# Response to Comments

For the Notice of Proposed Rulemaking and Notice of Data

Availability : 

# Protection of the Stratospheric Ozone: Alternative s 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 , 201 2

U.S. Environmental Protection Agency Office of Air and Radiation Stratospheric Protection Division 1200 Pennsylvania Avenue, NW Washington, DC 204601



### 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 ................................ ................................ .................... 122



### 1. List of Public Co mments Re ceived 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/Technic al 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 B COOL 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 Materials3



### 2. Response to Comments on Use Conditions Comment: Some commenters claimed that the proposed STEL of 3% CO 2 averaged ov er 15 minutes in the cabin free space is enough to protect passengers and ensure driver alertness . One commenter suggested to consider a 3% CO 2 concentration limit averaged over 15 minutes in the breath level (i.e., breathing zone) instead o f 3% in the cabin free space. The commenter considered the breat hing 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 CO 2 concentration limits but over a shorter period of time ( e.g ., 5.5 % CO 2 for 5 minutes and 9% CO 2 as a ceiling limit) . Other commenters requested that EPA include a CO 2 ceiling limit in the passenger breathing zone and one commenter considered appropriate a 4% CO 2 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% CO 2 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 limi t of 4% CO 2 , in addition to the CO 2 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 CO 2 refrigerant reaching concentrations above 3% averaged ove r 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 CO 2 ceiling limit of 4% is necessary because it is possible for a time - weighted average concentra tion, such as the STEL, to be under 3%, while peak concentrations could reach higher limits for a few minutes. As shown i n published data , CO 2 concentration peaks above 4% could result in effects on hearing and vision that could distract and endanger a dri ver , or other , potentially more severe adverse health effects ( EPA - HQ OAR - 2004 - 0488 - 0041 ). CFD modeling showed that during unmitigated refrigerant leak scenarios , CO 2 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) . T he U.S. Army risk analysis ’s FTA showed that unmitigated leak scenario occurrences for CO 2 systems (reaching concentrations above 4% CO 2 ) could reach 4,300 per ye ar for the vehicle fleet (EPA HQ - OAR - 2004 - 0488 - 0025.2). Th ese 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 f luoride) exposure above health based limits . 1 Several studies reported that exposure ranging from 7% to 10% CO 2 for few minutes (e.g., 2 - 3 minutes) resulted in loss of consciousness (EPA - HQ - OAR - 2004 - 0488 - 0441). 2 1 T he 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 multiplie d 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 CO 2 concentration rangi ng 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 - O AR - 2004 - 0488 - 0041.4

EPA disagrees with the commenter stating th at 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, w e believe that unmitigated exposure occurre nces 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 CO 2 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 CO 2 inside the passenger com partment 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 t he text of the prop osed use condition indicating the STEL for CO 2 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 avera ge consisting of the average CO 2 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% CO 2 ap plicable in any part of the free space inside the passenger compartment for a time period of 60 se conds when the car ignition is on . The suggested language reads: “Engineering strategies and/or devices shall be incorporated into the system such that forese eable 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. Arm y 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 CO 2 (EPA - HQ - OAR - 2004 - 0488 - 0025.2). Thus, that analysis recognized th at CO 2 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 CO 2 concentr ations 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. a n OEM publicly announced that it will be using HFO - 1234yf in some vehicles startin g 2013 model year (EPA - HQ - OAR 2004 - 0488 - 0062). 4 Fully occupied is defined as the maximum design occupancy de termin ed by the number of sets of seat belts (EPA HQ - OAR - 2004 - 0488 - 0025.1

