RISK ASSESSMENT FOR ALTERNATIVE REFRIGERANTS

HFO-1234yf and R-744 (CO2)

Phase III 

PREPARED FOR:

SAE INTERNATIONAL 

Cooperative Research Program 1234 

PREPARED BY: 

Thomas A. Lewandowski, Ph.D., DABT, ERT 

GRADIENT CORPORATION 

600 Stewart Street 

Seattle, WA 98101 

December 17, 2009 
 

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Table of Contents

Executive Summary ............................................................................................................. 1

Project Summary ................................................................................................................. 4

1 Introduction and Approach ..................................................................................... 12

1.1 European Union F-Gas Regulation ................................................................................... 12

1.2 U.S. EPA Significant New Alternatives Program (SNAP) ............................................... 12

1.3 U.S. EPA Sponsored Risk Assessment of R-152a and R-744 (CO2) ................................ 13

1.4 SAE Cooperative Research Program (CRP) 150 .............................................................. 15

1.5 SAE Cooperative Research Program 1234 (CRP1234) .................................................... 15

1.5.1 Phase I .................................................................................................................. 15

1.5.2 Phase II ................................................................................................................ 16

1.5.3 SAE CRP1234 Phase III (Current Effort) ............................................................ 17

1.6 Risk Assessment Approach ............................................................................................... 19

1.6.1 Fault Tree Analysis (FTA) ................................................................................... 21

1.6.2 Interpretation of FTA Results .............................................................................. 22

2 HFO-1234yf ........................................................................................................... 23

2.1 General Properties ............................................................................................................. 23

2.2 Toxicity Assessment for HFO-1234yf .............................................................................. 23

2.2.1 Evaluation of Available Toxicity Data from Manufacturer ................................. 23

2.2.2 Summary of Toxicological Findings ................................................................... 25

2.3 Flammability Properties .................................................................................................... 26

2.4 Consideration of Combustion Products ............................................................................ 27

2.5 Consideration of Hazards to be Quantitatively Addressed by FTA .................................. 28

2.5.1 Hazards to be Quantitatively Addressed via FTA ............................................... 28

2.5.2 Hazards Which Will Not Be Quantitatively Addressed via FTA ........................ 31

3 HFO-1234yf Data Collection ................................................................................. 42

3.1 Refrigerant Concentration Data ........................................................................................ 42

3.1.1 Concentration Measurements – Passenger Compartment .................................... 42

3.1.2 Modeled HFO-1234yf Concentrations in the Passenger Compartment ............... 43

3.1.3 Concentration Measurements – Engine Compartment ........................................ 44

3.1.4 Modeled HFO-1234yf Concentrations in Vehicle Service Areas ........................ 45

3.1.4.1 Uniform Mixing Model ........................................................................ 45

3.1.4.2 CFD Modeling of Local HFO-1234yf Concentrations ........................ 47

3.2 Flammability Testing ........................................................................................................ 49

3.2.1 Identification of Credible Ignition Sources.......................................................... 49

3.2.2 Bench Scale Ignition Testing ............................................................................... 50

3.2.3 Passenger Compartment Ignition Testing ............................................................ 53

3.2.3.1 Exponent .............................................................................................. 53

3.2.3.2 Hughes Associates Inc. ........................................................................ 53

3.2.4 Engine Compartment Testing .............................................................................. 54

3. 3 HF generation potential .................................................................................................... 55

3.3.1 Work Conducted by Honeywell .......................................................................... 55

3.3.2 Work Conducted by DuPont ................................................................................ 55

3.3.3 Work Conducted by INERIS ............................................................................... 56

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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3.3.4 Work Conducted by Hughes Associates Inc. ....................................................... 57

3.3.4.1 Passenger Compartment ....................................................................... 57

3.3.4.2 Engine Compartment ........................................................................... 59

3.3.5 Review and Selection of HF Exposure Limit ...................................................... 61

3.3.6 Summary of HF Generation Potential .................................................................. 65

4 FTA for HFO-1234yf ............................................................................................. 72

4.1 Refrigerant Ignition, Passenger Compartment Scenario ................................................... 72

4.2 Refrigerant Ignition, Engine Compartment Scenarios ...................................................... 73

4.3 Refrigerant Ignition, AC Service Scenarios ...................................................................... 74

4.4 HF Generation, Passenger Compartment Scenarios ......................................................... 75

4.5 HF Generation, Engine Compartment Scenarios .............................................................. 77

4.6 High Pressure Equipment Risks in Vehicle Service Areas ............................................... 78

4.7 Risks for Other Receptors ................................................................................................. 79

4.7.1 Potential Risks to Other Human Receptors ......................................................... 79

4.7.2 Ecological Risks .................................................................................................. 79

4.8 Sensitivity analysis and description of uncertainties ........................................................ 80

4.8.1 Sensitivity Analysis ............................................................................................. 80

4.8.2 Sources of Uncertainty in the Risk Assessment................................................... 83

4.9 Conclusions Regarding HFO-1234yf ............................................................................... 85

5 R-744 ...................................................................................................................... 92

5.1 General Properties ............................................................................................................. 92

5.2 Toxicity Assessment for CO2 ............................................................................................ 92

5.2.1 Summary of Relevant Data .................................................................................. 92

5.2.2 Identification of Health-Based Limits for R-744 ................................................. 97

5.3 Hazards to be Quantitatively Addressed by FTA ............................................................ 97

6 R-744 Data Collection .......................................................................................... 104

6.1 Refrigerant Concentration Measurements – Passenger Compartment ............................ 104

6.2 Refrigerant Concentration Modeling – Passenger Compartment ................................... 104

6.3 Modeling Refrigerant Concentrations for AC Service Scenarios ................................... 105

7 FTA for R-744 ...................................................................................................... 108

7.1 CO2 Exposure, Passenger Compartment Scenarios ........................................................ 108

7.2 CO2 Exposure, AC Service Scenarios ............................................................................. 109

7.3 High Pressure Equipment Risks in Vehicle Service Areas ............................................. 109

7.4 CO2 Exposure for Individuals Sleeping in Vehicles During a Refrigerant Leak ............ 109

7.5 Sensitivity analysis and description of uncertainties ...................................................... 110

7.6 Conclusions Regarding R-744 ........................................................................................ 111

8 References ............................................................................................................ 115

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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LIST OF TABLES 

Table ES-1 Probabilities of Various Adverse Events Compared to Probabilities of Events Associated 

with Vehicle Operation-Related Leaks of HFO-1234yf or R-744 

Table ES-2 Probabilities of Various Adverse Events Compared to Probabilities of Events Associated 

with Vehicle Service-Related Leaks of HFO-1234yf or R-744 

Table 2-1 Summary of Available Toxicity Data for HFO-1234yf 

Table 2-2 Comparison of Toxicity of HFO-1234yf to the Toxicity of R-134a 

Table 2-3 Flammability of HFO-1234yf Relative to Other Refrigerants 

Table 2-4 HFO-1234yf Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Table 2-5 Boundary Conditions Considered in Vehicle Operation/Fire Fault Trees 

Table 2-6 Boundary Conditions Considered in Vehicle Service Fault Trees 

Table 3-1 HFO-1234yf Concentrations Measured in Vehicle Cabins by Different OEMs after 

Simulated Refrigerant Release 

Table 3-2 Results of Modeling Conducted for HFO-1234yf Concentration in Passenger Compartment 

Table 3-3 Results of Air Modeling for AC Service Scenarios, HFO-1234yf 

Table 3-4 HF Concentration Measurements Obtained by INERIS in Bench Scale Tests 

Table 3-5 Results of In Compartment HF Concentration Measurements Obtained by Hughes 

Associates 

Table 3-6 Results of Engine Compartment HF Concentration Measurements Obtained by Hughes 

Associates 

Table 4-1 Ecological Toxicity Data for Selected Chemicals Compared to HFO-1234yf 

Table 4-2 Results of Sensitivity Analysis for HFO-1234yf FTA 

Table 4-3 Probability of Injury or Fatality from Various Events Compared to Probabilities Associated 

with Leaks of HFO-1234yf 

Table 4-4 Overall Comparison of HFO-1234yf to R-12 and R-134a 

Table 5-1 Summary of Available Toxicity Data for CO2 

Table 5-2 Toxicity Effects Associated with Carbon Dioxide Exposures for Different Time/Duration 

Combinations 

Table 5-3 Comparison of Toxicity of CO2 to the Toxicity of R-134a 

Table 5-4 Possible R-744 Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Table 6-1 HFO-1234yf Concentrations Measured in Vehicle Cabins by Different OEMs after 

Simulated Refrigerant Release 

Table 6-2 Results of Air Modeling Simulations for Passenger Scenarios, R-744 [Uniform Mixing 

Model] 

Table 6-3 Results of Air Modeling for AC Service Scenarios, R-744 

Table 7-1 Probabilities of Injury or Fatality from Various Events Compared to Probabilities Associated 

with Leaks of R-744 

Table 7-2 Overall Comparison R-744 to R-12 and R-134a 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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LIST OF ATTACHMENTS

Attachment A Input Values and Fault Tree Analysis Results for HFO-1234yf 

Attachment B Input Values and Fault Tree Analysis Results for R-744 

Attachment C List of Technical Terms and Acronyms Used in the Risk Assessment 

Attachment D Honeywell Report - Leak Rate Determinations for HFO-1234yf and R-744 

Attachment E Uniform Mixing Model Results for Vehicle Compartment Exposures 

Attachment F Modeling Results for AC Service Scenarios: HFO-1234yf, Uniform Mixing 

Attachment G DuPont Report – CFD Analysis of Potential Release of HFO-1234yf During 

Vehicle Service Scenario 

Attachment H Monforte and Caretto Paper on Automotive Refrigerant Evaluation 

Attachment I Ignition Source Evaluation 

Attachment J INERIS Report – Ignition of Refrigerant-Oil Mixtures on Hot Surfaces 

Attachment K DuPont Reports – Analysis of Potential Ignition of HFO-1234yf with Simulated 

Car Lighter (Glow Wire) During Accidental Refrigerant Leak and Analysis of 

Potential Ignition of HFO-1234yf by a Lit Cigarette During Accidental 

Refrigerant Leak 

Attachment L DuPont Report – HFO-1234yf Ignitability to Spark from 12-V Battery Short 

Circuit 

Attachment M Honeywell Report - Combustion Pressure Rise of Selected Refrigerants 

Attachment N Hughes Associates Report – Refrigerant Decomposition Tests, Part 1: In Car 

Tests 

Attachment O Hughes Associates Report – Refrigerant Decomposition Tests, Part 1: Passenger 

Car Engine Compartment Tests 

Attachment P Honeywell Report - Combustion Tests of HFC-134a and HFO-1234yf 

Simulating Cabin Leaks in Mobile AC Systems 

Attachment Q DuPont Report – Analysis of Inhalation of HFC-134a and HFO-1234yf Through 

a Cigarette During Accidental Refrigerant Leak 

Attachment R INERIS Report – HF Generation by a Refrigerant When Submitted to a High 

Temperature 

Attachment S Summary of Data on Vehicle Collisions 

Attachment T Toyota Data on Vehicle Collisions 

Attachment U Modeling Results for AC Service Scenarios: R-744 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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CONTRIBUTING PARTIES 

This risk assessment was developed under the direction of a Cooperative Research Project under the 

auspices of the SAE International (SAE). The following companies participated in Phase III of this risk 

assessment. 

Audi 

BMW 

Chrysler 

Daimler 

DuPont 

Ford/Volvo 

General Motors/Opel 

Honeywell 

Hyundai 

Porsche 

PSA 

Renault 

Jaguar/Land Rover/Tata 

Toyota 

Volkswagen 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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Executive Summary 

 Due to concerns about global warming, the use of refrigerant R-134a (i.e., 1,1,1,2-

tetrafluoroethane), will be banned for air conditioning systems for new vehicle platforms throughout the 

European Union (EU) on January 1, 2011. The replacement will have to be acceptable in terms of human 

and ecological toxicity, environmental effects, effectiveness, and compatibility with other air conditioning 

components. No regulatory scheme equivalent to that adopted by the EU currently exists elsewhere. A 

regulatory program under consideration in the United States that would offer tail-pipe emissions credits 

for reduced greenhouse gas emissions may also provide an incentive for developing a replacement for R134a. 

 To assist member companies with selection of an R-134a replacement, SAE International 

(formerly the Society of Automotive Engineers, SAE), in co-operation with E.I. du Pont de Nemours and 

Company (DuPont) and Honeywell International (Honeywell), have undertaken a cooperative research 

project (CRP1234) to evaluate two current replacement candidates for R-134a: 2,3,3,3-tetrafluoroprop-1-

ene (HFO-1234yf) and carbon dioxide (R-744, CO2). The evaluation addressed the risks to vehicle 

occupants as well as vehicle assembly, service, and service workers resulting from accidental releases of 

these refrigerants such as might result from damage to vehicle air conditioning systems or improper use of 

vehicle service equipment. The evaluation also addresses possible exposures to HF formed during 

combustion or thermal decomposition of this refrigerant. 

CRP1234 Phases I and II 

 This document represents an update of earlier risk assessments of HFO-1234yf completed in 

March of 2008 (CRP1234 Phase I) and March of 2009 (CRP1234 Phase II). The Phase I risk assessment, 

which focused on refrigerant toxicity and flammability, concluded that use of HFO-1234yf as an 

automotive AC system refrigerant would result in levels of risk that were well below levels of risk 

inherent in many commonly accepted activities (e.g., airplane travel, riding in elevators). The Phase I 

report noted several outstanding questions related to refrigerant flammability in the context of vehicle 

collisions. As part of the Phase II risk assessment, additional testing was conducted by various vehicle 

OEMs and INERIS, a French laboratory specializing in flammability analyses. The Phase II risk 

assessment also considered exposures to hydrogen fluoride (HF) that could occur during refrigerant 

combustion. It also included an assessment of R-744, using the same parameters used to evaluate HFOFinal Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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1234yf so as to provide OEMs with a basis for selecting an R-134a replacement. The Phase II risk 

assessment report concluded that use of HFO-1234yf as an automotive AC system refrigerant posed 

acceptable levels of risk in terms of toxicity, ignition potential and potential HF exposure. With respect 

to R-744, the Phase II risk assessment found that the anticipated probability of adverse events was of the 

same order of magnitude as those estimated for HFO-1234yf. 

CRP1234 Phase III 

The Phase III risk assessment constitutes a new body of work and focused on additional risk 

scenarios identified by CRP1234 members. The additional work included: 

Harmonization of the CRP1234 risk assessment with independent investigations 

undertaken by a group of German OEMs who joined the CRP1234 effort. 

Additional work related to potential HFO-1234yf ignition sources conducted at both 

DuPont and Honeywell. 

Research to obtain additional data on parameters contributing to vehicle collisions such 

as collision frequency, impact location, and collision severity (in particular the potential 

to damage the AC system). 

Analyses of HFO-1234yf releases in an actual vehicle. This included measurement of 

refrigerant concentrations in passenger compartment and engine compartment air, 

attempted ignition of the refrigerant, and measurement of HF produced by refrigerant 

ignition or thermal decomposition. 

Complementary studies of HF generation potential related to HFO-1234yf ignition or 

thermal decomposition. 

Determination of appropriate HF exposure limits for one-time accidental exposures by a 

workgroup of CRP toxicologists. 

Additional modeling to address potential risks for AC service technicians due to localized 

concentrations of HFO-1234yf that would not be predicted by a uniform mixing model. 

Consideration of the likelihood of potential injuries for AC service technicians in the 

event of a rupture of a high-pressure recharge hose. 

 The Phase III risk assessment incorporated various types of data including toxicity data, bench 

scale testing results (i.e., flammability and HF generation studies), modeled air concentrations, and 

vehicle measurements (i.e., measurements of passenger compartment and engine compartment 

concentrations of chemicals in air and, where applicable, ignition tests). These data were collected from 

various sources such as government databases as well as publicly available and confidential reports. Data 

not found in existing reports were generated by consulting groups and/or CRP1234 members. 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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 Overall, the CRP1234 Phase I, II, III risk assessments demonstrate that HFO-1234yf is suitable 

for use in direct-expansion air-conditioning systems. 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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Project Summary

 Due to concerns about global warming, the use of refrigerant R-134a (i.e., 1,1,1,2-

tetrafluoroethane), will be banned for air conditioning systems for new vehicle platforms throughout the 

European Union (EU) on January 1, 2011. Use of R-134a will be banned in the EU for all new vehicles 

by January 1, 2017 (European Fluorocarbon Technical Committee, 2007). Regulations developed by the 

European Union require that any refrigerant(s) intended to replace R-134a have a global warming 

potential (GWP) less than or equal to 150 based on a 100-year time horizon. The replacement will have 

to be acceptable in terms of human and ecological toxicity, environmental effects, effectiveness, and 

compatibility with other air conditioning components. No regulatory scheme equivalent to that adopted 

by the EU currently exists elsewhere. A regulatory program under consideration in the United States that 

would offer tail-pipe emissions credits for reduced greenhouse gas emissions may also provide an 

incentive for developing a replacement for R-134a. 

 To assist member companies with selection of an R-134a replacement, SAE International 

(formerly the Society of Automotive Engineers, SAE), in co-operation with E.I. du Pont de Nemours and 

Company (DuPont) and Honeywell International (Honeywell), has undertaken a cooperative research 

project (CRP1234) to evaluate two current replacement candidates for R-134a: 2,3,3,3-

Tetrafluoropropene (HFO-1234yf) and carbon dioxide (R-744, CO2). The evaluation addressed the risks 

to vehicle occupants as well as vehicle assembly, service, and service workers resulting from accidental 

releases of these refrigerants such as might result from damage to vehicle air conditioning systems or 

improper use of vehicle service equipment. The evaluation also addresses possible exposures to HF 

formed during combustion or thermal decomposition of this refrigerant. 

 The current risk assessment follows the general approach of the U.S. EPA-sponsored risk 

assessment of R-152a and CO2 (Blackwell et al., 2006) so as to employ a consistent methodology and 

generate a product familiar to regulatory agencies. The approach is also consistent with a previous risk 

assessment evaluating alternative refrigerants (CRP 150) prepared by Gradient Corporation in 2008 

(Gradient, 2008). The current risk assessment considers additional scenarios (e.g., risks of refrigerant 

ignition in the engine compartment, rupture of high pressure hoses during vehicle service) beyond those 

addressed in the earlier efforts and incorporates newer data related to the probabilities of various factors 

related to potential exposure. 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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CRP1234 Phases I and II 

 This document represents an update of earlier risk assessments of HFO-1234yf completed in 

March of 2008 (CRP1234 Phase I) and March of 2009 (CRP1234 Phase II). The Phase I risk assessment 

was based largely on CFD modeling and limited analyses of refrigerant concentrations in air and 

refrigerant ignition performed in an actual vehicle. Modeled and measured concentrations of HFO1234yf in the air of the passenger compartment never exceeded heath-based limits. Concentrations of 

HFO-1234yf in passenger compartment air were predicted to reach the flammable range (i.e., exceeded 

the lower flammable limit) in the event of a leak resulting from a vehicle collision which damages the 

evaporator but does not damage other parts of the AC system and does not break the vehicle windows or 

door seals. However, field testing showed it was not possible to ignite the refrigerant after a simulated 

leak in the passenger compartment. Field simulations of a ruptured tube in the engine compartment did 

indicate concentrations would exceed the LFL, although attempts to ignite the leaked refrigerant were 

unsuccessful. Modeled concentrations of refrigerant in the air of vehicle service areas in the event of an 

accidental release never exceeded the health-based limit nor the LFL. A fault tree analysis, focused on 

refrigerant toxicity and flammability, demonstrated that use of HFO-1234yf as an automotive AC system 

refrigerant would result in levels of risk that were well below levels of risk inherent in many commonly 

accepted activities (e.g., airplane travel, riding in elevators). The Phase I report noted several outstanding 

questions related to refrigerant flammability with respect to refrigerant releases that could occur after a 

vehicle collision. questions that were subsequently addressed in Phase II. 

