and not to governmental entities. Thus, E O 13132 does not 

apply to this rule.

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

This final rule does not have tribal implications, as specified in EO 13175 (65 FR 67249, 

November 6, 2000). It does not significantly or uniquely affect the communities of Indian tribal 

governments, because this regulation applies directly to facilities that use these substances and 

not to governmental entities. Thus, EO 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children from Environmental Health Risks and 

Safety Risks

This final rule is not subject to the EO 13045 (62 FR 19885, April 23, 1997) because it is 

not economically significant as defined in Executive Order 12866, and because the Agency does 

not have reason to believe the environmental health or safety risks addressed by this action 

present a disproportionate risk to children. This action’s health and risk assessments are 
48

discussed in sections VI and VIII of the preamble and in documents EPA-HQ-OAR-2004-0488-

0025.2, EPA-HQ-OAR-2004-0488-0041 and EPA-HQ-OAR-2004-0488-0051 in the docket for 

this rulemaking.

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution, 

or Use

This rule is not a “significant energy action” as defined in Executive Order 13211 (66 FR 

28355; May 22, 2001) because it is not likely to have a significant adverse effect on the supply, 

distribution, or use of energy. This action would impact manufacturers of CO2 MVAC systems

for new vehicles. Preliminary information indicates that these new systems are equally or more 

energy efficient than currently available systems in some climates. Therefore, we conclude that 

this rule is not likely to have any adverse effects on energy supply, distribution or use.

I. National Technology Transfer Advancement Act

Section 12(d) of the National Technology Transfer and Advancement Act of 1995, Public 

Law No. 104-113, Section 12(d) (15 U.S.C. 272 note) directs EPA to use voluntary consensus 

standards in regulatory activities unless to do so would be inconsistent with applicable law or

otherwise impractical. Voluntary consensus standards are technical standards (e.g., materials 

specifications, test methods, sampling procedures, and business practices) that are developed or 

adopted by voluntary consensus standards bodies. NTTAA directs EPA to provide Congress, 

through OMB, explanations when the Agency decides not to use available and applicable 

voluntary consensus standards. 

This rulemaking involves technical standards. EPA has decided to use, by reference, the 

2011 version of SAE standard J639 which is a voluntary consensus standard. This standard can 

be obtained from http://www.sae.org/technical/standards/. This standard addresses safety and 
49

reliability issues of CO2 MVAC systems. 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority

Populations and Low-Income Populations 

Executive Order 12898 (59 FR 7629; February 16, 1994) establishes federal executive 

policy on environmental justice. Its main provision directs federal agencies, to the greatest extent 

practicable and permitted by law, to make environmental justice part of their mission by

identifying and addressing, as appropriate, disproportionately high and adverse human health or 

environmental effects of their programs, policies, and activities on minority populations and lowincome populations in the United States.

EPA has determined that this final rule will not have disproportionately high and adverse 

human health or environmental effects on minority or low-income populations because it 

increases the level of environmental protection for all affected populations without having any 

disproportionately high and adverse human health or environmental effects on any population, 

including any minority or low-income population. The refrigerant CO2 is a non ozone-depleting 

substance with a GWP of 1.0. Based on the toxicological and atmospheric data described earlier, 

the use of CO2 subject to the use conditions specified in this final rule will not have any 

disproportionately high and adverse human health or environmental effects on any population, 

including any minority or low-income population. This final rule requires specific use conditions 

for MVAC systems, if motor vehicle manufacturers choose to market MVAC systems using this 

refrigerant alternative.

K. Congressional Review Act

The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the Small Business 

Regulatory Enforcement Fairness Act of 1996, generally provides that before a rule may take 
50

effect, the agency promulgating the rule must submit a rule report, which includes a copy of the 

rule, to each House of Congress and to the Comptroller General of the United States. EPA will 

submit a report containing this rule and other required information to the U.S. Senate, the U.S. 