– 0025.2).5

we did not specify whether the vehicle should be fully occupied to account for CO 2 ba ckground concentrations. We believe that CO 2 refrigerant concentrations may reach levels of concern (i.e., above 4% CO 2 ) 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, t o account for background CO 2 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 CO 2 STEL as a concentration limit averaged over 15 min utes, 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 sha ll 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 CO 2 concentration over the air volume of a fully occupied car an d 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 CO 2 STEL of 3% averaged over 15 minutes considers the average CO 2 concentration i n a passenger cabin o ver a total time period of 15 minutes during the event of refrigerant leak; and the ceiling limit of 4% CO 2 considers the total CO 2 in the passenger breathing zone at any one moment in a passenger compartment during the event of a leak. Regarding the alternativ e language suggested by the other commenter specifying a ceiling limit of 4% CO 2 applicable in any part of the free space inside the passenger compartment for a time period of 60 se conds when the car ignition is on, we note that the commenter did not provi ded 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 nee d for clarity on whether the use conditions apply whe n the ignition is o ff as well as when the ignition is o n . 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 comme nters 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 commen t period closed on the NODA, SAE issued a report, “ Risk Assessment for HFO - 1234yf and R - 744 (CO 2 ) Phase III” ( referred herein after as SAE CRP report), that evaluated toxicity effects and quantitative risks of CO 2 , 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 CO 2 exposure estimations due to leaks into the passenger compartment during different modeled 5 We refer here to the SNAP rule listing HFO - 1234yf as accept able 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 m ode, system failure, and during a collision (EPA 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 (EP A - 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 CO 2 refrigerant exposure above 6 % ( a threshold limit used by the CRP for this scenario ) to be in the order of 10 - 1 2 per vehicle/hour/occupant (EPA - HQ - OAR - 2004 - 0488 - 0051, EPA - HQ OAR - 2008 - 0664 - 0056.2). We believe that exposures of concern inside the passenger compartment ar e 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 - 2 004 - 0488 - 00 2 5 .2) and consistent with the SAE CRP report (EPA - HQ - OAR - 2004 - 0488 - 0051, EPA - HQ - OAR 2008 - 0664 - 0056.2). In addition, e ven 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 - 2 004 - 0488 - 00 2 5 .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 runni ng. Therefore, EPA finds that the overall risks to human health and the environment from CO 2 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 - 152a 6 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. 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 CO 2 co ncentration limit should focus on the driver breathing zone rather than t he 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 sitti ng 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 J 1052. Since the automotive indu stry 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 passeng er 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 CO 2 and other available refrigera nts found acceptable subject to use conditions under SNAP (73 FR 33304; June 12, 2008).7

EPA dis agrees with the comment indicating that a CO 2 concentration limit should only focus on the driver breathing zone rather than the passenger cabin free space . B ased 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% CO 2 STEL averaged over 15 minutes in the passenger cabin free space as an additional protective measure for passenger exposure to CO 2 . 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 CO 2 (EPA - HQ - OAR - 2004 - 0488 - 0025.2) , thus we believe it is necessary to set a limit that wou ld address risk to all people in the passenger compartment and not solely the driver . We also take into consideration that passengers may not be i n a normal sitting position all the time (e.g., passenger may rest in a reclined position ) and note CO 2 is hea vier 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 peri od of time (i.e., 15 minutes) without suffering adverse health risks. For these reasons we include both limits (i.e., 4% CO 2 ceiling limit in the passenger breathing zone and 3% CO 2 averaged over 15 minutes in the passenger cabin free space) in this final rule. Comment: Several commenters suggested that a CO 2 ceiling limit shoul d rely on exposure time since potential effects of CO 2 vary with both concentration and duration of the exposure. One commenter stated that if the ceiling limit is exceeded, it is li kely due to collision events. Response: EPA agrees the health effects of CO 2 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 CO 2 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 CO 2 levels to the exceed the ceiling limit establi shed in the use condition s , 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 CO 2 . Comment: Several commenters suggested considering ce iling limits of CO 2 above 4% (e.g., 6%, 9%) based on studies showing that visual disturbances occur at concentrations of 6% CO 2 . They stated that the SAE CRP report’s rationale suggest ed a 9% CO 2 concentration ceiling limit , based on studies showing centra l nervous system ( CNS ) effects at CO 2 exposure concentrations of 10% (100,000 ppm) . Response: Studies report that human exposures to 6% CO 2 for periods as short as 2 minutes can lead to hearing and visual disturbances, and that exposures to 7.5% for 5 min utes lead to significant reasoning and performance decrements (Gellhorn, 1936; Sayers, 1987 as cited in EPA - HQ - OAR - 2004 - 0488 - 0041). T o provide a margin of safety, EPA considers it necessary to require a ceiling limit of 4% CO 2 in the passenger breathing zo ne as indicated in the NODA and8

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 Lif

e and Health ( IDLH ) value of 4% CO 2 based on a 30 - minute exposure. Response: EPA disagrees with the commenter s to the extent they are suggesting that the 4% limit be based on a 30 minute exposure. T he 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 he alth effects, or would impair an individual's ability to escape from a dangerous atmosphere." W e believe NIOSH’s IDLH is inappropriate as a ceiling limit for the use of CO 2 as a ref rigerant in MVACs because, as indicated above, a ceiling limit is a limit t hat 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 CO 2 exposure at concentrations equal or higher than 4% . CO 2 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 no t ed 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 perpendicula r leak angle to the rupture cross - sectional area (EPA - HQ - OAR - 2004 - 0488 - 0025.1) . The 2006 analysis finds that an unmitigated discharge of CO 2 , in full recirculation mode, results in CO 2 concentration above 3% for more than 60 minutes. The U.S. Army also in dicate d that a 3 - second, rather than a 10 - second squi b valve, as originally determined, would be needed to ensure that CO 2 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 error s (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 CO 2 refrigerant.9