 For the Phase II risk assessment, additional testing was conducted by various vehicle OEMs and 

INERIS, a French laboratory specializing in flammability analyses. This new testing provided additional 

data on refrigerant concentrations in passenger compartment air for different types of vehicles. The 

INERIS work also better defined the specific conditions which could lead to refrigerant ignition (e.g., 

possible refrigerant ignition sources, vehicle operating conditions which could produce refrigerant 

concentrations above the flammable limits). In addition, testing was conducted by DuPont and 

Honeywell with regards to the generation of hydrogen fluoride (HF) when HFO-1234yf is subjected to an 

energy source at air concentrations below the LFL (i.e., thermal decomposition). The Phase I fault trees 

were revised and expanded to incorporate these new data. Finally, because the values and assumptions 

used in the fault tree analysis were substantially updated relative to those used in U.S. EPA’s analysis of 

R-152a and R-744 (Blackwell et al., 2006), the Phase II assessment also included an assessment of R-744, 

using the same parameters used to evaluate HFO-1234yf so as to provide OEMs with a comparable basis 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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for selecting an R-134a replacement.. The Phase II risk assessment report concluded that use of HFO1234yf as an automotive AC system refrigerant posed acceptable levels of risk in terms of toxicity, 

ignition potential and potential HF exposure. For example, the risk assessment found that the risk of an 

HFO-1234yf-related ignition event would be on the order of 1 event per year or less across an entire 

vehicle fleet (considering the vehicle fleets in Europe, Japan and the U.S. separately). The risk assessment 

also estimated fewer than 0.001 HF exposures per year above an appropriate health-based limit (equal to 

less than 1 such exposure every 1,000 years) across an entire vehicle fleet. Exposures of AC service 

technicians never exceeded health-based limits. With respect to R-744, the Phase II risk assessment 

found that the anticipated probability of adverse events was of the same order of magnitude as those 

estimated for HFO-1234yf. Exposures of AC service technicians to CO2 were not found to exceed healthbased limits, although details of R-744 servicing systems were noted to be uncertain. 

CRP1234 Phase III 

The SAE CRP1234 members recently undertook additional data gathering activities to further 

refine the results of the Phase II risk assessment. The additional work included: 

Harmonization of the CRP1234 risk assessment with independent investigations 

undertaken by a group of German OEMs who joined the CRP1234 effort. 

Additional work related to potential HFO-1234yf ignition sources conducted at both 

DuPont and Honeywell. 

Research to obtain additional data on parameters contributing to vehicle collisions such 

as collision frequency, impact location, and collision severity (in particular the potential 

to damage the AC system). 

Analyses of HFO-1234yf releases in an actual vehicle. This included measurement of 

refrigerant concentrations in passenger compartment and engine compartment air, 

attempted ignition of the refrigerant, and measurement of concentrations of HF produced 

by refrigerant ignition or thermal decomposition. Evaluations considered releases in both 

the passenger compartment and the engine compartment. The analyses were conducted 

by Hughes Associates, Inc., a firm specializing in fire scenario evaluations and HF 

analysis. 

Complementary studies of HF generation potential conducted in a specially constructed 

ignition chambers at the INERIS laboratories in France and the Honeywell laboratories in 

Buffalo, New York. These studies allowed for greater variability in testing conditions 

compared to the vehicle tests related to HFO-1234yf ignition or thermal decomposition. 

Additional modeling to address potential risks for AC service technicians due to localized 

concentrations of HFO-1234yf that would not be predicted by a uniform mixing model. 

Consideration of the likelihood of potential injuries for AC service technicians in the 

event of a rupture of a high-pressure recharge hose. 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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In addition, the Phase III risk assessment moved away from the region-specific (i.e., Europe, Japan, U.S.) 

estimation of risks used in Phase I and Phase II. This change reflects the fact that OEMs market vehicles 

in many different countries (including many beyond Europe, Japan and the U.S.) and individual OEMs 

are not confined in sales to a specific region. It therefore makes sense to develop a risk estimate that 

applies to a global vehicle fleet. Risks were estimated in terms of vehicle operation hours, a unit 

consistent with international vehicle performance standards. 

The Phase III risk assessment incorporated various types of data including toxicity data, bench 

scale testing results (i.e., flammability and HF generation studies), modeled air concentrations, and 

vehicle measurements (i.e., measurements of passenger compartment and engine compartment 

concentrations of chemicals in air and, where applicable, ignition tests). These data were collected from 

various sources including government databases as well as publicly available and confidential reports. 

Data not found in existing reports were generated by consulting groups and/or CRP1234 members. 

The results of the risk assessment indicate that HFO-1234yf is comparable to the current 

automotive refrigerant (R-134a) in terms of both human health effects and ecological effects. HFO1234yf has an ozone depletion potential (ODP) of 0 and a global warming potential (GWP) of 4 based on 

a 100-year time horizon, which meets the new European Union requirements. HFO-1234yf has been 

assigned a flammability classification of “Class 2”1

 by ASHRAE 34, as tested by SAE and ASHRAE 

reference test methods ASTM E681-04. Additional flammability testing conducted as part of the 

CRP1234 evaluation effort indicates the ignition potential of this chemical is far less than that of other 

flammable refrigerants (e.g., R-152a, which is assigned to the Class 2 category using the same 

definitions). 

Toxicity data for HFO-1234yf are available from a large number of studies conducted by 

independent testing laboratories commissioned by the manufacturers. These toxicity data, when 

combined with the results of air measurements obtained by the vehicle manufacturers after simulated 

refrigerant releases in different types of vehicles, indicate that potential HFO-1234yf exposures of vehicle 

passengers in the event of AC system leaks into the passenger compartment would be below levels 

associated with potential health effects. Similarly, HFO-1234yf exposures of AC service technicians 

would be below levels associated with potential health effects. Potential exposures of automotive 

 

Final Report on Risk Assessment for HFO1234yf and R-744 prepared for the SAE CRP1234
 

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assembly workers, which could occur due to small losses of refrigerant during vehicle assembly and/or 

service, were also estimated to be below levels of health concern based on data collected for R-134a. 

Thus, potential health effects from exposures to HFO-1234yf were not judged to be a source of concern in 

the risk assessment and were not evaluated in the fault tree analysis. 

As regards the potential ignition hazards of HFO-1234yf, the risk assessment considered a 

scenario in which the evaporator tubing is ruptured in the event of a vehicle collision and refrigerant 

enters the passenger compartment and is ignited. Smaller-sized leaks, such as might be due to corrosion 

of the evaporator surface, were judged to be too slow to produce significant concentrations of refrigerant 

in the passenger compartment air. The risk assessment also considered ignition scenarios in the under 

hood area involving both collision-related damage or a failure of the AC system during vehicle operation. 

When the risks for these potential ignition scenarios are combined, the fault tree analysis indicates that the 

risk of an occupant (or former occupant) being exposed to an open flame due to HFO-1234yf ignition will 

be extremely low, on the order of 9 x 10-14 events per hour of vehicle operation. Nearly all of this risk was 

attributable to release and ignition of refrigerant as a result of a vehicle collision. Note that not all 

ignition events leading to the presence of an open flame will lead to potential occupant injury. Risks for 

certified AC service technicians were also found to be inconsequential (1 x 10-26 per vehicle service hour) 

because modeled air concentrations of HFO-1234yf in the service areas only reached the LFL at a 

distance of 10 cm or less from the source of the AC system leak. The CRP1234 members judged that 

ignition sources would not be located this close to the AC system given the presence of other flammable 

materials in the engine compartment. Injury to AC service technicians from rupture of high pressure 

refrigerant recharge hoses was also evaluated in the FTA and found to result in 3 x 10-8 reportable vents 

per vehicle service hour. 

CRP1234 also evaluated potential exposures to HF that might occur in the event of HFO-1234yf 

ignition or thermal decomposition. The risk assessment considered potential HF exposures in the 

passenger compartment, either from refrigerant entering the compartment after a collision and being 

ignited or HF entering the compartment after being produced in the engine compartment. The risk 

assessment also considered potential HF exposures in the under hood area for various receptors (former 

occupants, good Samaritans) and under different scenarios (i.e., collision, AC equipment failures, vehicle 

fires related to non-AC equipment failure, vehicle fires to due vandalism). When the risks for these 

potential HF exposure scenarios are combined, the fault tree analysis indicates that the risk of an 

individual being exposed to HF above the relevant health-based limit will be extremely low , on the order 

of 5 x 10-12 events per hour of vehicle operation. The risk is almost entirely and equally attributable to 

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two scenarios: HF generation by vehicle fires arising from failure of non-AC vehicle components and 

decomposition of refrigerant in the engine compartment after a vehicle collision with subsequent 

aspiration of HF into the vehicle cabin. 

Overall, the CRP1234 Phase I, II, III risk assessments demonstrate that HFO-1234yf is suitable 

for use in direct-expansion air-conditioning systems. 

With respect to CO2, the available toxicology data indicate that acute adverse health effects 

involve effects on the central nervous system (CNS). One part of the risk assessment was therefore 

focused on exposures that might exceed the threshold for impairment of perception or reaction time. 

Passengers whose exposures exceed this level may not be able to take appropriate action to deal with 

emergency situations (e.g., escaping from the vehicle after a collision). The risk assessment also 

evaluated the case of individuals who might be sleeping in the vehicle during a CO2 release and might 

experience prolonged CO2 exposure and CNS depression. When the risks from each of the potential CO2

exposure scenarios are combined, the fault tree analysis estimated that the number of R-744 exposure 

events above the health based limits would be on the order of 2 x 10-10 events per hour of vehicle 

operation. This risk was almost entirely attributable to refrigerant release associated with a vehicle 

collision. Exposures of service workers were found to be in the range of 3 x 10-17 per vehicle service 

hour. Injury to AC service technicians from rupture of high pressure refrigerant recharge hoses was also 

evaluated in the FTA and found to result in 3 x 10-7 reportable events per vehicle service hour. 

 Tables ES-1 and ES-2 show that using a risk assessment approach involving a common set of 

fault tree assumptions, the chances of experiencing adverse events due to a refrigerant release can be 

compared for HFO-1234yf and R-744. These risks can also be compared to other risks individuals might 

encounter. Table ES-1 shows that the estimated probabilities for vehicle passengers of adverse events 

associated with accidental refrigerant release of either refrigerant are less than the probabilities of other 

commonly accepted risks. Table ES-2 shows that probabilities of injury of AC service technician are also 

less than the rate of other workplace injuries, the probability of injury from high pressure systems 

involving CO2 being one order of magnitude greater than the equivalent probability associated with HFO1234yf. 

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Table ES-1. Probabilities of Various Adverse events Compared to Probabilities of Events 

Associated with Vehicle Operation-Related Leaks of HFO-1234yf or R-744 

Event Probability per 

vehicle per 

operating hour* 

Citation 

Probability of being in a police reported vehicle collision1 5 x 10-5 NHTSA, 2007a 

Probability of dying during a 5000-mile plane trip2 4 x 10-7 Wilson and Crouch, 

1987 

Probability of vehicle collision due to vehicle brake failure3 3 x 10-7 New York State DMV 

Probability of highway vehicle fire (any cause)4 1 x 10-7 Ahrens, 2008; BTS, 

2004 

Probability of an airbag-related fatality associated with a vehicle collision5 2 x 10-10 NHTSA, 2007b

Probability of vehicle occupant exposure to R-744 above health based 

limits 2 x 10-10 Current analysis 

Probability of vehicle occupant/former occupant experiencing HF 

exposure above health based limits associated with an HFO-1234yf 

ignition event 

5 x 10-12 Current analysis 

Probability of vehicle occupant being exposed to an open flame due to 

HFO-1234yf ignition 9 x 10-14 Current analysis 

1 There were 6,024,000 police reported vehicle collisions in the U.S. in 2007. This is divided by the number of registered vehicles (255,748,000, 

NHTSA 2009) and the average number of hours each vehicle is operated (approximately 500 hours based on SAE J2766, Table 6).

2 Frequency per trip divided by 8 hours assuming plane travels between 600 and 650 mph. 3 The New York State Department of Motor Vehicles reports 1753 accidents in 2008 attributed to brake failure (NYSDMV, 2008) and roughly 11 million 

registered vehicles in New York State in 2008. Combining these data with the "operating hours per year" suggested in J2766 (~500 as an average) 

yields the an accident frequency per vehicle operating hour.

4 Ahrens reports that 33 highway vehicle fires are reported per hour. This is divided by the number of registered vehicles in the U.S. (255,748,000, 

NHTSA 2009) .

5 Based on NHTSA statistics for 1990 to 2001, the average number of confirmed air-bag related fatalities per year is 16. This is divided by the number 

of hours of vehicle operation per year (approximately 500 hours based on SAE J2766, Table 6) and the number of vehicle drivers in the U.S. (200 

million) to arrive at an average hourly risk rate.

* Probabilities are given in these units to be consistent with international performance standards used by the automotive industry. Results of the Phase I 

and Phase II risk assessment were given in units of "per year". Conversion between these units is possible by accounting for the number of vehicle 

operating hours per year (roughly 500 hours) and the size of the relevant vehicle fleet (e.g., roughly 250 million vehicles in the U.S.). Note that the 

outcomes of individual fault trees should not all be combined because some have different denominators (events per vehicle operating hour, events per 

year). The values shown here represent the highest estimates when combining individual fault trees with common units of the top level events. 

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Table ES-2. Probabilities of Various Adverse Events Compared to Probabilities of Events 

Associated with Vehicle Service-Related Leaks of HFO-1234yf or R-744 

Event Probability Citation 

Non-fatal recordable Injury at work (all occupations, per working hour)1 2 x 10-5 NSC, 2004

Probability an AC service technician is struck by object resulting in lost work 

days (per working hour)2 2 x 10-6 BLS, 2007 

Probability of recordable incident involving AC service technicians 

being struck with CO2 equipment hose (per hour of vehicle service) 3 x 10-7 Current analysis 

Probability of recordable incident involving AC service technicians 

being struck with HFO-1234yf equipment hose (per hour of vehicle 

service)

3 x 10-8 Current analysis

Probability of AC service technician exposure to CO2 above health 

based limits (per hour of vehicle service) 3 x 10-17 Current analysis 

Probability of AC service technician exposure to an open flame due to 

HFO-1234yf ignition (per hour of vehicle service) 1 x 10-26 Current analysis 

1 The rate (3.7 per 100 workers) for non-fatal workplace injuries reported for 2008 by the U.S. Bureau of Labor Statistics (BLS, 2008) divided by the 

number of working hours per year in the U.S. (i.e., 2080). 2 The U.S. Bureau of Labor Statistics (2007) reports a total of 3450 cases in 2005 of automotive repair technicians and mechanics being struck by an 

object at work resulting in an injury that required time off from work. This can be divided by the number of technicians and mechanics working in the 

U.S. (954,000, BLS, 2007) and 2,080 working hours per year to yield a risk per working hour. 

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1 Introduction and Approach 

1.1 European Union F-Gas Regulation 

 On June 14, 2006, the F-Gas Regulation and Mobile Air Conditioning (MAC) Directive were 

published in the official journal of the European Union. The objective of these regulations is to contain, 

prevent, and thereby reduce emissions of fluorinated greenhouse gases covered by the Kyoto Protocol. 

The measures became effective as of July 4, 2007. The F-gas regulation covers the use of HFCs, PFCs 

and SF6 (sulfur hexafluoride) in all their applications, except Mobile Air Conditioning, which is covered 

by the MAC Directive (European Fluorocarbon Technical Committee, 2007). 

 The MAC Directive covers the use of HFCs in Mobile Air Conditioning for passenger cars only 

(Directive 2006/40/EC). The main objectives of the Directive are the control of leakage of certain 

fluorinated greenhouse gases with a global warming potential (GWP) higher than 150 in MAC systems 

and the prohibition of MAC systems using those gases after a certain date (Kozakiewicz, 2007). The 

current automotive refrigerant, R-134a2

, will be banned for air conditioning systems for new vehicle 

models by January 1, 2011, and for all new vehicles by January 1, 2017 (European Fluorocarbon 

Technical Committee, 2007). No equivalent regulatory scheme currently exists in the U.S. or Japan, 

although the issue is currently being considered by the California Air Resources Board in the U.S. In 

addition, tail pipe emissions credits for reduced greenhouse gas emissions currently under consideration 

by the U.S. Federal government would also provide an incentive for developing a replacement for R134a. 

1.2 U.S. EPA Significant New Alternatives Program (SNAP) 

 Section 612 of the Clean Air Act (CAA) requires the U.S. EPA to establish a program to identify 

alternatives to Class I (CFCs, halons, carbon tetrachloride, methyl chloroform, methyl bromide, and 

HBFCs) and Class II (HCFCs) ozone-depleting substances and to publish lists of acceptable and 

unacceptable substitutes. U.S. EPA implements these requirements through the Significant New 

 

2

 Under the ASHRAE standards, refrigerants are designated by the letter R followed by an alphanumerical code determined by 

the chemical structure of the refrigerant molecule. A similar naming system in which the letter “R” is replaced by a chemical 

class designation (e.g., CFC, HFC, HFO) is used by chemical manufacturers to designate halogenated hydrocarbons. Thus, R134a is also known as HFC-134a (hydrofluorocarbon-134a). The ASHRAE naming convention is used throughout this 

document where applicable. An ASHRAE designation has not yet officially been assigned to HFO-1234yf but an action in this 

regard is anticipated in early 2010. 

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Alternatives Policy (SNAP) Program. The purpose of the program is to allow a smooth transition away 

from ozone-depleting compounds by identifying substitutes that offer lower overall risks to human health 

and the environment. SNAP evaluates substitutes for ozone-depleting compounds in a number of 

industrial sectors including the automotive industry. Although R-134a is not an ozone depleting 

substance, U.S. EPA will have jurisdiction over the evaluation of the R-134a replacements because these 

chemicals are, in terms of the regulations, substitutes for R-12 as well as R-134a. Substitutes are assessed 

based on the potential for human exposure, toxicity, flammability, ozone depletion, impact on global 

warming, and other environmental effects. The final rulemaking describing the process for administering 

the SNAP program and issuing the first acceptability lists for substitutes was published on March 18, 

1994. Under the rule it is illegal to replace a Class I or Class II substance with any substitute which U.S. 

EPA determines may produce adverse effects on human health or the environment where other substitutes 

have been identified that reduce overall risk and are currently or potentially available (U.S. EPA, 2007). 

 A draft SNAP rulemaking has been issued for HFO-1234yf which proposes exposure limits based 

on avoiding refrigerant ignition. However, this rule has not been finalized and EPA is requesting 

comments on the proposed usage conditions. A proposed SNAP ruling was published which addressed 

the use of R-744 in mobile air conditioning systems. However, this rule has not been finalized. 3

1.3 U.S. EPA Sponsored Risk Assessment of R-152a and R-744 (CO2)

 Blackwell et al. (2006) presented a risk assessment for alternative MAC systems employing 

either R-152a (1,1-difluoroethane) or R-744 (CO2). The risk assessment was sponsored by U.S. EPA, the 

Australian Greenhouse Gas Office; the European Commission; the Japanese Ministry of Environment; the 

Japanese Ministry of Economy, Trade and Industry; and Environment Canada. To examine these risks, 

U.S. EPA identified refrigerant release scenarios with the highest risks of an adverse outcome. These 

scenarios were the potential for a moderate to large leak into the passenger compartment, and the potential 

for technicians to be exposed during vehicle AC system service. For R-152a, Blackwell et al. defined 

unacceptable exposures as those exposures “above 3.5% in air, which is slightly less than the lower 

flammability limit of 3.7%” (Blackwell et al., 2006).4

 Potential toxic effects of R-152a were not 

 

3

 In the September 21, 2006 SNAP notice concerning the use of CO2 as a mobile air conditioning refrigerant, U.S. EPA proposed 

an exposure limit of 3% averaged over 15 minutes (Federal Register, Vol. 71, No. 183). The 3% concentration limit is identical

to the National Institute for Occupational Safety and Health (NIOSH) recommended exposure limit (REL) for CO2 and the 

American Conference of Governmental Industrial Hygienists (ACGIH) short-term exposure limit (STEL). To date, the SNAP 

notice concerning use of CO2 as a mobile air conditioning refrigerant has not been finalized. 4

 In the June 12, 2008 SNAP Final Rule concerning the use of R152a as a mobile air conditioning refrigerant, U.S. EPA ruled 

R152a was acceptable for use in MACS under the following use conditions: “Engineering strategies and/or devices shall be 

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addressed because the thresholds for these effects are above the lower flammability limit. For R-744, 

unacceptable exposures were defined as those above 4% for a period of an hour, or above 6% for 

exposures of one to two minutes or more. These levels were selected because human exposure to CO2 at 

and above these levels can reduce a driver’s capacity to handle a vehicle in an appropriate manner. 

