Response to Comments

For the Notice of Proposed Rulemaking and Notice of Data 

Availability:

Protection of the Stratospheric Ozone:

Alternatives for the Motor Vehicle Air 

Conditioning Sector under the Significant 

New Alternatives Policy (SNAP) Program

For the listing of Carbon Dioxide (R-744) as an alternative 

subject to use conditions for the Motor Vehicle Air 

Conditioning Sector

Docket: EPA-HQ-OAR-2004-0488

April 17, 2012

U.S. Environmental Protection Agency

Office of Air and Radiation

Stratospheric Protection Division

1200 Pennsylvania Avenue, NW

Washington, DC 20460
1

Contents

1. List of Public Comments Received and Summarized.............................................................. 2

2. Response to Comments on Use Conditions............................................................................. 3

3. Response to Comments on Risk Mitigation Strategies............................................................ 8

4. Response to Comments on Industry Standards...................................................................... 11

5. Response to Comments on Servicing .................................................................................... 12
2

1. List of Public Comments Received and Summarized

Public Comments Received by EPA on the Carbon Dioxide (R-744) for the Motor Vehicle 

Air Conditioning Sector under the SNAP Program

Document ID Title

EPA-HQ-OAR-2004-0488-0025

Comment submitted by Neal E. Blackwell, U.S. Army Research Development & 

Engineering Command, Communications-Electronics Research Development & 

Engineering Center 

EPA-HQ-OAR-2004-0488-0026 Comment submitted by John Noble, President, Cool Flow, Inc.

EPA-HQ-OAR-2004-0488-0027

Comment submitted by Ward Atkinson, Chair, Interior Climate Control 

Standards Committee, Society of Automotive Engineers (SAE)

EPA-HQ-OAR-2004-0488-0028 Comment submitted by Audi, BMW, Daimler-Chrysler, and Volkswagen 

EPA-HQ-OAR-2004-0488-0029

Comment submitted by William R. Hill, GM Technical Fellow HVAC, General 

Motors (GM)

EPA-HQ-OAR-2004-0488-0030

Comment submitted by Kjerstin Lien, Project Manager, OBRIST Engineering 

and Marc Chasserot, Marketing Manager, Shecco Technology

EPA-HQ-OAR-2004-0488-0032

Comment submitted by Douglas I. Greenhaus, Director, Environment Health 

and Safety, National Automobile Dealers Association (NADA)

EPA-HQ-OAR-2004-0488-0034 Comment submitted by Audi, BMW, DiamlerChrysler and Volkswagen

EPA-HQ-OAR-2004-0488-0035

Comment submitted by Frederick Sciance, Manager, General Motors Public 

Policy Center, General Motors Corporation (GM)

EPA-HQ-OAR-2004-0488-0036

Comment submitted by Ragiemra Amato, Manager Government/Technical 

Affairs, Delphi

EPA-HQ-OAR-2004-0488-0037

Comment submitted by Carloandrea Malvicino, Head, Thermal Systems 

Department, Vehicle Systems, Centro Ricerche Fiat (CRF), on behalf of BCOOL Consortium

EPA-HQ-OAR-2004-0488-0042 Comment submitted by Tony Zanders, Heritage Auto Repair

EPA-HQ-OAR-2004-0488-0043 Comment submitted by Michael Schneider, BMW Group

EPA-HQ-OAR-2004-0488-0044

Comment submitted by Gabriele Hoffmann, Umweltbundesamt (UBA) - Federal 

Environment Agency Germany

EPA-HQ-OAR-2004-0488-0045

Comment submitted by Frederick Sciance, Manager, Environment, Energy and 

Safety Policy, General Motors Public Policy Center, General Motors Corporation 

(GM)

EPA-HQ-OAR-2004-0488-0046 Comment submitted by J. Koehler

EPA-HQ-OAR-2004-0488-0047

Comment submitted by Ragiemra Amato, Manager, Government/Technical 

Affairs and Customer Contact, Innovation and Technology Office, Delphi 

Corporation

EPA-HQ-OAR-2004-0488-0048

Comment submitted by Christianna Papazahariou, Head, Government Affairs, 

Shecco 

EPA-HQ-OAR-2004-0488-0049 Comment submitted by Dr. Michael Sonnekalb, Konvekta

EPA-HQ-OAR-2004-0488-0050

Comment submitted by Steven H. Bernhardt, Global Director Regulatory 

Affairs, Honeywell Specialty Materials
3

2. Response to Comments on 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 

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. 

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.

1

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).2

 

1 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.

2 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.
4

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.3 

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.4 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 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, 

 

3 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 

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).

4

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).
5

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-occupiedvolume, 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 

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.

5 The report evaluates CO2

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

 

5 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)
6

scenarios such as different MVAC operation mode, system failure, and during a collision (EPAHQ-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-152a6as 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. 

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). 

 

6 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).
7

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 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. 

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 
8

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. 

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 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.

3. Response to Comments on 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 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.
9

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 

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 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.
10

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 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. 
11

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 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. 

4. Response to Comments on 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 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). 
12

5. Response to Comments on 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 follow-on 

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 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.