House of Representatives, and the Comptroller General of the United States prior to publication 

of the rule in the Federal Register. A Major rule cannot take effect until 60 days after it is 

published in the Federal Register. This action is not a “major rule” as defined by 5 U.S.C. 

804(2). This rule will be effective [insert date 60 days after publication in the Federal 

Register].

IX. References

The documents below are referenced in the preamble. All documents are located in the 

Air Docket at the address listed in section titled ``ADDRESSES'' at the beginning of this 

document. Unless specified otherwise, all documents are available in Docket ID No. EPA-HQOAR-2004-0488 at http://www.regulations.gov.

ACGIH, 2005a. TLVs® and BEIs® Based on the Documentation of the Threshold Limit 

Values for Chemical Substances and Physical Agents & Biological Exposure Indices. 

American Conference of Governmental Industrial Hygienists (ACGIH), Cincinnati, Ohio.

ACGIH, 2005b. Documentation of the Threshold Limit Values for Chemical Substances: 

Carbon Dioxide. American Conference of Governmental Industrial Hygienists (ACGIH), 

Cincinnati, Ohio.

Aero Medical Association, 1953. Committee on Aviation Toxicology, Blakiston, New York. 

As cited in CDC IDLH documentation for carbon dioxide. Available online at:

http://www.cdc.gov/niosh/idlh/124389.html.

Al-Delaimy W.K., Hay S.M., Gain K.R., Jones D.T., Crane J., 2001. The effects of carbon 

dioxide on exercise-induced asthma: an unlikely explanation for the effects of Buteyko 

breathing training. Med J Aust. 15,174(2):64-5.

ANSI/ASHRAE, 2007. ASHRAE Standard 62.1-2007: Ventilation for Acceptable Indoor Air 

Quality. American National Standard Institute, American Society for Heating, 

Refrigerating, and Air-Conditioning Engineers (ASHRAE), Atlanta, GA. Available 

online at: http://www.sae.org/altrefrigerant/presentations/sumantran.pdf
51

ASHRAE, 2006. BSR/ASHRAE Addendum to ANSI/ASHRAE Standard 34-2004 –

Designation and Safety Classification of Refrigerants, Publication Draft. American 

Society for Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), Atlanta, 

GA.

Atkinson, W., 2002. Consumer Use of A/C Systems. SAE Automotive Alternate Refrigerant 

Systems Symposium 2002. Phoenix, Arizona. Available online at: 

http://www.sae.org/events/aars/2002/consumeruse.pdf

Amin, J., B. Dienhart, and J. Wertenbach., 1999. Safety Aspects of an A/C System with 

 Carbon Dioxide as Refrigerant, SAE Subcommittee Safety of Refrigerant Systems.

 Available online at: http://www.sae.org/misc/aaf99/visteon.pdf

Birgfeld, E., 2003. Risk Analysis for CO2, HFC-152a, and Hydrocarbon Refrigerants in 

 MACs, presented at The Earth Technologies Forum, Washington, DC, April 2003.

Beck J.G., P.J. Ohtake, and J.C. Shipherd., 1999. Exaggerated anxiety is not unique to CO2 in 

panic disorder: A comparison of hypercapnic and hypoxic challenges. J Abnorm Psychol 

108:473-482.

Blackwell, N., L. Bendixen, E. Birgfield., 2006. Risk Analysis for Alternative Refrigerant in 

Motor Vehicle Air Conditioning. 

Brown. E.W., 1930. The value of high oxygen in preventing the physiological effects of noxious 

concentrations of carbon dioxide. U.S. Naval Med. Bull. 132, 523-553. As cited in Wong 

1992.

Bureau of Labor Statistics (BLS), 2003. “Workplace Injuries and Illnesses in 2002,” December 

 18, 2003.

CATAM., 1953. Aviation Toxicology, An Introduction to the Subject and a Handbook of Data. 