Computational fluid dynamic (CFD) modeling simulated a reasonable worst-case scenario leak into the 

passenger compartment and the potential for passenger exposures. The simulated exposure 

concentrations were then used in a fault tree analysis (FTA) to estimate and broadly quantify the potential 

for passenger exposure and technician exposure in a range of scenarios. FTA is a method of combining 

the frequencies and/or probabilities of all factors contributing to a failure of a mechanical system to 

understand how, and with what frequency, the system fails. More detail on FTA is provided in Section 

1.6.1. 

 Taken together, the fault tree analysis and the CFD modeling indicated that a reasonable worstcase scenario leak for both R-152a and R-744 systems resulted in unacceptable exposures to vehicle 

occupants. For R-152a, U.S. EPA estimated 50 events per year where flammable concentrations of R152a might be present in the passenger compartment along with an ignition source. U.S. EPA also 

estimated 1,600 events per year where flammable concentrations of R-152a might be present in service 

areas along with an ignition source. For R-744, U.S. EPA estimated 4,300 passenger exposures per year 

above the health-based limit and 2,400 AC service technician exposures per year above the health-based 

limit.5

 The authors found that proper engineering to reduce the amount of refrigerant that can leak into 

the passenger compartment would reduce estimated refrigerant concentrations in passenger compartment 

air to acceptable levels. For example, a directed release system (squib valve) where the refrigerant charge 

is released outside the passenger compartment in the event of a large leak was modeled and shown to 

effectively reduce refrigerant concentrations in the air of the passenger compartment to below levels of 

concern for both R-744 and R-152a. Other risk mitigation strategies were also described. 

 incorporated into the system such that foreseeable leaks into the passenger compartment do not result in R-152a concentrations 

of 3.7% v/v or above in any part of the free space inside the passenger compartment for more than 15 seconds when the car 

ignition is on.” U.S. EPA also required that manufacturers adhere to the safety requirements specified in SAE Standards J639 

and SAE Standard J2773. 

5

 Note that the results of U.S. EPA’s analysis of R-152a and CO2 cannot be directly compared to the results of the CRP150 and 

CRP1234 analyses due to some differences in fault tree parameters. In addition, U.S. EPA assumed that all repair worker 

exposures would be above the health-based limit and did not conduct modeling to confirm that this was the case. Modeling for 

repair worker exposures in CRP150 and CRP1234 consistently indicated exposures were below health-based limits. 

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1.4 SAE Cooperative Research Program (CRP) 150 

 SAE International (formerly the Society of Automotive Engineers, SAE), in conjunction with the 

German Association of the Automotive Industry (VDA), requested that Gradient Corporation (Gradient) 

conduct a risk assessment of alternative refrigerants from several different chemical manufacturers (SAE 

CRP150). The two proposed refrigerants were Honeywell’s Fluid H and IneosFluor’s Auto AC-1. An 

assessment of potential risks associated with the use of R-12 as an automotive refrigerant was also 

included as a baseline evaluation. The risk assessment followed the approach of the U.S. EPA-sponsored 

risk assessment of R-152a and R-744 (Blackwell et al., 2006) so as to allow comparison with the risks 

from the other alternatives. In a number of cases, the SAE CRP150 risk assessment also considered 

additional scenarios or input values which were believed to provide a more robust or realistic estimate of 

passenger or worker risks. Because the risk assessment included proprietary information from a number 

of chemical manufacturers, the details of the SAE CRP150 risk assessment remain confidential to 

CRP150 members. Neither refrigerant is being proposed for use in automotive AC systems at the present 

time. 

1.5 SAE Cooperative Research Program 1234 (CRP1234) 

1.5.1 Phase I 

 In 2008, SAE International, in conjunction with E.I. DuPont de Nemours and Company (DuPont) 

and Honeywell International Inc. (Honeywell), undertook an additional risk assessment of a new 

automotive refrigerant, HFO-1234yf. The risk assessment generally followed the approach used in the 

U.S. EPA-sponsored risk assessment of R-152a and R-744 (Blackwell et al., 2006) and the assessment 

conducted for CRP150. The first phase of the risk assessment included a comparison of risks between 

HFO-1234yf and R-12 because information concerning R-12 was readily available. The Phase I HFO1234yf risk assessment was completed in February 2008 and concluded that HFO-1234yf posed only 

minimal risks (regarding health concerns and potential ignition) and was therefore an appropriate 

replacement for R-134a. The fault tree analysis estimated that the probability of exposure to HFO-1234yf 

above human health-based criteria would be 0.02 events per year across the whole vehicle fleet 

(attributable to exposures resulting from a leak associated with vehicle collision) and the risk of HFO1234yf ignition was estimated to be 0.004 events per year across the whole vehicle fleet (also attributed to 

the vehicle collision scenario). These event frequencies compared very favorably to the risks estimated 

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for R-12, which were 11 exposures above the human health-based criterion per year. The risks estimated 

for HFO-1234yf were also below the levels of risk inherent in many commonly accepted activities. The 

risk assessment noted that the vehicle collision scenario (the risk driver for HFO-1234yf) was based on 

rough extrapolation from the modeling results for evaporator corrosion leaks and was therefore uncertain. 

In addition, there were several yet-to-be-answered questions regarding HFO-1234yf flammability and the 

risk assessment concluded that additional testing should be conducted. 

1.5.2 Phase II 

A second phase of the CRP1234 risk assessment program was initiated after February 2008. As 

with the earlier risk assessment, the Phase II risk assessment followed the same approach as used in 

earlier risk assessments but incorporated more recent testing data as well as newer data on boundary 

conditions relevant to the fault tree analysis. Additional data on the collision scenario were obtained to 

provide a better estimate of risks associated with this event. The risk assessment also addressed concerns 

regarding potential HF generation and exposure which could result from ignition of HFO-1234yf during 

accidental release of refrigerant under various scenarios. In addition, the Phase II risk assessment 

included a parallel evaluation of carbon dioxide (CO2, R-744), the other alternative refrigerant currently 

under consideration for use in mobile air conditioning. To the extent possible, the R-744 assessment used 

the same boundary conditions as used for assessing HFO-1234yf. The Phase II assessment did not 

evaluate risks of R-12 since the comparison between HFO-1234yf and R-744 was the relevant 

consideration. The Phase II risk assessment report concluded that use of HFO-1234yf as an automotive 

AC system refrigerant posed acceptable levels of risk in terms of toxicity, ignition potential and potential 

HF exposure. For example, the risk assessment found that the risk of an HFO-1234yf-related ignition 

event would be on the order of 1 event per year or less across an entire vehicle fleet (considering the 

vehicle fleets in Europe, Japan and the U.S. separately). The risk assessment also estimated fewer than 

0.001 HF exposures above an appropriate health-based limit per year (equal to less than 1 such exposure 

every 1,000 years) across an entire vehicle fleet. Exposures of AC service technicians never exceeded 

health-based limits. With respect to R-744, the Phase II risk assessment found that the number of R-744 

exposure events above the health-based limits would be on the order of l event per year across an entire 

vehicle fleet. Exposures of service technicians were not found to exceed health-based limits, although 

details of R-744 servicing systems were noted to be uncertain. 

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1.5.3 SAE CRP1234 Phase III (Current Effort) 

To further refine the results of the Phase II risk assessment, the CRP1234 members undertook 

additional research to resolve several outstanding items that were noted in the Phase II report. These 

included: 

Harmonization of the CRP1234 risk assessment with independent investigations 

undertaken by a group of German OEMs who joined the CRP1234 effort. 

Additional work related to potential HFO-1234yf ignition sources conducted at both 

DuPont and Honeywell. 

Research to obtain additional data on parameters contributing to vehicle collisions such 

as collision frequency, impact location, and collision severity (in particular the potential 

to damage the AC system). 

Analyses of HFO-1234yf releases in an actual vehicle. This included measurement of 

refrigerant concentrations in passenger compartment and engine compartment air, 

attempted ignition of the refrigerant, and measurement of concentrations of HF produced 

by refrigerant ignition or thermal decomposition. Evaluations considered releases in both 

the passenger compartment and the engine compartment. The analyses were conducted 

by Hughes Associates, Inc., a firm specializing in fire scenario evaluations and HF 

analysis. 

Complementary studies of HF generation potential conducted in a specially constructed 

ignition chambers at the INERIS laboratories in France and the Honeywell laboratories in 

Buffalo, New York. These studies allowed for greater variability in testing conditions 

compared to the vehicle tests related to HFO-1234yf ignition or thermal decomposition. 

Additional modeling to address potential risks for certified AC service technicians due to 

localized concentrations of HFO-1234yf that would not be predicted by a uniform mixing 

model. 

Consideration of the likelihood of potential injuries for certified AC service technicians 

in the event of a rupture of a high-pressure recharge hose. 

As with the earlier Phase II assessment, this report also describes the results of an assessment of R-744 as 

an automotive refrigerant. The revised R-744 analysis incorporates all FTA input parameter changes that 

were developed in the revised assessment for HFO-1234yf. 

It should be noted that the Phase III risk assessment moved away from the region-specific (i.e., 

Europe, Japan, U.S.) estimation of risks used in Phase II. OEMs market vehicles in many different 

countries (including many beyond Europe, Japan and the U.S.) and individual OEMs are not confined in 

sales to a specific region. 

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Reports or data developed by other groups relied upon in the risk assessment included the 

following: 

1. Concentration measurements of HFO-1234yf after release into passenger vehicles (BMW, 2008; 

GM, 2008; Renault/Fiat/PSA, 2008; Toyota, 2009) 

2. Concentration measurements of R-744 after release into passenger vehicles (GM, 2008; Daimler 

2008) 

3. CFD Analysis of Potential Release of HFO-1234yf during Vehicle Repair Scenario (DuPont, 

October 27, 2009) 

4. Ignition of refrigerant oil mixtures on hot surfaces (INERIS, January 15, 2009), 

5. Combustion Tests of HFC-134a and HFO-1234yf Simulating Cabin Leaks in Mobile AC Systems 

(Honeywell, March 24, 2008) 

6. Analysis of Potential Ignition of HFO-1234yf by a Lit Cigarette During Accidental Refrigerant 

Leak (DuPont, July 6, 2009a) 

7. Analysis of Potential Ignition of HFO-1234yf from Short of 12-volt Battery (DuPont, August 29 

2009) 

8. Analysis of Potential Ignition of HFO-1234yf with Simulated Car Lighter (Glow Wire) During 

Accidental Refrigerant Leak (DuPont, July 6, 2009b) 

9. Analysis of Inhalation of HFC-134a and HFO-1234yf Through a Cigarette During Accidental 

Refrigerant Leak (DuPont, September 21, 2009) 

10. Combustion Pressure Rise of Selected Refrigerants (Honeywell, November 1, 2009) 

11. Refrigerant decomposition tests - Part I: Passenger car cabin tests (Hughes Associates Inc., 

August 19, 2009) 

12. Refrigerant decomposition tests – Part II: Passenger car engine compartment tests (Hughes 

Associates Inc., August 24, 2009) 

13. Data on HF generation from HFO-1234yf and R-134a (INERIS, December 15, 2009) 

14. An analysis of refrigerant leak volume and direction related to the location of an under hood hot 

surface (Honeywell, January 12, 2009), 

15. Data on failure rates for AC system evaporators collected by OEMs and various suppliers, 

16. Data on collision frequencies, locations, speeds and damages extracted from the GIDAS (German 

In-Depth Accident Study) database as well as other sources 

17. Concentration and flammability testing conducted on a full-scale vehicle as part of the Phase I 

analysis (Exponent, January 21, 2008), 

18. Data on R-134a concentrations measured in automotive assembly plants (Bureau Veritas, 2008), 

19. Estimate of refrigerant leakage rates for HFO-1234yf and R-744 with leaks of various diameters 

(Honeywell, October 22, 2008), 

20. Toxicology test data for HFO-1234yf provided by DuPont and Honeywell, 

21. Toxicology test data for CO2 obtained from published studies and governmental reports, 

22. Toxicology test data for HF obtained from published studies and governmental reports, 

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23. Estimates of certain fault tree boundary conditions and vehicle accident data obtained in 

consultation with Delphi Corporation and Japanese automotive manufacturers, 

24. Data on accident rates, vehicle operating patterns and vehicle ownership obtained from various 

U.S. government agencies, 

25. Risks of various types of accidents and hazards obtained from published studies and 

governmental reports. 

 The details of the risk assessment are described in the following sections of this report. It 

should be noted that the risk assessment incorporated various types of data including toxicity data, bench 

scale testing results (i.e., flammability metrics), modeled air concentrations, vehicle measurements (i.e., 

measurements of passenger and engine compartment concentrations, ignition tests) and vehicle operating 

parameters. These data were collected from various industry sources, using publicly available and 

confidential reports. Data not found in existing reports were generated by consulting groups and/or 

individual CRP1234 members. Toxicity data were reviewed by a Gradient toxicologist for completeness 

and to identify appropriate health-based limits for use in the risk assessment. All other data received from 

CRP1234 team members were assumed to be accurate and complete and were used as supplied in 

conducting the risk assessment. 

1.6 Risk Assessment Approach 

 As used in the context of this evaluation, risk is the likelihood or probability of an adverse event 

which, while not necessarily injurious, is nonetheless undesirable (e.g., failure of a mechanical 

component, placing an individual in a potentially dangerous situation).6

 Risks are evaluated and 

quantified through the process of risk assessment. Like all risk assessments, the risk assessment of a 

potential alternative refrigerant for R-134a is a multi-step process. An early step in the process is to 

consider the possible hazard scenarios under which an individual could be harmed. It is then necessary to 

gather data to support a quantitative estimation of the risk associated with that particular hazard. Often, a 

risk assessment is an iterative approach, where the data collected may either suggest some hazards need 

not be quantitatively evaluated or, alternatively, may suggest other hazards that were not previously 

considered. Once all of the potential hazard scenarios are identified and the necessary data collected, the 

data are brought together to develop a mathematical estimate of potential risk. 

 

6

 Note that in some contexts (for example under the guidelines of the German Technical Certification Institute) risk is defined as 

the combination of the probability of the event occurring and the degree of harm that is sustained if the event occurs. Because 

different jurisdictions require different methods for evaluating injury severity, the OEMs decided to take the alternative approach 

of estimating the probability of events that have potential to cause injury (e.g., exposure to an open flame) rather than estimating 

actual injury rates. 

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 The risk assessment procedure employed in this evaluation consisted of the following steps: 

1. A review of the possible hazards associated with each refrigerant (i.e., potential effects on 

human health or flammability) and development of the scenarios under which each 

hazard could result in harm to an individual. 

2. An assessment of potential refrigerant concentrations in air in the event of a refrigerant 

release in the passenger compartment, engine compartment, or vehicle service areas. 

3. A review of toxicity data available for each refrigerant. When studies were sponsored by 

the manufacturer to obtain new (i.e., unpublished) data, the study reports were reviewed 

to ensure the studies were conducted appropriately. In such cases, our review included 

evaluating the study design, the results, any deviations noted from the study protocol, and 

the conclusions drawn by the testing laboratory from the data. 

4. Determining the toxicological effects for potential refrigerant exposures, the levels at 

which these occur, and any human health criteria related to these effects, including any 

safety factors considered to be necessary. 

5. An evaluation of ecological toxicity data in the context of limits developed by regulatory 

agencies for classifying chemicals of ecological concern. 

6. Based on the available toxicity data, an evaluation of the absolute toxicity as well as the 

relative toxicity of the proposed refrigerant compared to R-134a, the refrigerant to be 

replaced. The toxicity of a candidate refrigerant relative to R-134a is expected to be a 

criterion used by U.S. EPA in their SNAP evaluation. 

7. An assessment of the flammability of the candidate refrigerant, including, if applicable, 

determining the upper and lower flammable limits, the minimum ignition energy, the 

autoignition temperature, and the fundamental burning velocity. 

8. For fluorine containing refrigerants, an assessment of hydrogen fluoride generating 

potential, both from refrigerant ignition (if applicable) and refrigerant thermal 

decomposition. 

9. Research on the probabilities and frequencies of events contributing to accidental releases 

of refrigerant. Where specific data were not available, consensus values were developed 

based on the expertise of OEM representatives familiar with automotive engineering and 

system failure mechanisms. Values chosen to characterize accidental refrigerant releases 

were made based on conservative assumptions so as to overstate rather than understate 

actual risks. The analysis of accidental releases also excluded the impact of mitigation 

strategies which may be developed by individual OEMs. 

10. Data from the previous nine steps were then combined to estimate specific probabilities 

relating to chemical exposure above health-based limits or exposure to an open flame, as 

applicable, through the use of fault tree analysis (FTA). The results were then considered 

in the context of other commonly encountered life events which may involve adverse 

effects. 

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1.6.1 Fault Tree Analysis (FTA) 

To quantify the potential adverse effects associated with a refrigerant release, we used FTA. FTA 

is a methodology that graphically presents the sequences and combinations of failures that lead to a 

particular outcome. It utilizes a "top-down" approach, starting with the undesired effect as the top event 

of a tree of logic. Each situation that could cause that effect is added to the tree as a series of logic 

expressions. This technique analyzes the potential combinations or sequences of events by which an 

undesired event may occur. 

FTA provides insight on failure sequences and serves as a guide for identifying system 

components, use patterns, or failure modes that are most likely to contribute to an undesirable outcome. 

Though fault trees can be labeled with quantitative failure probabilities, not all fault trees are quantified. 

When quantified, absolute accuracy is often secondary to the identification of failure sequences and the 

determination of the effectiveness of various mitigation measures, particularly in daily safety management 

activities (Blackwell et al., 2006). The overall goal of fault tree analysis is to assess an upper bound of 

adverse event frequencies . 

FTAs consist of various event "boxes", which reflect the probability or frequency of key events 

leading up to a system failure. The event boxes are linked by connectors ("gates") which describe how 

the contributing events may combine to produce the failure. Events may be combined in different ways: 

in cases where a series of events must all occur to produce an outcome (e.g., ignition source and sufficient 

oxygen to support combustion), the probabilities or frequencies of the individual contributing events are 

multiplied via an "AND" gate; in cases where only one of a series of events is needed to produce an 

outcome (e.g., a spark, a lighted cigarette, a hot surface all possibly leading to ignition), the probabilities 

are added via an "OR" gate.7

 More complex combinations are possible (e.g., conditional situations in 

which a series of contributing events must occur in a specific order to produce a failure) but these are not 

required by the present analysis. 

 Fault trees were constructed by CRP1234 Phase III members using the program FaultTree+ 

(Isograph Ltd, Irvine CA). Fault trees were developed iteratively in detailed collaboration with CRP1234 

representatives, particularly over the course of several meetings held during 2009. 

 

7

 In the special case when the inputs to an OR gate are probabilities of events that could occur simultaneously (e.g., driver 

fatigued/vehicle operated at night) and which can each by themselves cause the failure, the math is A+B-A*B. When the 

probabilities are mutually exclusive (vehicle stationary/vehicle moving) the math is A+B. 