Comment: One comment er stated that secondary loop techno logy is not a viable risk mitigation strategy for CO 2 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 speci fic risk mitigation strategy; however, EPA notes that secondary loop technology could potentially reduce the risks of exceeding the ceiling limit of CO 2 in the passenger compartment because the refrigerant charge stays separate from the pa ssenger compartme nt. 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. H owever, 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 CO 2 sensor performance . Additionally , the commenter stated that during sharp increases in CO 2 concentrat ion 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 i n the proposal, we intended for the squib valve activation time to include : 1) the time the sensor takes to dete ct a significant leak that would cause CO 2 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 time s (i.e., 30, 10 and 3 seconds) of squib valve during modeled scenarios of CO 2 refrigerant leak. The results showed higher effectiveness of the valve preventing high refrigerant concentration reaching the passenger compartment during the shorter activation ti me. EPA believes that sharp increases in CO 2 concentration in the passenger compartment will likely occur only when a significant amount of CO 2 refrigerant leaks into the passenger compartment. Risk assessments showed that CO 2 buildup due to passenger res piration 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: EP A did not propose the use of odorants, and this final action neither requires nor prohibits the use of odorants in new CO 2 MVAC systems. Odorized CO 2 may be an effective means to alert the driver and passengers to a refrigerant leak into the passenger comp artment. 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 odorize d CO 2 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 c abin as a mitigation strategy. Ano ther 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 Re sponse: As noted previously, EPA is not specifying the risk mitigation strategies that must be used to ensure CO 2 levels do not exceed the levels established in the use conditions. W e do not believe that a sensor alone would be sufficient to provide effec tive protection to the passengers and to ensure that concentrations inside the passenger compartment and passenger breathing zone do not exceed the established CO 2 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 CO 2 concentrations do not exceed the exposure limits established in the use conditions . Thus, a n 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 de vices because of cost . The commenter stated that close - coupled and hermetically sealed sys tems are technically feasible and noted that a n automatic increase in air exchange is a possible strategy that is technically feasible. Ano ther commenter suggested th at switching the MVAC blower to operate on outside air mode on high, rapidly after CO 2 refrigerant is released, could reduc e 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 CO 2 exp osure 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 negat e the risk s associated with potentially elevated CO 2 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 c ommenters indicated that SAE is developing standards for safety and servicing of CO 2 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 note s and agrees with the important role industry standards play particularly for t he MVAC sector. In addition, we note that the regulatory text reference s the relevant SAE technical standards to promote consistency with established industry practices. Specifi cally, use conditions in this final rul e 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 J 2 77 2 standard, “Measurement of Passenger Compartment Refrigerant Concentrations under system refrigerant leakage conditions” (EPA - HQ - OAR - 2004 - 0448 - 0054) and SAE J 2 773 standard, “Standard f or 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 J 2772 and J 2 773 standards as well as other available standards such as SAE J1052, Motor V ehicle Driver and Passenger Head P osition (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 th e 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 d id not see any servicing concerns in the proposal that would impact the service technicians they represent , but would be interested in any follow - on rule making 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 MV AC 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 5514 4 September 21, 2006) . Additional details regarding our evaluation of risk associated with MVAC service can be found in Section V I of th is final rule preamble (Why is EPA listing CO 2 acceptable subject to use conditions?). As explained in more detail i n Section VI above , we do not believe it is necessary to establish any use conditions regarding servicing because the overall envi ronmental and human health risks posed by the use of CO 2 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 CO 2 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 CO 2 as an acceptab le substitute for CFC - 12 only for new MVAC systems. We have never receive d a SNAP submission requesting consider ation of CO 2 in retrofitted MVAC systems . EPA understands that the higher working pressure of CO 2 compared to CFC - 12 and other SNAP acceptable r efrigerants could raise significant issues with retrofitting such systems to CO 2 . Because we have not received a request to use CO 2 in retrofitted systems, which would include the technical and other analyses necessary to determine whether such use would p resent more risk than other available substitutes, t his final rule only applies to the use of CO 2 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 retrofittin g to CO 2, we will consider CO 2 for use as a refrigerant to retrofit existing MVAC systems.