Committee on Aviation Toxicology, Aero Medical Association. The Blakiston Co.: New 

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List of Subjects in 40 CFR Part 82

Environmental protection, Administrative practicable procedure,

Air pollution control, Reporting and recordkeeping requirements, Stratospheric ozone layer.

 Dated: ______________________________________

_____________________________________________ 

Lisa P. Jackson,

Administrator
56

For the reasons set out in the preamble, 40 CFR Part 82 is amended as follows:

PART 82 - PROTECTION OF STRATOSPHERIC OZONE

1. The authority citation for Part 82 continues to read as follows:

Authority: 42 U.S.C. 7414, 7601, 7671 - 7671q.

Subpart G--Significant New Alternatives Policy Program

2. The first table in Appendix B to Subpart G of Part 82 is amended by adding 1 new entry to

the following at the end of the table to read as follows:

Appendix B to Subpart G of Part 82 - Substitutes Subject to Use Restrictions and 

Unacceptable Substitutes

REFRIGERANTS-ACCEPTABLE SUBJECT TO USE CONDITIONS

Application Substitute Decision Conditions Further Information

 * * * * * * * 
57

Application Substitute Decision Conditions Further Information

 * * * * * * * 

CFC-12 Motor 

Vehicle Air 

Conditioning

(New equipment 

only)

Carbon 

dioxide (CO2)

as a substitute 

for CFC-12

Acceptable

subject to 

use 

conditions

Engineering strategies and/or 

mitigation devices shall be 

incorporated such that in the 

event of refrigerant leaks, the 

resulting CO2 concentrations do 

not exceed: 

The short term exposure

level (STEL) of 3% or 30,000 

ppm averaged over 15 minutes 

in the passenger free space34; 

and;

The ceiling limit of 4% or 

40,000 ppm in the passenger 

breathing zone35.

Vehicle manufacturers must 

keep records of the tests 

performed for a minimum 

period of three years 

demonstrating that CO2

refrigerant levels do not exceed 

the STEL of 3% averaged over 

15 minutes in the passenger free 

space, and the ceiling limit of 

4% in the breathing zone.

The use of CO2 in MVAC 

systems must adhere to the

standard conditions identified in 

SAE Standard J639 (2011 

version) including: 

Installation of a high pressure

system warning label; 

Installation of a compressor 

cut-off switch; and

Use of unique fittings with:

Outside diameter of 16.6 +0/-

0.2 mm (0.6535 +0/-0.0078 

inches) for the MVAC low-side; 

Outside diameter of 18.1 +0/-

0.2 mm (0.7126 +0/-0.0078 

inches) for the MVAC highside; and 

 Outside diameter of 20.955 

+0/-0.127 mm (0.825 +0/-0.005 

inches) and right-hand thread 

direction for CO2 refrigerant 

service containers.

36

Additional training for 

service technicians is 

recommended.

In designing risk 

mitigation strategies 

and/or devices, 

manufacturers should 

factor in background 

CO2 concentrations in 

the passenger cabin 

potentially contributed 

from normal respiration 

by the maximum number 

of vehicle occupants.

Use of the standards 

SAE J1052, SAE J2772, 

and SAE J2773 is 

recommended as 

additional reference.

Manufacturers should 

conduct and keep on file 

Potential Failure Mode 

and Effects Analysis in 

Design [Design FMEA], 

Potential Failure Mode 

and Effect Analysis in 

Manufacturing and 

Assembly Process 

[Process FMEA] on the 

MVAC as stated in SAE 

J1739. 
58

* * * * *

[FR Doc. ] BILLING CODE 6560-50-P

 

34 The passenger free space is the space inside the passenger compartment excluding the space enclosed by the 

ducting in the HVAC module.

35 Area inside the passenger compartment where the driver’s and passengers’ heads are located during a normal 

sitting position. Refer to SAE J1052 for information on determining passenger head position.

36 The refrigerant service containers fitting requirement applies only to refrigerant service containers used during 

servicing of the MVAC, in accordance with the provisions established for MVAC servicing under 40 CFR part 82, 

subpart B.