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1.6.2 Interpretation of FTA Results 

The results of FTA alone are of little value unless they can be placed in proper context. Clearly, 

in an idealized world we would all opt for situations which present no risk whatsoever. Unfortunately, all 

facets of life involve some element of risk. Life involves the identification and selection of activities that 

present an acceptable level of risk based on the knowledge available about the risk involved. With this in 

mind, the risks associated with refrigerant exposure developed in the FTA analysis are compared to risks 

related to other aspects of vehicle use or the workplace that can be calculated from data reported in 

government or scientific publications. This allows the automotive manufacturers not only to weigh risks 

of one refrigerant against another, but also to consider the significance of individual refrigerant risks in an 

appropriate context. For most fault trees (those involving vehicle operation) calculations were made on 

the basis of events per hour of vehicle operation, as this is the most common metric used by OEMs for 

evaluating failure rates and is consistent with international performance standards. For fault trees related 

to vehicles not in operation, calculations were based on events per year. Finally, for fault trees related to 

vehicle service, calculations were based on events per service hour. Equivalent values for comparison 

events (e.g., risk of vehicle collision due to brake failure, risk of workplace injury) were obtained by 

dividing annual estimates by the number of hours an individual would typically engage in the activity in 

question. These comparison risks are shown in Tables ES-1, ES-2, 4-3 and 7-1. 

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2 HFO-1234yf 

2.1 General Properties 

HFO-1234yf is a new proprietary chemical being developed by the companies and has been 

subjected to an extensive battery of evaluations. HFO-1234yf is slightly flammable; the LFL is 6.2% 

(62,000 ppm) and the UFL is 12.3% (123,000 ppm) at 21°C and the minimum ignition energy is greater 

than 5,000 mJ (Monforte and Caretto, 2009, DuPont, 2007). Thus, in addition to toxicity, the potential 

ignition of HFO-1234yf is evaluated in the risk assessment. 

 The WEEL committee of the AIHA adopted a workplace environmental exposure limit (WEEL) 

value of 500 ppm for HFO-1234yf (AIHA, 2008), which is expected to be published in the 2009 set of 

WEEL values. Studies have indicated that HFO-1234yf is a non ozone depleting substance and that the 

GWP of the HFO-1234yf is 4 (Hurley et al., 2008; Nielsen et al., 2007). Thus, the chemical will satisfy 

the European Union F-gas directive. 

2.2 Toxicity Assessment for HFO-1234yf 

2.2.1 Evaluation of Available Toxicity Data from Manufacturer 

 DuPont and Honeywell have provided results for a large number of studies conducted on HFO1234yf. These consisted of the following laboratory reports: 

Mammalian Toxicity

4-hour inhalation range finding study in the mouse (Huntingdon Life Sciences, March 11, 2004) 

4-hour inhalation screening study with micronucleus assay in rats and mice (Huntingdon Life 

Sciences, March 30, 2004) 

14-day inhalation study in rats (TNO, November 4, 2005) 

Chromosomal aberration test in cultured human lymphocytes (TNO, November 8, 2005) 

Bacterial reverse mutation test (Ames Assay) (TNO, November 15, 2005) 

Micronucleus test in bone marrow cells of mice treated via inhalation (TNO, November 23, 2005) 

4-hour inhalation toxicity test in rats (TNO, January 17, 2006) 

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Toxicogenomic assessment of carcinogenic potential (Hamner Institutes for Health Sciences, May 

26, 2006) 

Cardiac sensitization study in dogs (WIL Research Laboratories, August 23, 2006) 

4-week (28-day) inhalation toxicity study in rats, including unscheduled DNA synthesis and 

micronucleus test results (TNO, September 5, 2006) 

Prenatal developmental inhalation toxicity study in rats (TNO, April 17, 2007) 

13-week (90-day) inhalation toxicity test in rats (TNO, May 11, 2007) 

Prenatal developmental inhalation toxicity study in rabbits (WIL Research Laboratories, 

summary provided by G. Jepson, February 16, 2009) 

Two generation reproductive toxicity test in rats (summary provided by Honeywell, October 9. 

2009) 

Ecological Toxicity

Preliminary experiment for developing methods to maintain test concentrations in aquatic toxicity 

tests (TNO, May 5, 2006) 

Freshwater algal growth inhibition test (TNO, May 8, 2006) 

Acute toxicity study in Daphnia magna (TNO, May 8, 2006) 

96-hour acute toxicity study in carp (TNO, June 15, 2006) 

At the time of this report, DuPont and Honeywell have completed a full range of toxicity testing on HFO1234yf. In lieu of a full 2-year carcinogenicity study, DuPont and Honeywell have conducted a 13-week 

inhalation exposure in rats and mice followed by a genomic carcinogenicity examination. Honeywell has 

reviewed the study design with both the U.S. EPA and with European (i.e., Belgian) regulatory agencies 

before initiating the toxicology program, and has not received any objections to the approach. The 

companies have indicated they would perform a 2-year carcinogenicity study if required by regulatory 

authorities; however, given the lack of activity in the mammalian mutagenicity assays, the lack of specific 

organ toxicity in the 13-week inhalation study, and the lack of a positive carcinogenic response in the 

genomic assays, the probability that HFO-1234yf would show a positive response is considered by the 

Honeywell Director of Toxicology to be low. 

 Toxicity testing was conducted primarily at TNO Laboratories in the Netherlands. TNO is one of 

the largest toxicology research laboratories and has a solid reputation. Some work (including the cardiac 

sensitization study of HFO-1234yf) was conducted at WIL Research Laboratories in the U.S. WIL is 

another well recognized toxicological testing laboratory with experience in cardiac sensitization testing 

(Chengelis, 1993; 1997). The final research laboratory used, the Hamner Institutes for Life Sciences, 

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conducted the genomics study. Hamner (formerly the Chemical Industry Institute of Toxicology) is also 

well regarded and specializes in cutting edge research. 

 For the toxicity studies identified above, Gradient reviewed the methodology, the data generated, 

and the conclusions drawn by the testing laboratory. Based on the information provided in the study 

reports, the studies appear to have been conducted appropriately. The cardiac sensitization study 

conducted for HFO-1234yf may have been somewhat different in design than tests previously published 

in the literature (note that there is no prescribed method for this type of study). The study used baseline 

epinephrine doses in some dogs that did not produce any arrythmiogenic effect ["the highest dose level 

tested during the pre-study evaluation that did not elicit significant ECG findings (e.g., PVC)"] rather than 

a minimally arrythmiogenic effect (the term used in previously published studies). In addition, 

Honeywell/WIL used the criterion of eleven or more premature ventricular contractions in 10 seconds to 

identify an adverse effect. This particular criterion has not been cited elsewhere. These study design 

differences would be expected to have fairly limited effects on the final outcome of the study. Given that 

the cardiac sensitization NOAEL of HFO-1234yf is 120,000 ppm (the highest dose tested), these study 

design differences should not be a barrier to adoption of the chemical. It should be noted that in 

discussions with Honeywell about the cardiac sensitization study, the company indicated that the study 

was conducted in the same manner as previous studies and that differences in terminology ("minimally 

arrythmiogenic" vs "no significant ECG findings") do not indicate differences in study design. 

2.2.2 Summary of Toxicological Findings 

 The available data concerning the toxicity of HFO-1234yf are summarized in Table 2-1. Overall 

the acute and chronic toxicity of HFO-1234yf appear to be low. ASHRAE has assigned a toxicity safety 

group classification of “A” to HFO-1234yf and an ATEL of 100,000 ppm for acute exposures less than 

30 minutes (ASHRAE, 2008). ISO 817 has not assigned safety classifications to any refrigerant because 

the ISO standard is still in draft status and is not expected to be published until 2010 (ISO, 2007). 

 According to the U.S. EPA SNAP regulations, the Agency will evaluate each new alternative 

refrigerant in terms of potential health risk relative to the current MAC refrigerant, R-134a. A 

comparison of key health indices for HFO-1234yf relative to R-134a is included in Table 2-2. Like R134a, HFO-1234yf displays low acute and chronic toxicity. To assess potential health risks of HFO1234yf exposure Gradient adapted the ANSI/ASHRAE RCL methodology for use in this risk assessment. 

Gradient therefore established health-based limits for HFO-1234yf exposures as follows: 

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For exposure scenarios involving potentially stressful stimuli we used the lower of (1) the 

cardiac sensitization NOAEL (120,000 ppm) as compared to a 5-minute TWA exposure 

concentration or (2) 100,000 ppm as compared to a 30-minute TWA exposure 

concentration. The latter was calculated as 50% of the lowest observed effect level 

(LOEL) for anesthetic effects in the 4-hour acute toxicity study in rats. 

For exposure scenarios not involving potentially stressful stimuli we used a value of 

100,000 ppm as compared to a 30-minute TWA, which was calculated as described 

above. 

2.3 Flammability Properties 

 The manufacturers of HFO-1234yf have conducted considerable research to understand its 

potential for combustion. These properties are summarized in Table 2-3, along with comparative data for 

some other common refrigerants. ASTM E-681-04 testing indicates that the flammable range of HFO1234yf (without PAG oil) lies between 6.2 and 12.3 percent (i.e., 62,000 ppm and 123,000 ppm). This 

range is narrower than the comparable ranges for R-152a and the LFL is substantially higher, indicating a 

higher accumulation of gas is required before ignition is possible. The minimum ignition energy of 

HFO-1234yf appears to be exceptionally high among refrigerants that would be assigned to ASHRAE 

Classification 2. Tests conducted at DuPont using an electrical spark as an ignition source showed that 

the minimum ignition energy for HFO-1234yf was between 5,000 mJ and 10,000 mJ (DuPont, 2008). 

Testing at INERIS also confirmed earlier work done by Chilworth Technology (U.K.) for Honeywell 

showing that the autoignition temperature of HFO-1234yf is 405°C using EC testing method A15. 

However, the autoignition temperature test conditions require that the refrigerant gas, the hot surface, and 

the surrounding air environment must be at the same temperature and the refrigerant and air must be 

uniformly mixed and quiescent at the stoichiometric refrigerant/air composition before ignition can occur. 

Such conditions are unlikely to be present in a real-world situation. INERIS (INERIS, January 15, 2009) 

found that the ignition temperature of pure HFO-1234yf when exposed to a hot body is in the order of 

1050o C. However, INERIS also carried out tests on mixtures of 1% to 7% PAG oil mixed with HFO1234yf, and achieved ignition with HFO-1234yf and oil mixtures with hot body surface temperatures at 

or above 750o C. As noted previously, INERIS could not ignite R-744 or R-134a when mixed with oil up 

to the temperature limit of their tests, 800o C. Separate tests run by Honeywell on R-1234 vapor and 

HFO-1234yf/oil aerosol streams sprayed on hot steel surfaces up to 800o C did not show an indication of 

ignition and behaved similarly to R-134a. Only when Honeywell ran these tests on hot surface 

temperatures >900o C did Honeywell observe ignition of both HFO-1234yf/oil and R-134a/oil mixtures. 

Other groups have observed ignition at somewhat lower temperatures, for example Hughes Associates 

observed a very brief period of HFO-1234yf ignition in one test at 700o C, indicating that the exact hot 

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body ignition temperature is dependent on test conditions. Finally, the heat of combustion and burning 

velocity of HFO-1234yf are both fairly low, particularly in comparison to R-152a. The low burning 

velocity suggests that if HFO-1234yf is ignited, the flame could be extinguished by wind or drafts 

moving at fairly minimal speeds. This also suggests HFO-1234yf has a weak potential to sustain a fire if 

the gas were to be ignited. 

 ASHRAE has assigned a flammability safety group classification to HFO-1234yf of “Class 2” 

(Lower Flammability) and has approved an LFL value of 6.2% v/v in air (at standard temperature and 

pressure) and a heat of combustion of 10.73 kJ/g, both determined using ASHRAE standard test methods. 

ISO 817 has not assigned safety classifications to any refrigerant to date, since the ISO Standard is still in 

the draft stage and will not be published until 2010. However, both ISO and ASHRAE are moving to 

establish a new flammability safety classification of 2L for low burning velocity refrigerants. When the 

revised standards are published, HFO-1234yf would be assigned a "2L" flammability classification, 

having a measured burning velocity less than 10 centimeters/second (i.e., HFO-1234yf has a burning 

velocity of 1.5 centimeters/second) that would indicate lower flammability risk than R-152a, which has a 

burning velocity of 23 cm/sec. A burning velocity of 10 centimeters/second is the International ISO DIS 

817 2008 Standard criteria for separating Class 2L refrigerants from Class 2 refrigerants, and is planned 

for adoption by ASHRAE in the future to harmonize the two standards. Flammability tests conducted at a 

number of laboratories and listed in Table 2-3 show that HFO-1234yf has significantly higher minimum 

ignition energy and a significantly lower burning velocity than other commercial “Class 2” “Lower 

Flammability” refrigerants. 

2.4 Consideration of Combustion Products 

 For fluorocarbon refrigerants, exposure to a sufficient energy source may lead to either refrigerant 

ignition/combustion or thermal decomposition. In such cases, refrigerants like R-134a and HFO-1234yf 

will produce hydrogen fluoride (HF), carbonyl fluoride (COF2) as well as other very short lives 

intermediates. The potential for production of fluorinated breakdown products from fluorinated 

refrigerants is a well known property that has been studied due to the use of these chemicals as flame 

suppression agents. In theory, one molecule of HFO-1234yf (tetrafluoropropene) would produce roughly 

the same amount of HF upon complete combustion as one molecule of R-134a (tetrafluoroethane) but the 

flammability of HFO-1234yf indicates it will ignite and possibly decompose more readily than R-134a. 

Initial testing conducted by Honeywell indicated that the concentrations of HF produced by combustion 

of HFO-1234yf were not significantly different from those produced by R-134a (Honeywell, March 24, 

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2008). However, these tests were carried out with HFO-1234yf concentrations of 4% or less and 

CRP1234 members agreed that higher HFO-1234yf concentrations and HF levels could potentially occur 

in the event of a large release which might be associated with events such as a serious collision or car fire. 

Evaluating potential HF exposures was therefore a major focus of the Phase III risk assessment and both 

bench scale tests (conducted primarily at INERIS) and field tests (conducted by Hughes Associates) were 

conducted to better understand this possible hazard. The results of these tests are described in Section 

3.0. 

2.5 Consideration of Hazards to be Quantitatively Addressed by FTA 

 A critical stage of the risk assessment is to identify those scenarios in which an individual could 

feasibly be harmed due to an exposure to the chemical of interest. For HFO-1234yf, three types of hazard 

were initially considered. First, in the event of an accidental refrigerant release individuals could 

potentially be exposed to refrigerant above a health based limit. Second, individuals could potentially be 

exposed to an open flame if there is a release, refrigerant concentrations are between the LFL and UFL, 

and an ignition source of sufficient energy is present at the same time and for a sufficient duration to 

transfer the requisite energy to the refrigerant. Third, individuals could potentially be exposed to 

hydrogen fluoride (HF) if the refrigerant is released and a sufficient energy source is present such that it 

can either ignite the refrigerant locally or cause thermal decomposition. Exposure to HF could potentially 

occur either by inhalation of HF gas or dermal contact with HF deposited on surfaces. Each of these 

potential hazards also involves other contributing factors (e.g., the presence of closed windows in the 

vehicle) which also must be considered in the analysis. 

2.5.1 Hazards to be Quantitatively Addressed via FTA 

 Considering the three types of hazards noted above, the CRP members then considered the 

various triggering events which could cause refrigerant to be released, the location of the release, and the 

specific type of individual (e.g., vehicle occupant, bystander, service worker) that might be involved. 

Based on available data and detailed discussions among the CRP members, each scenario was evaluated 

to determine whether evaluation via FTA was required or whether the scenario could be excluded from 

the FTA based on already existing knowledge about the refrigerant or vehicle operation. The results of 

this deliberation are shown in Table 2-4. The vehicle operation scenarios that are evaluated in the FTA 

can be summarized as follows: 

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A collision causing release of refrigerant into the passenger compartment with the vehicle 

occupants present (evaluated in Phase I and Phase II). This scenario was evaluated in the 

FTA for the potential for refrigerant ignition (i.e., exposure to an open flame) as well as 

potential for HF exposure. In the ignition case, the source of ignition energy was 

assumed to be an electrical short which occurs as a result of the collision or an open 

flame generated by passenger use of a match or butane lighter. Note that this scenario 

involves a break in the AC system that occurs within the passenger compartment (i.e., the 

portion associated with the evaporator). 

A collision causing release of refrigerant into the engine compartment and the occupant 

leaving the vehicle and looking under the hood for signs of damage. This scenario was 

evaluated in the FTA for the potential for refrigerant ignition (i.e., exposure to an open 

flame) as well as potential for HF exposure. 

A collision causing release of refrigerant into the engine compartment and a trapped 

occupant being exposed to HF which was generated in the engine compartment and then 

aspirated into the passenger compartment via the air intakes located just above the hood. 

A collision causing release of refrigerant into the engine compartment and a good 

Samaritan attempting to either rescue a trapped occupant or move the car and therefore 

being present immediately adjacent to the engine compartment.8

 This scenario was 

evaluated in the FTA for HF exposure. It was not evaluated for refrigerant ignition since 

the good Samaritan would not need to enter the engine compartment while attempting to 

free the trapped occupant. 

An AC system leak causing refrigerant to be released into the engine compartment and 

then aspirated into the passenger compartment via the air intakes located just above the 

hood. This scenario was evaluated in the FTA for potential HF exposure. 

A scenario involving a vehicle fire due to failure of a non-AC component which, while 

not originating in the AC system, eventually compromises the AC system and causes 

refrigerant to be combusted. This could involve exposure of occupants traveling in a 

vehicle (very minor fire) or an occupant who stops the vehicle to investigate the fire. 

These scenarios are evaluated only in terms of potential for HF exposure because data 

collected in CRP1234 Phase I and II indicate that HFO-1234yf would not contribute 

insignificantly to an existing vehicle fire due to its low burning velocity and low flame 

temperature. 

A similar scenario involving a fire started due to vandalism which the owner of the 

vehicle then attempts to extinguish. Again, only potential HF exposure is evaluated in 

the FTA. 

A description of the boundary conditions (i.e., operating conditions, environmental conditions, individual 

actions) evaluated in the vehicle scenarios is provided in Table 2-5. 

 

8

 Note that the good Samaritan is distinguished in this risk assessment from a bystander by being actively involved with and in 

very close proximity to the vehicle. Implicit in this understanding of the term is the idea of someone who consciously chooses

not to move away from the vehicle to avoid the strong irritant effect of HF. Good Samaritans also include volunteer firemen 

who may arrive at the scene of an accident before protective equipment arrives. 

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 Regarding HF exposure hazards, the CRP1234 members agreed that HF could be generated either 

due to HFO-1234yf ignition or thermal decomposition (i.e., decomposition due to exposure to an energy 

source not sufficient to cause ignition). While thermal decomposition will generate less HF than 

refrigerant ignition, the conditions associated with thermal decomposition may occur more frequently. 

However, HF generation via thermal decomposition is based upon hot body cross-sectional area, and 

temperature and duration of contact with hot body. The CRP1234 members agreed that a hot body (such 

as an exhaust manifold) could potentially cause significant thermal decomposition of HFO-1234yf. The 

members also agreed that the limited cross-sectional area and duration of electrical shorts and sparks 

would not cause thermal decomposition of HFO-1234yf. Hot bodies (i.e., those with temperatures above 

100oC) are not a relevant source of HF generation in the passenger compartment, because the generated 

amount of HF due to the small surface of the existing components will not exceed the HBL. The CRP 

members therefore agreed that thermal decomposition was only a possible concern for HFO-1234yf 

releases in the engine compartment. 

 In addition to the vehicle operation scenarios, the CRP1234 members also decided to evaluate 

potential risks for certified AC service technicians. Uniform mixing models used as part of the CRP1234 

Phase I and Phase II efforts indicated that HFO-1234yf concentrations in service areas would not exceed 

health-based limits for HFO-1234yf nor would they exceed the LFL. CFD modeling conducted to predict 

localized concentrations of HFO-1234yf in a service shop leak indicated the LFL would be exceeded only 

in the immediate vicinity of the leak, beyond a few cm from the hole the concentration decreases 

significantly. Although certified AC service technicians are unlikely to perform activities like welding or 

grinding in the engine compartment (where other more flammable materials are present) this unlikely case 

was evaluated in the FTA. HF exposure was not evaluated for certified AC service technicians because 

unlike vehicle passengers, service technicians will be knowledgeable about the potential for HF 

generation and will immediately move away from the area when they perceive the irritancy of HF prior to 

being exposed above a health-based limit. The FTA also considered repair workers who could be subject 

to physical injury from a rupture of a high pressure service line. This risk will be evaluated for both 

HFO-1234yf and R-744 even though HFO-1234yf has operating pressures similar to R-134a. A 

description of the boundary conditions (i.e., operating conditions, environmental conditions, individual 

actions) evaluated in the vehicle scenarios is provided in Table 2-5. 

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2.5.2 Hazards Which Will Not Be Quantitatively Addressed via FTA 

 The earlier CRP1234 Phase I and Phase II risk assessments included evaluation of small, medium 

and large corrosion leaks of the evaporator and resulting leaks into the passenger compartment. The 

findings from those earlier assessments indicated that only the large (e.g., 0.5 mm) leak might release 

enough refrigerant to achieve concentrations exceeding the LFL in the passenger compartment (Gradient, 

2009). After further consideration, the CRP1234 members concluded that these scenarios were 

unrealistic. In reality, corrosion leaks start out quite small and grow gradually over time. By the time an 

evaporator hole reaches a "large" size, the refrigerant charge will have been already lost and the HFO1234yf concentration in the passenger compartment will not exceed the LFL or any relevant health-based 

limit. It is also the case that leak testing methods that will be specified under SAE standard J2843 for use 

during AC system service would be able to detect leaks greater than 0.1 mm and such a leak would be 

repaired prior to recharging the system. Refrigerant releases due to corrosion-type leaks of the evaporator 

were therefore not included in the FTA for Phase III except for the special case of a slow CO2 leak while 

an individual is sleeping in the vehicle (see Section 5.0 for a discussion of CO2 scenarios). 

 The CRP1234 Phase I and Phase II risk assessments also indicated that HFO-1234yf exposure 

concentrations never exceeded health-based limits (i.e., those related to anesthetic effects, frank toxicity, 

cardiac sensitization or oxygen deprivation) in any of the vehicle concentration measurement studies. 

Therefore, while evaluation of potential toxicity will be discussed qualitatively in this Phase III report, 

health effects due to exposure to the refrigerant were not evaluated in the FTA. 

 

 Risks for DIYers are also not being addressed via FTA in the Phase III risk assessment because 

the new vehicle service standards (J639, J2845, and J2911) will require that servicing be performed by 

certified AC service technicians. Warning labels will be included on the AC unit indicating that only 

service by certified technicians is acceptable. By designing the standards this way it is the expressed 

intention of the OEMs to prevent AC service to the vehicle by anyone but a properly training technician 

from servicing the AC system as this is already required for other vehicle systems. Therefore it is not 

appropriate to evaluate the risks of individuals who intentionally disregard the manufacturer's warnings 

and instructions. 

 A number of other scenarios were considered by the CRP1234 team members but were excluded 

from the FTA based on logical arguments demonstrating they were not credible hazards. These scenarios 

were: 

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Bystanders who might be exposed to an open flame or HF if they are present near a 

stationary vehicle when a refrigerant release occurs and if the refrigerant is ignited and 

HF is generated. CRP members decided not to include this scenario in the FTA because 

(1) the extent of an open flame due to HFO-1234yf ignition would be too limited to affect 

anyone standing along the side of the vehicle, and (2) a bystander would move away 

from the vehicle due to the irritating nature of HF before they could possibly be exposed 

to concentrations above the HBL. 

Emergency responders (e.g., firemen, ambulance workers) might respond in the event of 

a vehicle fire or collision and could potentially be exposed to HF. Due to the low 

burning velocity of HFO-1234yf, ignition of the refrigerant will not contribute 

substantially to a pre-existing fire. Professional first responders also have training in 

chemical hazards and possess appropriate gear which will prevent them from receiving 

HF exposures above health-based limits. 

Because HFO-1234yf can be ignited, some CRP members questioned whether an ignition 

event could generate sufficient pressure to result in physical injuries. As described in 

Section 3, testing conducted by Honeywell to investigate the pressure rise associated with 

HFO-1234yf indicated a negligible amount of pressure is generated when this refrigerant 

is ignited, consistent with its low burning velocity. Risk of pressure-related injuries was 

there not addressed in the FTA. 

 

Some CRP members questioned whether an AC system release occurring in a tunnel 

could pose an increase HF hazard due to the enclosed nature of such structures. 

However, tunnels and similar structures (e.g., underground parking garages) are designed 

with ventilation systems to remove carbon monoxide and other combustion byproducts 

and such systems should be capable of rapidly diluting the HF generated from the release 

of refrigerant in a single vehicle to below health-based limits. In the case of a vehicle fire 

in a tunnel, including a fire involving multiple vehicles, the HF (which is lighter than air) 

would rise towards the roof of the tunnel (where ventilation systems are located) and not 

pose a risk for persons present in the tunnel. A more important hazard would be heavierthan-air combustion by-products (carbon dioxide, particulates, plastic combustion 

products) which may be toxic or contribute to asphyxiation. Production of these heavierthan-air combustion by-products during a vehicle fire would not be significantly affected 

by the presence of HFO-1234yf, which would contribute only minimally to a fire. 

CRP1234 members also considered whether dermal contact with HF which has deposited 

on wet surfaces could pose a potential risk to individuals who might come into contact 

with vehicle surfaces subsequent to a refrigerant release and ignition that produces HF. 

A review of the toxicology literature and a discussion among CRP1234 member 

toxicologists indicated that health effects due to dermal exposure to HF occur only at 

solution concentrations in the single percentage range (i.e., 10,000 ppm) or higher. Such 

high concentrations would not be expected due to a short accidental release, as 

subsequently confirmed in vehicle testing (refer to Section 3.3.4). 

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 Other non-vehicle specific risks that were qualitatively evaluated in the Phase I and Phase II 

assessments (ecological risks, risks to assembly plant workers, etc.) without the use of FTA were 

similarly evaluated in the Phase III assessment. 

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Table 2-1. Summary of Available Toxicity Data for HFO-1234yf 

Endpoint HFO-1234yf

Acute (LC50) (ppm) >405,800 (TNO, 2006a) 

Anesthetic effects (ppm) 201,600 (TNO, 2006a) 

Cardiac sens. No effect level (ppm) 120,000 (WIL, 2006) 

Worker exposure limit (ppm) 500 (AIHA WEEL value) 

28-day NOAEL (ppm) 50,000 (TNO, 2006f) 

90-day NOAEL (ppm) 50,000 (TNO, 2007b) 

Developmental toxicity NOAEL (ppm) 50,000 (TNO, 2007a) [rat study] 

4,000 (Jepson, 2009) [rabbit study] 

Genotoxicity Negative (Huntingdon, 2004;2005b,c,d; 2006f) 

Carcinogenicity Screen Negative (Hamner, 2006) 

Aquatic toxicity (mg/L) Acceptable (TNO 2006c,d,e) 

Health Based Limit (ppm) 120,000 (based on card. sens. NOAEL) 

100,000 (based on anesthesia) 

ODP 0 (Nielsen et al., 2007) 

GWP 4 (Nielsen et al., 2007) 

Table 2-2. Comparison of Toxicity of HFO-1234yf to the Toxicity of R-134a 

Endpoint HFO-1234yf R-134a

Acute (LC50) (ppm) >405,800 >500,000 

Anesthetic effects (ppm) 201,600 81,000 

Cardiac sensitization no effect level (ppm) 120,000 50,000 

Worker exposure limit (ppm) 500 (AIHA WEEL value) 1,000 (PEL) 

1,000 (MAK) 

28-day NOAEC (ppm) 50,000 50,000 

90-day NOAEL (ppm) 50,000 50,000 

Developmental toxicity NOAEL (ppm) 50,000 [rat study] 

4,000 [rabbit study] 

10,000 [rat study#] 

40,000 [rabbit study##] 

Genotoxicity Negative Negative 

Carcinogenicity Screen Negative Negative 

Aquatic toxicity (mg/L) >83 (Daphnia) 450 

Health-Based Limit (ppm) 

120,000 (based on cardiac sensitization 

NOAEL) 

100,000 (based on anesthesia) 

50,000 (cardiac sensitization) 

141,000 (acute toxicity) 

64,800 (anesthetic effects) 

ODP 0 0 

GWP 4 1,430* 

Shading highlights values indicating greater toxicity/environmental hazard compared to R-134a. 

*Based on a 100-year time horizon and relative to the global warming potential of CO2. IPCC, 2007 # Hodge et al., 1980 ## Collins et al, 1995 

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Table 2-3. Flammability of HFO-1234yf Relative to Other Refrigerants 

Property HFO-1234yf R-12 R-134a R-152a R-32 

LFL (% volume in air) 6.2 NA NA 3.9 14.4 

UFL (% volume in air) 12.3 NA NA 16.9 29.3 

Minimum Ignition Energy (mJ) 5,000-10,000a NA NA 0.38 30-100 

Autoignition temperature (oC) 405 >750c 743e 455f 648g

Heat of combustion (kJ/g) 10.7 -0.8d 4.24 NA NA 

Burning velocity (cm/s) 1.5 NA NA 23 6.7 

Safety Classification A2Lb A1 A1 A2 A2 

NA – Property not relevant or not available for this material. 

Source: Monforte et al., 2009 unless otherwise indicated. a DuPont, 2008

b Anticipated ISO safety classification based on known flammability properties. c

 DuPont MSDS for R-12 

d U.S. EPA, 2007. http://www.epa.gov/ozone/snap/refrigerants/safety.html 

e Underwriters Laboratories, 1995 

f IPCS, 2005

g Air Liquide MSDS for R-32

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Table 2-4. HFO-1234yf Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Triggering Event

Location of 

Refrigerant 

Release 

Location of 

Potential Exposure

Exposed 

Individual

Type of Hazard Fault 

Tree 

ID 

Refrigerant 

exposure 

(toxicity) 

Exposure to 

open flame 

(ignition) 

HF exposure 

High Pressure 

Equipment 

Injury 

Evaporator leaks due to corrosion 

Passenger compartment 

Passenger compartment Vehicle occupant 

No evaluation 

via FTA. Leak is 

too slow, 

concentrations 

will not exceed 

HBL. 

No evaluation via 

FTA. Leak is too 

slow, 

concentrations will 

not exceed LFL.

No evaluation 

via FTA. Leak is 

too slow to 

generate HF in 

significant 

amounts. 

NA NA 

Collision Passenger compartment Passenger compartment Vehicle occupant 

No evaluation 

via FTA. 

Concentrations 

will not exceed 

HBL. 

Evaluate via FTA Evaluate via FTA(1) NA I3, HF3 

Collision Engine compartment Engine compartment 

Former occupant investigating engine or good Samaritan assisting trapped occupants 

No evaluation 

via FTA. 

Concentrations 

will not exceed 

HBL. 

Evaluate via FTA Evaluate via FTA(2) NA I5, HF4, HF5 

Collision Engine compartment 

Passenger compartment (aspiration from damaged engine compartment via air 

intakes) 

Vehicle occupant 

No evaluation 

via FTA. 

Concentrations 

will not exceed 

HBL. 

No evaluation via 

FTA. Dilution with 

outside air during 

intake would mean 

LFL would not be 

reached. 

Evaluate via

FTA(2) NA HF4.2 

AC system leak (non-collision) 

Engine compartment Engine compartment Former occupant investigating engine 

No evaluation 

via FTA. 

Concentrations 

will not exceed 

HBL. 

Evaluate via FTA Evaluate via FTA(2) NA I4, 8.2 

AC system leak (non-collision) 

Engine compartment 

Passenger compartment (via aspiration) 

Vehicle occupant 

No evaluation 

via FTA. 

Concentrations 

will not exceed 

HBL. 

No evaluation via

FTA. Dilution with 

outside air during 

intake would mean 

LFL would not be 

reached. 

Evaluate via

FTA(2) NA HF8.1 

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Table 2-4. HFO-1234yf Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Triggering Event

Location of 

Refrigerant 

Release 

Location of 

Potential Exposure

Exposed 

Individual

Type of Hazard Fault 

Tree 

ID 

Refrigerant 

exposure 

(toxicity) 

Exposure to 

open flame 

(ignition) 

HF exposure 

High Pressure 

Equipment 

Injury 

AC system leak (non-collision) 

Engine compartment 

Adjacent to engine compartment Bystander 

No evaluation 

via FTA. 

Concentrations 

outside of 

engine 

compartment 

would not 

exceed HBL. 

No evaluation via 

FTA. 

Concentrations 

outside of engine 

bay would not 

exceed LFL. 

No evaluation 

via FTA. 

Concentrations 

outside of engine 

bay would not 

exceed healthbased limit. 

Bystander will move away at 

low HF 

concentrations 

due to irritancy. 

NA NA 

Vehicle fire due to failure of non-AC components later involves refrigerant 

Engine compartment 

Passenger compartment 

Occupant driving vehicle with minor fire, aspiration of HF into vehicle. 

NA 

No evaluation via 

FTA. Low burning 

velocity of HFO1234yf indicates 

minimal 

contribution to fire. 

Evaluate via

FTA(1) NA HF7.1 

Vehicle fire due to failure of non-AC components later involves refrigerant 

Engine compartment Engine compartment 

Former occupant who investigates in the engine compartment 

NA 

No evaluation via 

FTA. Low burning 

velocity of HFO1234yf indicates 

minimal 

contribution to fire. 

Evaluate via

FTA(1) NA HF7.2 

Vehicle fire due to vandalism later involves refrigerant 

Engine compartment Engine compartment Owner NA 

No evaluation via 

FTA. Low burning 

velocity of HFO1234yf indicates 

minimal 

contribution to fire. 

Evaluate via

FTA(1) NA HF6 

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Table 2-4. HFO-1234yf Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Triggering Event

Location of 

Refrigerant 

Release 

Location of 

Potential Exposure

Exposed 

Individual

Type of Hazard Fault 

Tree 

ID 

Refrigerant 

exposure 

(toxicity) 

Exposure to 

open flame 

(ignition) 

HF exposure 

High Pressure 

Equipment 

Injury 

Vehicle fire related to collision, equipment failure or vandalism 

Engine compartment Engine compartment First Responder NA 

No evaluation via 

FTA. Low burning 

velocity of HFO1234yf indicates 

minimal 

contribution to fire.

No evaluation 

via FTA. HF 

will have 

dispersed by 

the time first 

responders 

arrive.

NA NA 

Certified service technician 

Engine compartment Service shop Worker 

No evaluation 

via FTA. 

Concentrations 

in service areas 

will not exceed 

HBL. 

Evaluate via FTA

No evaluation 

via FTA. 

Workers will 

move away from 

location to avoid 

irritation caused 

by HF. 

Evaluate via

FTA I6 

Do-It-Yourselfer (DIY) 

Engine compartment Home garage Owner No evaluation via FTA. New SAE equipment service standards will mandate service by certified individuals only. NA 

NA – Hazard is not relevant to the triggering event/exposure scenario described. 1 – Generation f HF via refrigerant ignition only. 2 – Generation of HF via refrigerant ignition or thermal decomposition. 

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Table 2-5 Boundary Conditions Considered in Vehicle Operation/Fire Fault Trees

Condition Consideration Significance 

Vehicle Blower Mode Is vehicle operating in recirculation mode or outside air 

mode? 

Blower operating mode is highly relevant to the FTA. For example, if only the blower is in outside air mode (OSA) can material be drawn into the vehicle from the engine compartment. The preferred operation mode of the HVAC system is OSA. Operation in recirculation (recirc) mode occurs primarily at very high ambient temperatures or when driving in areas with very poor air quality (e.g., smog). 

Vehicle Blower Speed Is the vehicle blower on or off? If the vehicle blower is operating in OSA during a release in the passenger cabin, the 

refrigerant will be diluted. If the vehicle blower is operating in OSA mode during a release in the engine compartment, HF may be aspirated 

Vehicle Windows Are vehicle windows broken/open or closed? The status of the vehicle windows has an important effect on the concentration of refrigerant 

or HF in the passenger cabin. Windows can be opened due to a conscious decision by vehicle occupants or may brake due to collision-related deformation or intrusion. 

Collision Speed Is collision speed high (> 50 km/hr), medium (16-50 km/hr) 

or low (<16 km/hr)? 

Collisions at higher speeds will be more likely to cause refrigerant leaks but also damage other components. Occupants' behavior will be different in collisions of different speeds. The collision speed categories used are those for which crash test data are available: A low impact auto accident is generally defined as an incident that takes place at speeds less than 16 km/h or 10 mph, in such cases the deformation is slightly visible ("fender bender"). Collisions that take place at speeds above 50 km/h (30 MPH) represent crashes that have a significant potential to cause fatalities. The automotive industry has data for 30 mph crash tests. 

Ignition sources Are there shorts in vehicle cabin of sufficient energy, are 

the sufficiently hot surfaces in the engine compartment? 

Under what conditions are these produced? 

As noted in Section 3, only a very limited number of ignition energy sources can ignite R1234yf. The conditions under which these would be present are critical to the FTA. 

Location/Timing of concentrations 

Is the refrigerant or HF present at a sufficient concentration where the ignition source or occupant is also present? Are they co-located at the same time? 

Relevant location and timing are necessary aspects of the FTA. 

Shielding Is shielding around hot surface absent/insufficient? Many OEMs place shielding around the exhaust manifold which may affect the probability of 

refrigerant ignition. 

Dynamic airflow Does airflow disperses refrigerant released in engine 

compartment? 

If the vehicle is moving or the engine fan is running, refrigerant released into the engine compartment may be dispersed to the extent that it never achieves concentrations that can be ignited or produce significant amounts of HF. 

Human behavior/capacity 

Do occupants leave the vehicle? Do they investigate the engine compartment? Do they take action to avoid HF exposure? Does a good Samaritan become involved? 

Because human exposures (either to an open flame or HF above a health-based limit) are the top events in the FTA, human actions are a critical element of the analysis. 

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Table 2-5 Boundary Conditions Considered in Vehicle Operation/Fire Fault Trees

Condition Consideration Significance 

Component integrity How will AC system components respond in collisions of 

different velocities? 

Evaporators are embedded in the HVAC in the centre of the vehicle. At low speed the vehicle integrity is fully maintained in this area of the vehicle. So fare no evaporator rupture is considered at low speed. AC components in the engine compartment break due intrusion or relative movement of the component. The refrigerant leaks in the engine compartment. In frontal crashes the condenser is damaged and leaking with a probability that increases with collision speed. In side crashes the extent of deformation of the engine compartment is much less but remains speed dependent. Side crashes therefore have a lower likelihood of loss of integrity of the AC system components. 

Accident scenarios What happens if a leakage occurs in the evaporator or 

other system component locations? Where does the refrigerant go and how much refrigerant is involved? 

In case of an AC component failure parallel in the passenger or the engine compartment there will be a split of the refrigerant flows. The fraction of refrigerant mass released in either direction is dependent on AC operation conditions, on full load or part load and system operating pressures, and the opening cross-section of the ruptures. 

Vehicle hood integrity In a collision, does the hood buckle in such a way as to 

create a favorable flowpath from the engine compartment to the air intakes for the passenger cabin? 

OEMs design vehicles to prevent aspiration of engine compartment gases into the passenger compartment. It is only when this design is compromised that an aspiration hazard would be present. 

Air flow path What is the path of air flow from the external environment 

into the passenger compartment? 

In outside air mode the air flows between the base of the windshield and the engine hood through a wide mesh grid in the air collector downstream of the air intake. The air collector is a closed part system (component) or is built as part of the frame with the engine hood which is sealed against the engine compartment. The hood seals might become weakened due to wear or mechanical deformation in the case of an accident. 

Dilution of HF How might wind and rain contribute to dispersal of HF 

generated in the engine compartment before it is aspirated into the passenger cabin? 

In one type of air collector (the closed system) the HF can be diluted by wind or can come into contact with rain drops and the HF can be dispersed in the water and drained. In the other type of air collector system (the open system) the hood seals might be damaged and by-passed in the case of an accident. In such a case, the air is taken directly from the engine compartment, is much less influenced by wind and is not influenced by rain. 

HF deposition on evaporator surface 

 Does HF condense on the wet evaporator? HF could be dispersed in the fine droplets of water present on the surface of the evaporator 

due to condensation. This would serve to attenuate HF aspiration into the passenger compartment from the engine compartment. This requires that the AC system had been operating prior to the release. 

Passenger cabin size Does refrigerant or HF concentration in the passenger 

cabin depend on differences in cabin size? 

A larger cabin size would provide greater potential for dilution. OEMs collected measurements from different types of vehicles to gain a sense of the impact of cabin size on this dilution potential. 

Other non-AC related events 

Vehicle fires due to non-AC component failure or vandalism 

Such fires do have the potential to damage the AC system and release and ignite the refrigerant. However, not all fires will progress to the point of doing so (e.g., rear wheel well fires). 

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Table 2-6. Boundary Conditions Considered in Vehicle AC Service Fault Trees 

Condition Consideration Significance 

Condition of service Is the work carried out by certified professionals? 

New SAE standards will require that all AC service be conducted by certified professionals who will have training concerning the flammability and HF generation potential of R-1234yf. 

Equipment leak Does a refrigerant leak occur from service equipment while in operation? OEMs considered a number of factors, including the design of current AC service equipment (coupler attachments, pressure gauges and warning devices, hose integrity) in addressing this probability. Shop ventilation Is ventilation inadequate? Facility ventilation should be sufficient to disperse refrigerant such that it does not reach or exceed the LFL. Presence of sufficient ignition sources 

Are sufficient ignition sources present in area where refrigerant is released and above LFL? 

Modeling conducted by DuPont (Section 3) demonstrated that in the event of minimal ventilation, the region where the refrigerant would exceed the LFL would be very small. The OEMs considered the likelihood that certified service technicians would introduce high energy activities (grinding, welding, lighting a match) into this area. 

 Working time per service How long is a typical AC service event? An average vehicle service at the AC system take 1 hour Person exposed to a flame What probability occurs for a flame exposure? 

In case of a leakage and refrigerant ignition the hazard to become injured is low, due to the fact that the refrigerant is heavier than air, ignition sources are not normally located low in the workspace, and the refrigerant is difficult to ignite in the presence of even low wind velocity. 

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3 HFO-1234yf Data Collection 

3.1 Refrigerant Concentration Data 

3.1.1 Concentration Measurements – Passenger Compartment 

Potential HFO-1234yf concentrations associated with a leak of refrigerant into the passenger 

compartment were measured in actual vehicles by several OEMs. Testing involved different vehicle 

types with different cabin volumes, refrigerant charges, and vehicle geometries. All tests involved 

placing a number of sensors within the vehicle, releasing the refrigerant into the passenger compartment 

at a set rate and evaluating the potential effects of different blower speeds and air distribution modes on 

the measured HFO-1234yf concentration measured at different locations in the vehicle. Sensor placement 

included the occupants’ breathing zones, the occupants’ lap area, and the vehicle floor. Data from various 

manufacturers are summarized in Table 3-1. The table shows results for releases associated with what 

was previously described as a large leak (0.5 mm diameter, 1.14 g/s) as well as data for a very large leak 

that might be associated with evaporator tube rupture in the event of a severe collision (6.35 mm 

diameter, 12 .4 g/s). The table shows data collected at locations relevant to the scenarios considered in 

the risk assessment; some OEMs collected additional data which are not shown to facilitate comparison 

across vehicles. The results confirm that concentrations in the occupants’ breathing zone never exceed 

the health-based limits for HFO-1234yf even in the event of a very large leak. With respect to possible 

refrigerant ignition, the data indicate that if the vehicle blower is off or set on low, the HFO-1234yf 

concentration in air could exceed 6.2% (the LFL as measured by ASHRAE test methods) near the vehicle 

floor in the event of the very large leak. One test from also indicated an HFO-1234yf concentration of 

approximately 6.6% in the driver’s lap area in the event of a very large leak and a low blower speed 

directed at the vehicle floor. Data from two OEMs also indicated concentrations exceeding 6.6% could 

be reached with an 0.5 mm leak in the foot and lap area in some cases. This was not found to be the case 

in the measurements made another OEM. The data suggest that refrigerant concentrations in such cases 

would only be found within a few centimeters of the vehicle floor, a location where it might be very 

difficult to situate an ignition source. As noted in Section 2.4, a sudden release of full charge from a 0.5 

mm diameter hole in the evaporator is not considered a credible scenario in the risk assessment because 

the refrigerant charge will have already been lost as the hole increased in size. 

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The results of vehicle testing confirm that collision-related leaks of refrigerant into the passenger 

compartment could result in concentrations of HFO-1234yf that could exceed 6.2% in certain locations 

and possibly present a hazard, assuming other conditions necessary for refrigerant ignition and/or HF 

generation are also present at the same time. 

3.1.2 Modeled HFO-1234yf Concentrations in the Passenger Compartment 

 As part of CRP1234 Phase II, modeling was conducted to evaluate potential HFO-1234yf 

concentrations in the passenger compartment. This data was used in lieu of actual concentration 

measurements which were not available at the time. Although actual measurement data are now 

available, it is useful to consider the modeling results here to determine whether they agreed with the 

results of the later measurement studies. 

Modeling for the passenger exposure consisted of a two step process: estimation of the mass of 

refrigerant released through the hole/orifice over time followed by dispersion modeling to estimate 

refrigerant concentrations in the passenger compartment. As reported by Honeywell, the emission leak 

rates were determined using a system bench arrangement of vehicle components built around an HVAC 

system extracted from a midsize European model automobile. Leak rates were measured from a variety 

of operating and soak ambient conditions for an 0.5 mm sharp orifice representing a maximum sized leak 

and a 1⁄4” ball valve representing a severed line. The maximum leak rate measured from a 0.5 mm 

diameter corrosion leak was 1.14 g/sec at an I50 running condition (50oC condenser inlet air, 35oC 

evaporator inlet air, 900 RPM). The maximum line severe leak rate was found to be 12.4 g/s for a soak 

condition of 45oC engine compartment and 50oC passenger compartment. Note that these leak rates 

correspond to the leak rates used by the OEMs in obtaining measurements of HFO-1234yf in passenger 

compartments. These leak rates were then used as the input to the perfect mixing analysis. Details on the 

leak rate determinations are included in Attachment D. 

 Dispersion modeling for HFO-1234yf and R-744 was conducted by CRP1234, using a virtual 

passenger compartment size of 2.5 m3

 and assuming that refrigerant was uniformly mixed in the 

passenger compartment. Because pooling of refrigerants that are heavier than air near the vehicle floor 

may present a particular risk as regards ignition, CRP1234 also estimated the HFO-1234yf concentrations 

when two thirds of the refrigerant charge was distributed into the lower one third of the passenger 

compartment. 

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The modeling assumed approximately 1.7 air changes per hour (ACH) which approximates the 

vehicle static with no blower operating. This ACH value is slightly different from the values used in 

earlier work (e.g., Blackwell et al., 2006 used a value of 1.8 ACH, the CFD modeling by Honeywell 

conducted as part of the Phase I risk assessment used an ACH value of 2.5, and modeling by DuPont for 

Phase I used an ACH value of 1.8). A recently published study by Ott and colleagues (Ott et al., 2008) 

indicated that air exchange rates in stationary vehicles with the ventilation off can range widely, from 1.0 

to 4.9. The ACH is strongly affected by vehicle speed and thus wind speed along the outside of the 

vehicle. Most of the studies evaluated by Ott et al. do not make mention of wind speed during testing. If 

the studies evaluated by Ott et al. were conducted in an enclosed space, they may have underestimated the 

typical ACH for a vehicle on the road. The chosen value of 1.7 is 35% of the maximum value reported by 

Ott et al. and therefore represents a conservative but not extreme selection. Results for the uniform 

mixing modeling are provided in Attachment E. 

 The results of the modeling effort are shown in Table 3-2. The results support the findings of the 

concentration measurement studies conducted by the OEMs. The maximum concentration predicted for 

the very large (6.35 mm) leak associated with the collision scenario was approximately 7.5%. The 

maximum concentration predicted for a 0.5 mm leak was 6.5% (therefore above the ASHRAE LFL value 

of 6.2%). These modeling results are consistent with the findings in the measurement studies if one takes 

into account that concentration measurements are highly dependent on vehicle geometries and test 

conditions. 

3.1.3 Concentration Measurements – Engine Compartment 

As part of CRP1234 Phase I, concentrations of HFO-1234yf resulting from a release in the engine 

compartment were assessed by Exponent. The leak involved a 14.2 to 14.6 g/s refrigerant leak rate with a 

total charge amount of 690-708 grams. The refrigerant was released into the engine space through a 1⁄4” 

nylon tube. After the complete release of the charge measurements of the refrigerant concentration were 

recorded at locations in the engine compartment near the exhaust manifold, the anticipated location of 

highest temperature. Exponent conducted 3 different tests where the HFO-1234yf concentration was 

measured. In one test, concentrations never exceeded the LFL, in another they just exceeded the LFL, 

while in a third test one sensor reported a peak concentration of 87,000 ppm detected near the exhaust 

manifold. Although this concentration exceeded the LFL attempts to ignite the refrigerant in this test 

were unsuccessful. However it was later noted that Exponent used a hot surface with a maximum 

temperature of approximately 600°C, which later testing at INERIS showed may not have been sufficient 

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to fully test ignition potential of pure HFO-1234yf. Greater detail on the field testing can be found in the 

Exponent report HFO-1234yf Refrigerant Concentrations and Ignition Tests in Full-Scale Vehicle 

Passenger Cabin and Engine Compartment (Exponent, 2008). 

3.1.4 Modeled HFO-1234yf Concentrations in Vehicle Service Areas 

3.1.4.1 Uniform Mixing Model 

 For the certified AC service technician situation CRP1234 members considered that service 

technicians could be at risk in the event of an equipment leak during refrigerant removal and recharging. 

Leaks in the service equipment hoses could allow for a leak of refrigerant of various magnitudes, 

depending on the size of the hole in the hoses. This scenario requires that the service technician fails to 

monitor the equipment or note that the equipment is malfunctioning. This would be possible if the 

service technician were to hook up the equipment and then leave to perform other tasks while the system 

was being evacuated and/or recharged. If system monitoring were neglected for a sufficient duration, the 

entire draining and recharging cycle could be involved in a release. For this scenario, we assumed that 

15% of the transferred refrigerant would be released, theorizing that a higher release rate would produce a 

sound that would be noticed by service personnel. 

 For the certified AC service technician scenario air modeling was conducted using a simple 

perfect mixing model (also called a box model). The box modeling approach is commonly used to 

estimate concentrations of a released or leaking chemical in an air space. It assumes the air space is in the 

shape of a box and that the gases inside the box are homogeneously mixed with air. Air concentrations 

were calculated iteratively assuming a constant emission rate over the time of the release (i.e., the total 

amount of refrigerant involved in the release is divided by the time of the release). The following formula 

was used in the calculations: 

  C AER t

V

ER t Ct C    

	 




   
 * *

*

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Where 

Ct = the concentration at time point of interest (g/m3

) 

C = the air concentration of the chemical at the proceeding time step (g/m3

) 

ER = the emission or release rate of the chemical (g/minutes) 

V = the volume of the room or airspace of interest (m3

) 

AER = the air exchange rate for the room or space of interest (minutes-1) 

t = the time step interval (minutes) 

Although this model is limited in its ability to account for complex air dispersion patterns, the approach is 

reasonable for small spaces such as those likely to be encountered in vehicle service. Furthermore, unlike 

the passenger compartment of an automobile, where an individual is confined to one position, individuals 

conducting service are likely to move about within the "box" and thus experience exposures that are 

closer to an average chemical concentration. Modeling was not conducted to address accumulation of 

refrigerant in a service cavity or pit because these areas would be considered confined spaces and by law 

are required to have sufficient ventilation before personnel can enter them. These requirements are 

implemented to prevent accumulation and exposure to other gases (e.g., carbon monoxide) that are more 

likely to be present in a service location than an AC refrigerant, which will only be present as a result of 

an accidental release. 

 Using professional service equipment (e.g., Snap-on Dual II, Kool Kare, and ECO-12), the 

refrigerant in the AC system is first removed and stored in the service unit reservoir. After any necessary 

service and subsequent leak check, the system is then recharged to ensure that a full refrigerant charge is 

present. Data from various equipment manufactures indicate that it takes approximately 15 minutes to 

remove the refrigerant from the system and 10 minutes or less to recharge the system with fresh 

refrigerant. 

 The certified AC service technician exposure scenario is assumed to occur in a small service 

station with only two service bays, where the doors and windows are closed. This results in a relatively 

small air space with limited ventilation. A review of property sales records suggested that a typical 

volume for the service area of a small station is approximately 1,000 square feet (approximately 93 m2

). 

Assuming an interior area height of 9.5 feet (approximately 2.9 m), the total work area volume would be 

270 m3

. If the estimated volumes of two vehicles (16 m3) and miscellaneous equipment (8 m3) are 

deducted, this leaves an open air volume of approximately 230 m3

. We assumed an air exchange rate of 1 

room volume per hour (0.017/minute), which is a low-end estimate for a commercial facility (U.S. EPA, 

2002). It should be noted that service shops have good ventilation by design and are often open to 

outside air during much of the year. Finally, we assumed that only one vehicle is having the AC system 

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serviced at a time and that the time between different vehicles being serviced is such that accumulation of 

refrigerant does not occur. 

 Two other theoretically possible AC service scenarios were not evaluated in the risk assessment 

because they were judged to be implausible. One of these would involve a release which could occur if 

an improper seal were made between the vehicle and the service equipment, allowing the refrigerant 

charge to escape. It was assumed that all service units will be equipped with auto shut-off 

adapters/couplers which prevent release of refrigerant during connection/disconnection and also ensure a 

locked seal to the vehicle AC system. Thus we judged this scenario to be implausible. In addition, this 

scenario is unlikely to result in greater exposure than the system rupture scenario described above. The 

other implausible scenario involved release of residual refrigerant from the 30-pound storage canisters 

once they were changed out of the charging equipment. Results from the Disposable Container Heel 

Testing Study Report [Heel Study] (Perrin Quarles, 2007) showed that only minimal residual refrigerant 

remained in the 30 pound canisters after charging. Further, due to the design of recharge systems, a 

release from the canister itself is unlikely. Therefore, this type of release was not included in the risk 

assessment. 

 It should be noted that visits to facilities where AC systems were being evaluated and discussions 

with industry professionals indicated that refrigerant exposures to certified AC service technicians are 

likely to be very rare. Most service shops must be well ventilated to be permitted and, as noted above, 

AC service equipment is designed to provide a tight seal between the service unit and the vehicle AC 

system. The scenarios evaluated should therefore be considered worst-case and unlikely to occur in the 

vast majority of service locations. 

 The result of the air modeling simulations for the AC service worker scenarios is summarized in 

Table 3-3. A graph, indicating the predicted air concentration over time predicted for this exposure 

scenario is included as Attachment F. 

3.1.4.2 CFD Modeling of Local HFO-1234yf Concentrations 

 To evaluate the possibility of localized areas of refrigerant concentration above the LFL in 

vehicle service areas, DuPont used CFD modeling work to simulate a potential release of HFO-1234yf 

for three potential AC service scenarios. The goal was to determine the refrigerant concentration, 

particularly the LFL region, at different distances from the origin of the leak. The first scenario simulated 

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a release of HFO-1234yf from a ruptured service line in an unbounded or open space, intended to be 

simulate the case of car service done outside. A second scenario simulated a leak in a bounded space, 

similar in size and shape to a one-car service garage. The third and final scenario simulated a leak in a 

bounded space replicating a one-car garage, but added the element of an impingement surface, similar to 

what might be expected from a leak in the engine compartment which is directed at the raised vehicle 

hood. 

 CFD modeling of releases in bounded areas was based upon work done by Venetsanos et al.

(2007). Initial modeling indicated that both pure vapor and 2-phase liquid-vapor models gave essentially 

similar results. Subsequent CFD modeling therefore used a single phase release to simulate the 

refrigerant concentration profile. Assumptions used for all three scenarios included a release of a full 

refrigerant charge of 600 grams and a leak rate of 12.4 g/s. The air exchange rate in the two bounded 

(i.e., inside garage) scenarios was assumed to be zero and the wind speed for the unbounded (i.e., outside) 

scenario was assumed to be low and on the order of 0.5 m/sec. 

 For the unbounded leak the resulting HFO-1234yf plume extended approximately 0.35m (~1 ft) 

from the leak hole. The portion of the plume with refrigerant concentrations above the LFL extended 

only 15 cm from the leak hole. For the bounded leak the resulting HFO-1234yf plume extended 

approximately 1.0 m (~3 ft) from the leak hole. The portion where the refrigerant was above the LFL 

consisted of the first 10 cm in the lateral direction and the first 2 cm (+/-1 cm) in vertical direction. The 

small size of the flammable portion of the plume results from the entrainment of air into the plume by the 

refrigerant as it is rapidly released. This dilutes the HFO-1234yf further from the leak source to 

concentrations below the LFL. 

 For the bounded leak impinging on a surface, the resulting refrigerant plume development and 

shape is affected by the surface distance from leak source. When the leak source is within 0.1 m from the 

impingement surface, the resulting HFO-1234yf plume is continually dispersed. Therefore, the 

flammable zone is quite limited, extending approximately 2-4 cm from the leak point. When the leak 

source is further away from the impingement surface, 0.22 m or less, quick initial plume dispersion 

occurs and the resulting HFO-1234yf plume feeds back within itself with almost no flammable zone 

developing, (~ 2cm around leak point). When the leak source is within 0.85 m from the impingement 

surface, a plume does form, but there is less extension compared to the non-impingement scenario 

described above. The portion of the plume where the HFO-1234yf is above the LFL is less than the first 

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10 cm extending outward. It should also be remembered that the actual results will depend on the 

size/shape of the hole and direction of the release versus air currents. 

 The results of the CFD modeling indicate a very limited ignition hazard posed by HFO-1234yf in 

AC service scenarios. AC service technicians would need to place a sufficient ignition source (e.g., 

welding arcs, parts grinding machinery) within centimeters of the origin of the leak in order to ignite the 

refrigerant. CRP members judged this to be highly unlikely given that certified service technicians will 

be aware of the flammability of not only HFO-1234yf but other more flammable fluids present in the 

engine compartment. This low likelihood of HFO-1234yf ignition during vehicle service was considered 

in developing inputs for the vehicle service portions of the FTA. 

 Results of the CFD modeling are described in more detail in Attachment G. 

3.2 Flammability Testing 

3.2.1 Identification of Credible Ignition Sources 

A key step in evaluating potential ignition hazards is the determination whether potential ignition 

sources are present. If no ignition source is present which can produce sufficient energy (either via open 

flames, hot surfaces or electrical sparks) then there is no ignition hazard. Furthermore, it is necessary that 

the ignition source be present in the same location as that where the refrigerant occurs in the flammable 

range, and at the same time as the flammable concentration occurs. To evaluate the potential ignition 

sources for the exposure scenarios of interest, SAE CRP1234 relied on the expert opinion of team 

members and outside experts who developed a potential list of ignition sources and reviewed these within 

context of usage parameters. For example, the figure below identifying potential electrical ignition 

sources in the passenger compartment was developed by INERIS under contract to Renault/PSA/Fiat and 

shared with the CRP1234 team (INERIS, June 20, 2008). More detail concerning this work is described 

in Monforte and Caretto (2009) which is included as Attachment H. 

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The results of CRP1234's own deliberations are provided in Attachment I. The members of CRP1234 

concluded that the most credible ignition sources capable of igniting an automotive refrigerant were (1) 

butane lighters or matches that might be used to light cigarettes or cigars, (2) butane lighters or matches 

that might be used to look for lost objects near the vehicle floor, (3) sparks generated by the short of a 12-

volt vehicle battery located on the floor of the passenger compartment9

, and (4) sparks generated by a 

short of a PTC heater located within the dashboard. These were believed to be the worst-case ignition 

sources (as described below subsequent laboratory testing showed that two of these sources, PTC heaters 

and 12-volt batteries, were not sufficient to ignite the material) (INERIS, June 20, 2008; 

DuPont/Honeywell, October 21, 2008). Based on previous reports, a number of other ignition sources 

(e.g., lit cigarettes/cigars, electrical cigarette lighters, vehicle clocks and radios) do not have the potential 

to ignite HFO-1234yf. (INERIS, June 20, 2008) 

3.2.2 Bench Scale Ignition Testing 

Work conducted by INERIS (INERIS, January 25, 2009) further evaluated the potential of 

refrigerants (including refrigerant/oil mixtures to be ignited by a hot surface). INERIS first conducted a 

review to identify the key “process” parameters involved in refrigerant ignition (including consideration 

 

9

 CRP1234 members decided to not consider 300-volt batteries such as might be found in a hybrid vehicle as a possible ignition 

source in this risk assessment. The design of hybrid vehicles and their battery systems is currently highly variable among OEMs 

so it would be impossible to evaluate these systems in a risk assessment intended to represent conditions across the general 

vehicle fleet. 

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of refrigerant/PAG oil mixtures). The review suggested that hot surfaces within the engine compartment 

would be the most likely causes of refrigerant/oil ignition. To evaluate this further, INERIS constructed a 

test apparatus that simulated the shape and internal geometry of a typical engine compartment. The hot 

surface was simulated with a 10 cm diameter, 50 cm long electrical heated rod. Both the temperature of 

air inside the apparatus as well as the temperature of the heating rod were externally controlled. After 

first demonstrating the effectiveness of the apparatus with propane, INERIS then tested various 

refrigerant-oil mixtures of HFO-1234yf-oil, R-134a-oil, and R-744-oil. The oil was a typical 

polyalkylene glycol (PAG) oil used in AC systems and the oil percentages studied (1-7%) were in the 

range of normal vehicle design. INERIS found that the presence of the oil was a critical component of the 

ignition scenario. PAG oil itself was ignited by the hot body at a temperature of 400°C. HFO-1234yf 

alone was not ignited until hot body temperatures exceeded 1000°C. When HFO-1234yf was combined 

with oil, the ignition of the oil is suppressed but at a sufficient temperature (approximately 750°C) the oil 

mist, in contact with the hot surface, ignites and then subsequently triggers ignition of the refrigerant. 

Neither the R-744-oil mixture nor the R-134a-oil mixture could be ignited in these studies (using 

temperatures up 800°C). The finding that a HFO-1234yf-oil mixture could be ignited at 750°C was used 

by the OEMs in establishing probabilities for the fault trees evaluating the under hood fire scenarios. A 

copy of the INERIS report on refrigerant ignition testing is included as Attachment J. 

 

Dupont also conducted a number of studies looking at the potential for different ignition sources 

to ignite HFO-1234yf. These included a study in which an 8.1% mixture of HFO-1234yf in air was 

drawn for approximately 20 seconds through a lit cigarette in a 8.5 liter glass tube. No refrigerant 

ignition was observed. DuPont conducted another study along similar lines using a 17 cm, 20 gauge, 

coiled nichrome wire as the ignition source. The wire was intended to simulate the ignition potential of a 

vehicle cigarette lighter. Other than the ignition source, the experimental setup was similar to that used 

with the lit cigarette. Current was applied to the wire until it was "white hot" at which time an 8.1% 

refrigerant-air mixture was passed over the wire. No visible indication of refrigerant ignition was 

observed. Further details regarding these studies can be found in Attachment K. 

DuPont also evaluated the potential for vehicle batteries to produce sparks that could ignite HFO1234yf. DuPont tested 8.13, 8.5 and 9% mixtures of HFO-1234yf in air in a 12 liter spherical flask at 20 

and 60°C and 50% relative humidity. The tests involved discharging a 12-V automotive battery across 

two 9.5 mm diameter copper electrodes as they were moved apart within the sphere. The electrodes were 

connected to the battery using standard automotive battery cables. HFO-1234yf did not ignite during any 

of the spark tests. Additional detail regarding this study can be found in Attachment L. 

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Honeywell also conducted a study to examine the pressure rise generated by ignition of HFO1234yf and other present or previous refrigerants. The results of this study were relevant to the CRP's 

assessment of whether HFO-1234yf posed an explosion hazard in the event of ignition. To conduct the 

study Honeywell established refrigerant atmospheres of know concentration inside a 28.3 liter Lexan®

box which was in turn situated in a much larger 230 liter box equipped with a pressure relief bladder. 

Small 9.5 mm holes were made in the top of the smaller box and each was covered with a small 

polyethylene ball ("ping pong ball") weighing 2.7 grams. Changes in air pressure within the smaller box 

forced air out of the small holes, causing displacement of the ping pong balls. Honeywell ignited the 

refrigerant gases in the box using different ignition sources and observed the effects using high speed and 

video cameras. 

Honeywell carried out the tests with HFO-1234yf and three other gases with notably different 

ignition energies and burning velocities: R-152a, isobutane, and ammonia. Each gas was used at a 

concentration between the LFL and UFL and close to the ideal ratio for combustion with oxygen (i.e., 

stoichiometric volume). Ignition of atmospheres of 4% isobutane and 7.5% R-152a, achieved using an 

approximately 1000 mJ electric spark source, resulted in an audible pressure rise and rapid displacement 

of the balls off the surface of the smaller box, causing them to hit the top of the larger box and then 

rebound to the box floor. Ignition of an atmosphere of 18% ammonia, which was achieved with the same 

spark source but only after repeated attempts, resulted in a much less dramatic event with the ping pong 

balls remaining stationary on the box (indicating a minimal pressure rise). Honeywell observed it was not 

possible to ignite HFO-1234yf with the spark source used for the other gases which was not surprising 

given the high minimum ignition energy of HFO-1234yf (reported to be greater than 5000 mJ as 

discussed in Section 2). Ignition of HFO-1234yf was also not achieved using a standard butane lighter. 

Only when butane from the lighter was ignited with the spark source was HFO-1234yf ignition achieved. 

In this case the pressure rise was unable to dislodge the balls from the top of the box. Based on these 

results, the CRP members concluded that HFO-1234yf would not present an explosion hazard in the event 

refrigerant from an AC system release is ignited. Further details concerning the pressure rise tests 

conducted at Honeywell are provided in Attachment M. 

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3.2.3 Passenger Compartment Ignition Testing 

3.2.3.1 Exponent 

To verify that refrigerant could be released and ignited in the passenger compartment, a number 

of actual vehicle tests were conducted. Testing was conducted by Exponent (2008) using a large size 

vehicle. This vehicle (3.1 m3

 interior volume, 2.3 m3 with passengers included) was selected to be 

consistent with that used in the U.S. EPA analysis of R-152a and R-744 (Blackwell et al., 2006). A full 

system charge was released into the vehicle simulating a large diameter evaporator leak (0.5 mm) and 

refrigerant concentrations were determined at different locations in the vehicle interior (driver and front 

seat passenger breathing zones, driver and passenger lap areas, driver and passenger foot areas, AC vents, 

and inside the AC ductwork). The vehicle fan was operated at low speed, using an outside source of 

electric power. Exponent tested only refrigerant and not a refrigerant-oil mixture (Exponent, 2008). 

Exponent attempted to ignite the refrigerant using a butane lighter but the test was unsuccessful. More 

detail regarding the field testing is provided in the Exponent report HFO-1234yf Refrigerant 

Concentrations and Ignition Tests in Full-Scale Vehicle Passenger Cabin and Engine Compartment

(Exponent, 2008). 

 

3.2.3.2 Hughes Associates Inc. 

More recently, CRP1234 members contracted with Hughes Associates to perform additional 

testing. Although the focus of the testing by Hughes was related to HF generation (as discussed below), 

the work also is informative for HFO-1234yf ignition. A typical refrigerant charge was released into a 

standard midsize vehicle with vents sealed to maximize refrigerant accumulation in the passenger 

compartment. Once concentrations had reached a maximum, Hughes attempted to light the refrigerant 

using a remote controlled butane lighter. Hughes found that a standard butane lighter was unable to be lit 

at refrigerant concentrations greater than 1.7%, suggesting the refrigerant could not be successfully lit by 

a cigarette lighter at concentrations at or above the LFL. HFO-1234yf may act as a fire suppression agent 

in this context. Hughes was only able to light the lighter in the presence of higher HFO-1234yf 

concentrations (up to 5%) if a spark ignitor was used to augment the lighter, which supplied an additional 

92 watts of energy to ignite the butane. This was not considered a realistic scenario for vehicle use. This 

finding supports the idea that refrigerant ignition in the passenger compartment will only occur if a lighter 

is lit in a region of low refrigerant concentration and then moved to a region of high concentration. 

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A copy of the Hughes Associates passenger compartment testing report is included as Attachment 

N. 

3.2.4 Engine Compartment Testing 

As part of CRP Phase I, part of the testing conducted by Exponent with an actual vehicle included an 

evaluation of HFO-1234yf ignition in the engine compartment. As described previously, the ignition 

source used by Exponent in this work was a heating element (hot body temperature, approximately 

600oC) and the engine compartment was open to the surrounding atmosphere although the entire vehicle 

was stationed in a closed shed. The leak involved a 14.2 to 14.6 g/s refrigerant leak rate with a total 

charge amount of 690-708 grams. Attempts to ignite the refrigerant in this test were unsuccessful even 

though the HFO-1234yf concentration exceeded the LFL. Greater detail on the field testing can be found 

in the Exponent report HFO-1234yf Refrigerant Concentrations and Ignition Tests in Full-Scale Vehicle 

Passenger Cabin and Engine Compartment (Exponent, 2008). 

Subsequent work conducted by INERIS (INERIS, January 15, 2009) suggested that the test 

conditions used by Exponent, specifically the use of refrigerant alone (as opposed to a refrigerant/oil 

mixture) and a hot surface with a temperature of approximately 600°C, were not sufficient to fully 

evaluate HFO-1234yf ignition. As noted previously, the temperature at which HFO-1234yf alone was 

found to ignite was over 1000°C and thus the ignition source used by Exponent could not have ignited the 

refrigerant. 

Based on the work at INERIS, for the purposes of the risk assessment CRP1234 members 

assumed that a hot body surface of approximately 750°C (as might be found in engines under extreme 

conditions) would be sufficient to ignite HFO-1234yf when mixed with PAG oil at the typical 

percentages. Some CRP1234 members were uncertain as to whether the INERIS finding was 

representative of all worst-case situations and whether the refrigerant-oil mixture might ignite at lower 

temperatures. It was decided to explore this issue via a sensitivity analysis in the FTA. 

A copy of the Hughes Associates engine compartment testing report is included as Attachment 

O. 

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3. 3 HF generation potential 

3.3.1 Work Conducted by Honeywell 

To evaluate potential HF exposures, Honeywell established atmospheres containing 0.1%, 2% 

and 4% refrigerant (R-134a and HFO-1234yf) through a 230-liter Lexan® chamber and then introduced 

two ignition sources into the chamber: a lighted match (burning time, 6 seconds) and a butane lighter 

(burning time, 5 seconds). The air inside the chamber was subsequently withdrawn and passed through 2 

water baths to trap fluoride breakdown products. Total fluoride, measured by ion chromatography, was 

used as an indication of the mass of hydrogen fluoride that was produced during refrigerant ignition. The 

mass of HF was then mathematically distributed into the volume of the passenger compartment to 

estimate a potential HF exposure concentration for passengers (assuming uniform mixing). For 

refrigerant concentrations in air up from 0.1% to 3%, Honeywell reported that the relationship between 

refrigerant concentration in air and the amount of HF generated was equivalent to that from R-134a 

(Honeywell, January 22, 2009). At HFO-1234yf concentrations higher than 3%, other testing has shown 

that an externally assisted electrical ignition source had to be used in combination with a butane lighter to 

combust or ignite HFO-1234yf, or else a butane lighter or match had to be lit outside the zone of the leak 

and then brought into the zone of higher refrigerant concentration. The maximum concentration of HF 

produced at the 4% concentration level would equate to 45 ppm if distributed in a 2.3 m3

 vehicle cabin 

(vs 15 ppm for the same concentration atmosphere of R-134a). A report summarizing the Honeywell 

work is included as Attachment P. 

3.3.2 Work Conducted by DuPont 

 DuPont studied the potential for an individual to be exposed to HF if fluorocarbon refrigerant (in 

this case either R-134a or HFO-1234yf) were inhaled through a lit cigarette. The temperature of the 

burning region of a cigarette may reach 700°C during inhalation by the smoker which would be sufficient 

to cause refrigerant ignition and/or thermal decomposition. To conduct their test, DuPont used a 

Massachusetts smoking machine, a standard device used by government regulators to evaluate 

individuals' exposures to chemical substances from inhaled cigarette smoke. The smoking machine 

simulates inhalation through a cigarette ("puffs") using draw pressures similar to that of an actual smoker. 

Air was drawn through a lit cigarette located inside a glass chamber into which a set concentration of 

refrigerant had been released. Each test involved nine 2 second puffs (it was assumed that this was the 

longest a smoker might tolerate the irritancy of any HF that would be produced by the cigarette). After 

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passing through the burning cigarette, the air stream was passed through a trap filled with distilled water 

which captured HF and other water soluble fluorine compounds. The total fluoride concentration in the 

sampling solution was then measured with an ion selective electrode and mathematically related back to 

the HF concentration in air drawn through the sampling fluid. 

 DuPont conducted the tests with refrigerant in air concentrations of 0.1% and 2 % for each 

refrigerant. For R-134a, tests conducted with a refrigerant in air concentration of 0.1% resulted in an 

average HF concentration of 5.8 ppm (standard deviation, 1.8) in air drawn through the cigarette. 

Increasing the R-134a concentration to 2% did not lead to a notably different HF concentration result 

(mean HF concentration 3.8 ppm, standard deviation 0.8 ppm). DuPont hypothesized that the limiting 

factor in HF generation is the time the refrigerant is in contact with the small "hot zone" of the burning 

cigarette. Even at the 0.1% refrigerant concentration only a small fraction of the available refrigerant 

fluorine is converted to HF. Thus increasing refrigerant concentration does not lead to more 

combustion/thermal decomposition. 

 DuPont obtained similar results for HFO-1234yf. At a concentration of 0.1% HFO-1234yf in air, 

the average HF concentration in the air drawn through the lit cigarette was 5.2 ppm (standard deviation, 

1.1 ppm). As with the case of R-134a, increasing the refrigerant concentration to 2% did not notably 

effect the HF concentration produced by the cigarette (average, 8.0 ppm; standard deviation, 1.3 ppm). 

 Further details concerning the DuPont cigarette smoking study are provided in Attachment Q. 

3.3.3 Work Conducted by INERIS 

CRP1234 members also engaged INERIS to study the potential of HFO-1234yf and R-134a to 

generate HF when subjected to thermal decomposition. INERIS constructed a new experimental 

apparatus based on modifications to the design used to test refrigerant ignition (described in Section 

3.3.2) in order to study HF generation and quantify HF concentrations in the test apparatus. For example, 

INERIS subjected different refrigerant-air mixtures to two different heat sources (one with an area of 

1,400 cm2

 and the other an area of 35 cm2) in an effort to better understand the theory behind HF 

generation. INERIS used sampling pumps to extract the air from the test chamber after refrigerant 

thermal decomposition and passed the air through a serial pair of water bubblers to extract the water 

soluble fluorine compounds. The resulting solutions were analyzed for total fluoride content via ion 

selective electrode (ISE) and then confirmed with ion chromatography. It is important to note that the 

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work done by INERIS used artificial test conditions intended to control experimental variables; the work 

was not intended to replicate field situations (see discussion of Hughes Associates tests below). 

The key results of the INERIS work are shown in Table 3-4. The INERIS findings concerning 

the role of various test conditions of HF formation may be summarized as follows: 

Effect of PAG oil. The amount of HF generated when PAG oil was present in the 

released refrigerant (at 4% by weight) was several times greater than the amount of HF 

generated by refrigerant alone. 

Hot body temperature - The amount of HF generated through thermal decomposition of 

refrigerant was highly dependent on the temperature of the hot body. HF concentrations 

declined by somewhat more than 50% when hot surface temperatures decreased from 

700°C to 630°C. When hot surface temperatures were lowered to 550°C, HF 

concentrations were less than 3% of the concentration observed at 700°C. 

Hot body surface area - When the area of the hot surface was decreased from 1400 cm2

 to 

35 cm2

, the concentration of HF in the vessel decreased by 40-fold. 

Leak geometry - Tests also suggested that when refrigerant was sprayed at the chamber 

ceiling near the hot surface rather than directly onto the heated rod, the amount of HF 

was decreased by 15 to 35 percent. 

Tests conducted with R-134a showed about 25-40% of the HF concentration observed 

with HFO-1234yf under similar test conditions. 

A copy of the INERIS report for this work is provided as Attachment R. 

3.3.4 Work Conducted by Hughes Associates Inc. 

3.3.4.1 Passenger Compartment 

 To evaluate HF production due to release of HFO-1234yf in the passenger compartment, Hughes 

introduced HFO-1234yf via the AC ducts into the passenger compartment of a Cadillac CTS (interior 

volume 2.77 m3

) and exposed the refrigerant to the flame of a butane lighter (supplemented in some 

instances with a spark igniter apparatus). While refrigerant was being released to the passenger 

compartment, the pressure relief valves were sealed and the AC blower was operated to ensure a well 

mixed refrigerant concentration and to attain the maximum concentration possible near the driver's 

breathing zone. However, even with an exceptionally high charge mass (1000 grams), the maximum 

HFO-1234yf concentration achievable in air near the driver's breathing zone was 5%, possibly because 

the refrigerant is heavier than air and sinks towards the floor despite operation of the fan. Hughes also 

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experienced difficulty igniting the butane lighter when the refrigerant leak occurred rapidly and 

conducted some tests with a slower rate of discharge. 

A series of air sampling pumps were positioned near the driver's breathing zone which collected 

air samples for various time intervals after ignition of the lighter (ranging from 3 to 30 minutes). The air 

from the passenger compartment was drawn through collection fluid which was later analyzed for total 

dissolved fluoride ion concentration using a fluoride ion selective electrode. The analytical results were 

therefore time-weighted average air concentrations covering time frames ranging from 3 to 30 minutes. 

Because this analytical method reports total dissolved fluoride ion, the result includes not only HF but 

also other fluoride species (e.g., COF2) which can be dissolved in the collection fluid. 

 The results of the Hughes passenger compartment tests are shown in Table 3-5. HF 

concentrations in the air of the passenger compartment (2.77 m3 ) decreased with time, with the highest 

HF concentrations observed in the 0-3 minute samples.10 The highest HF concentration measured in the 

driver's breathing zone, 34.6 ppm, was attained with a charge of 1000 grams, a HFO-1234yf 

concentration of 4.2% at the time of lighter ignition and the use of a spark augmented lighter. Using a 

typical vehicle charge (575 grams), the HFO-1234yf concentration in the passenger compartment at 

lighter ignition was 2 to 3 percent and the resulting HF concentration in passenger compartment air 

during the first 3 minutes was 10 ppm or less (depending on other test conditions). 

 Hughes also assessed the potential for HF to be deposited on interior surfaces when HF was 

generated by exposure of HFO-1234yf to the lighter. Gauze wipes moistened with distilled water were 

used to sample HF concentrations across a 100 cm2

 area in various parts of the passenger compartment 

(e.g., dash board, driver's side door). This method is similar to a surface sampling protocol recommended 

by the U.S. EPA (U.S. EPA, 1985) and is intended to mimic the amount of chemical that might be 

transferred to a moist hand. After sampling, the wipes were evaluated for total fluoride ion concentration 

by rinsing the wipes in distilled water and then analyzing the resulting solution with the fluoride ion 

selective electrode. Samples were also collected from several control vehicles which had not been 

exposed to refrigerant. The results are also shown in Table 3-5. The highest HF concentration measured 

on an interior surface in the test vehicles was 0.18 g/cm2

 which was obtained in a test using 900 grams 

of HFO-1234yf (range across all tests 0.04 to 0.18 g/cm2

). For comparison, the highest HF 

concentration measured in control vehicles was 0.15 g/cm2

 (range 0.03 to 0.15 g/cm2). Based on these 

 

10 The only exception was test SAE 8 where the HF concentration from 0-3 minutes was 1.28 ppm and the concentration from 0 

to 7 minutes was 1.69 ppm. This small difference in values may be due to analytical variation. 

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results, the CRP team members concluded that HF exposures on vehicle surfaces after a refrigerant 

release would not be a credible health hazard to evaluate in the FTA. Further details of the tests can be 

found in the Hughes Associates report, provided as Attachment N. 

3.3.4.2 Engine Compartment 

 As noted in section 2.4, HF that may be generated in the engine compartment may also be a 

concern, either due to aspiration of HF into the passenger compartment11 or if passenger attempt to 

investigate engine malfunctions by looking directly into the engine compartment after a refrigerant 

release. To evaluate HF production due to release of HFO-1234yf in the engine compartment, Hughes 

released a full charge (575 g) of HFO-1234yf mixed with PAG oil (3%) into the engine compartment of a 

Cadillac CTS in the direction of an artificial "hot body" meant to simulate a hot engine manifold. A 

similar test was conducted using R-134a for comparison purposes. In all tests, the release rate was 12 

g/second which was intended to simulate a major rupture of the AC system piping. The hot body was a 

stainless steel pipe 36.8 cm long and 6.3 cm in diameter which was heated from the inside using a coiled 

tubular heater. The hot body was located in the engine compartment directly in front of the passenger 

compartment air intake (thus representing a worst-case location). Tests were run with the hot body at 450 

and 700°C, the former being characteristic of manifold temperature under typical operating conditions, 

the later of operation under extreme conditions. The refrigerant was released directly towards the hot 

body from a distance of 5 cm, which also represented a worst case situation. CRP members felt that in 

most cases, a greater distance and less direct path would separate the exhaust manifold from the 

refrigerant leak. 

 HF concentrations were measured by locating sampling pump lines in five areas: in the HVAC 

module inside the passenger compartment, the breathing zone of the occupant, just below the grill of the 

air intake for the passenger compartment, and above and below the hot body in the engine compartment. 

The sampling pumps drew air through collection fluid for later analysis by ion selective electrode 

measurement as described previously. In addition, a Fourier transform infrared spectrometer (FTIR) was 

used to monitor HF concentrations along the side of the car where, for example, a bystander might be 

standing (FTIR was not used inside the engine and passenger compartments because the sample beam 

length would be too small to obtain sufficiently sensitive measurements). 

 

11 CRP1234 considered movement of HF into the passenger compartment from the engine compartment due to active ventilation 

of the AC blower. Other routes of HF migration (e.g., due to damage to the windshield) were not considered in this scenario 

because, in the absence of active ventilation, only a very small amount of HF would be expected to move laterally into the 

passenger cabin rather than upwards from the warmer engine compartment. 

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 Movement of gases and vapors generated in the engine compartment into the passenger 

compartment is normally blocked by the engine hood and associated hood seals. However, in the event of 

a collision, the hood may buckle and the hood seals may be compromised. To simulate such an effect, 

testing was carried out with the hood open, closed or partially open in order to estimate the impact of the 

degree of hood buckling on HF exposure. The AC fan was operated at high speed and in outside air mode 

to maximize air intake into the passenger compartment. Further details of the tests can be found in the 

Hughes Associates report, provided as Attachment O. 

 The results of the Hughes engine compartment tests are shown in Table 3-6. For HFO-1234yf the 

highest HF concentrations in both the passenger and engine compartments occurred in test 4U with the 

hood closed, the rubber hood seals removed (to simulate vehicle damage), and the hot body temperature 

at 700°C. In this case, the HF concentration in the occupant's breathing zone was 49.6 ppm while the 

concentration in the engine compartment was 118.8 ppm (measured above the hot surface). This test was 

unique in that videos of the test show a short ignition of refrigerant/oil which was likely responsible for 

the higher HF measurements. In all other tests involving HFO-1234yf, the measured HF concentrations 

in air were below 10 ppm. Results of FTIR measurements along the side of the vehicle indicated a lack of 

any detected HF above the detection limit of approximately 90 ppm. Results for test 5U, involving a 

similar charge of R-134a indicated lower but still measurable HF concentrations; 7.5 ppm in the 

occupant's breathing zone and 36.1 ppm in the engine compartment (measured below the hot surface). 

Two additional engine compartment tests contracted separately by GM and Ford Motor Company (7U 

and 9U) explored the impact of fan operation and refrigerant discharge orientation on the level of HF 

generated. Test 7U showed that when the vehicle fan is not operating, concentrations of HF in the 

passenger cabin are dramatically reduced (2.3 ppm in test 7U). Test 9U showed that HF concentrations 

are also reduced when the refrigerant is not discharged directly onto the hot surface; the HF concentration 

in occupant's breathing zone was 12.0 ppm versus 49.6 ppm in the direct discharge test (test 4U). Note 

that all of the HF concentrations just discussed are above the irritation threshold for HF. The absence of 

reports of HF irritation associated with R-134a releases in vehicles in which this refrigerant has been used 

since 1992 suggests the conservative nature of the Hughes tests.12

 

12 It should be acknowledged that cases of HF irritation have not specifically been investigated by the manufacturers; it is 

possible that a small number of cases could occur without being reported. The absence of reports does suggest that if HF 

exposures are associated with the current use of R-134a, the problem must be fairly limited. 

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 Hughes also assessed the potential for HF to be deposited on engine compartment surfaces. Due 

to the water solubility of HF, this could be a concern for automotive components which may be covered 

with a thin layer of water while operating. Shallow dishes filled with 50 ml of water were placed at 3 

locations in the engine compartment. The fluid in the dishes was subsequently evaluated for total fluoride 

ion concentration using the fluoride ion selective electrode. The results are also shown in Table 3-6. The 

highest HF concentration measured was 46.8 g/cm2

 which was obtained on the side of the vehicle 

nearest the hot surface (range across all tests: 0.01 to 46.8 g/cm2

). Back calculating the results into the 

HF concentration in the 50 ml of sampling fluid indicates maximum HF concentrations of approximately 

0.0001%. This is about 100 times less than the HF solution concentration which is reported to be 

minimally irritating to the skin or eyes (Hjortsjö et al., 2009). Based on these results, the CRP team 

members concluded that HF exposures on vehicle surfaces after a refrigerant release would not be a 

credible health hazard to evaluate in the FTA. 

3.3.5 Review and Selection of HF Exposure Limit 

HF is a corrosive and irritant gas (IPCS, 1995). The highly irritating nature of HF upon 

inhalation exposure may limit exposures because individuals who are able will actively seek to escape 

from the irritating effects. The odor threshold for HF varies among individuals but has been reported to 

be 0.02 to 0.13 ppm (NAC/NRC, 2004). Because HF is water soluble and highly reactive, inhaled HF 

will exert its greatest effect in the upper airways (nose and throat) due to the moist conditions found in 

this area and the ”scrubbing” action of the nasal passages on inhaled air. Prolonged exposure to HF can 

lead to bone fluorosis but this effect would not be a consequence of acute exposures encountered during a 

sudden refrigerant leak and ignition event. 

Appropriate guidelines for assessing acute exposures to HF in an emergency situation include the 

Emergency Planning Response Guideline (EPRG) values published by the American Industrial Hygiene 

Association (AIHA) and the Acute Exposure Guideline Limits (AEGLs) established by the U.S. National 

Advisory Council for AEGLs and National Research Council (NRC).13 Occupational exposure values 

such as the U.S. Occupational Health and Safety Administration (OSHA) Short Term Exposure Limit 

(STEL) or the National Institutes of Occupational Health and Safety (NIOSH) Recommended Exposure 

Limits (REL) are not appropriate values for consideration because these values are intended for workers 

 

13 Provisional AEGLs are developed and proposed by the U.S. National Advisory Council for AEGLs with administrative 

support from U.S. EPA. They are then reviewed, finalized and published by the National Research Council through the National 

Academies Press. 

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who may be exposed repeatedly to these values over the course of their job tenure (i.e., multiple brief 

exposures per work week or even per day). Such a value would not be consistent with the emergencytype situation envisioned by release and ignition of a refrigerant. 

The ERPGs and AEGLs consist of three specific values for each chemical of interest, 

corresponding to increasing severity of health effects. By definition, exposures that remain below the 

first level (i.e., AEGL-1 or EPRG-1) will result in no adverse health effects whatsoever, including effects 

in members of susceptible population groups. Exposures that remain below the second level (i.e., AEGL2 or EPRG-2) mean that individuals may experience mild and reversible effects, such as irritation or 

discomfort, but the effects will not impair the ability to escape or result in permanent health 

consequences. Exposures maintained below the third level (i.e., AEGL-3, ERPG-3) will avoid lifethreatening concentrations but may nonetheless result in long term health consequences. 

The AEGL-2 and ERPG-2 values represent reasonable health-based limits for use in this risk 

assessment. The key concern should be to prevent situations in which the individual is unable to safely 

operate the vehicle or take action (e.g., stopping, opening windows) to avoid significant and permanent 

injury. AEGL-1 or EPRG-1 values, which represent a threshold for minor discomfort, would be overly 

restrictive for evaluating scenarios which by their very nature involve other hazards such as collisionrelated trauma, exposures to smoke or flames in car fires, etc. 

AEGL-2 values are available for both 10-minute and 30-minute time frames (NAC/NRC, 2004). 

The NAC/NRC based the AEGL-2 value for 10 minutes on a rat study conducted by Dalbey et al. (1998). 

This study was designed specifically to establish an acute exposure value for humans. In this study, rats 

were exposed for 10 minutes to different concentrations of HF (135, 271, 950 and 1764 ppm). The 

exposures were administered directly into the rats' lungs via a tracheal tube. This is a highly conservative 

means of administration since it avoids the "scrubbing" effect on air moving through the nasal passages, 

and essentially mimics the exposure of someone who is a "mouth-breather". Rats were evaluated for 

signs of lung inflammation as well as effects on lung function. Rats exposed for 10 minutes to 950 ppm 

HF showed only minimal signs of lung inflammation and no deficits in lung function tests.14 The 

NAC/NRC applied an uncertainty factor of 10 to the exposure concentration of 950 ppm (to account for 

both inter-species differences and sensitive individuals) to arrive at the AEGL-2 value of 95 ppm. 

 

14 In a separate sub study, some animals exposed to 1764 ppm were allowed to recover for 2 weeks prior to being evaluated. No 

signs of the toxicity observed immediately after exposure were observed in these animals, suggesting reversibility even at this

higher concentration. 

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The AEGL-2 for a 30-minute exposure is based on a study involving exposures of dogs to 157 or 

243 ppm HF for 1 hour (Rosenholtz et al., 1963). Animals were observed for 14 days after the exposures. 

At 243 ppm, the dogs exhibited "signs of irritation and discomfort including blinking, sneezing, and 

coughing" (NAC/NRC, 2004). The NRC suggested these effects were consistent with the "threshold for 

impaired ability to escape" that is the intention of the AEGL-2. The NRC applied an uncertainty factor of 

10 to the exposure concentration of 243 ppm (to account for both inter-species differences and sensitive 

individuals) to arrive at the AEGL-2 value of 24 ppm for 60 minutes. This was time-adjusted using 

Haber's Rule15 to arrive at a 30-minute AEGL-2 value of 34 ppm. Thus, time-weighted exposures to HF 

over a 30-minute period should not exceed the 30-minute AEGL-2 value of 34 ppm. 

During the course of testing at Hughes, the CRP1234 members became interested in identifying a 

very short term (i.e., 1 minute) exposure limit for HF. A teleconference was convened among OEM and 

independent toxicologists to review the available HF toxicology data. It was the consensus of this group 

to use the 95 ppm 10-minute AEGL-2 value as a conservative limit for a 1-minute exposure. The group 

noted that dose-time extrapolation (e.g., using Haber’s Rule) is possible and would result in a higher 1-

minute exposure limit but such extrapolation would introduce additional uncertainty. Studies conducted 

in animals (Dalbey et al. and humans (Machle et al., 1934) support a 95 ppm 1-minute exposure limit 

consistent with the intent of AEGL-2. 

The EPRG-2 values developed by the AIHA are similar to the AEGL values established by the 

NRC. The 10-minute ERPG-2 value for HF is 50 ppm (and thus lower than the corresponding AEGL) 

while the 60-minute EPRG-2 value is 20 ppm. A 30-minute ERPG value has not been established but, 

using time-adjustment, would be expected to be similar to the 30-minute AEGL-2. The ERPG values 

appear to have been most recently updated in 1999. Because the AEGL values are more recent (i.e., 

published in 2004), were developed by a national consensus body, and were developed with the support 

of U.S. EPA, they are the most scientifically supported values for use in this risk assessment. The 10-

minute AEGL-2 value also has the benefit of being derived from a study which included 10-minute 

exposures, thus avoiding the uncertainty inherent in dose-time extrapolation. It is important to note that, 

like all values based on interpretation of biological test data, the AEGLs and ERPGs represent scientific 

judgments concerning safety and contain some degree of uncertainty (generally believed to be biased 

towards overestimating potential health risks). In risk assessment such values are typically evaluated by 

looking at the ratio of the anticipated exposure concentration to the health limit (termed the margin of 

 

15 Haber’s Rule states that exposure concentration and exposure time can be related by the formula Cn

 x T = constant, where n is 

a chemical specific exponent. The NRC indicated that the value of n for HF is 2. 

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exposure approach). Ratios should be less than 1.0 and the smaller the ratio, the greater the level of 

confidence that adverse effects will not be experienced in an exposed human population. 

The current NIOSH immediately dangerous to life and health (IDLH) value for HF is 30 ppm, a 

value that should not be exceeded for a 30-minute exposure to ensure the individual remains able to 

escape from a dangerous situation without suffering irreversible health effects. This value is just below 

the 30-minute AEGL-2 value. The NIOSH value appears to be based on a limited number of animal 

studies published in the 1960s and has not been revised since it was first set in the early 1970s. The IDLH 

is therefore not the most scientifically appropriate criterion for use in the risk assessment but does provide 

support for the AEGL-2. 

We note that a set of human studies conducted by Lund et al. (1997, 1999) were used by the 

NAC/NRC to establish the AEGL-1 value for HF (1 ppm). Workers in this study were exposed to 

varying HF concentrations for 1-hour, including during a period of moderate exercise. After exposure, 

the workers rated their degree of nasal, throat and pulmonary symptoms, were assessed with pulmonary 

function tests, and were evaluated for signs of lung inflammation by bronchioalveolar lavage. Individuals 

exposed in the range of 0.85–2.9 ppm reported no to low sensory irritation and evidenced no change in 

pulmonary function, but had slight increases in some parameters assessed by bronchioalveolar lavage 

(specifically increases in the percentage of CD3 cells and myeloperoxidase). The NAC/NRC applied an 

uncertainty factor of 3 to this value to arrive at the AEGL-1 value of 1 ppm (applicable to 10-minute, 30-

minute and 1-hour exposures). As noted above use of the AEGL-1 value is not consistent with the level 

of other hazards present in the accidental release scenarios being evaluated in this risk assessment. 

Finally, the California Environmental Protection Agency (Cal-EPA) has established an inhalation 

exposure reference level for HF of 0.24 mg/m3

 (0.3 ppm) (Cal-EPA, 1999). This level is intended to be 

protective against "mild adverse effects" and is also based on a study (Lund et al, 1997) in which human 

subjects reported symptoms of mild irritation. This value would be comparable to the AEGL-1 value 

described above but is lower due to the use of a larger uncertainty factor (i.e., 10 versus 3). Cal-EPA has 

also established a 1-hour exposure limit for “more severe effects” of 1 ppm. This is based on effects 

observed in rats at 103 ppm (for 1 hour) in the Rosenholtz et al. (1963) study and use of an uncertainty 

factor of 100. In the NAC/NRC review of this study, the effects observed at this concentration are 

summarized as "mild, nondisabling effects or only occasional signs of ocular and nasal irritation " which 

would not be consistent with the definition of the AEGL-2 value. The difference between the NRC and 

Cal-EPA values therefore results from differences in interpretation of the severity of findings in the 

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animal studies. The Cal-EPA analysis appears to have been conducted before the Dalbey et al. (1998) 

study became available. That well-conducted study would suggest Cal-EPA's values are overly 

restrictive. 

3.3.6 Summary of HF Generation Potential 

 The data collected by the various efforts described in Sections 3.3.1 through 3.3.5 were used by 

the CRP1234 members in developing inputs for the fault trees relating to potential HF exposure. The 

work conducted by DuPont related to HF generation via cigarette smoking led the CRP1234 members to 

exclude this scenario from the FTA because the data indicated it would not be possible for a smoker to 

experience exposures above the AEGL-2 value before the irritant properties of HF caused them to stop 

smoking. Similarly, the work conducted by Honeywell demonstrated the difficulty in producing HF by 

igniting HFO-1234yf with a butane lighter when concentrations were in the flammable zone; a smoker 

would need to light the lighter in one location and then move it into the area where the HFO-1234yf was 

above the LFL (most likely near the vehicle floor). This data factored into the probabilities used in the 

FTA for HF production by a smoking-related ignition source. 

 Hughes Associates conducted a number of tests to evaluate potential HF generation via thermal 

decomposition of HFO-1234yf. The data obtained in the vehicle tests conducted by Hughes Associates 

demonstrated that HF concentrations in the passenger compartment did not exceed the AEGL-2 value for 

HF when HFO-1234yf at concentrations below the LFL was exposed to a potential thermal 

decomposition source. This was true even in tests where nearly twice the expected refrigerant charge was 

used to maximize HFO-1234yf concentrations in the vehicle cabin. Engine compartment tests were also 

conducted where the refrigerant was exposed to a hot surface at temperatures below the reported ignition 

temperature of 750°C. In these tests, HF exceeded the AEGL-2 value in only 1 test and only by a slight 

amount (118 ppm measured as a 3-minute TWA versus the 10-minute AEGL-2 value of 95 ppm). This 

test was conducted with the hood closed and the hood seals compromised (thus most relevant to the 

passenger cabin aspiration scenario and not relevant to a case where the former occupant is exposed to HF 

in the engine compartment by opening the hood). It was the opinion of the CRP toxicology group that the 

distinction between 118 ppm and 95 ppm is negligible given the 10-fold safety factor used in deriving the 

AEGL-2 value. In the test conducted with the hood open (relevant to the scenario involving the occupant 

investigating the engine compartment), the measured HF concentrations were less than one tenth of the 

AEGL-2 value. Based on the results of these tests, the CRP1234 members concluded that the potential 

for generating HF above the AEGL-2 value due to thermal decomposition of HFO-1234yf would be 

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extremely low. This knowledge was then used in developing input values for the fault trees (refer to 

Attachments A and B). Note that in the fault trees CRP1234 members did assume that in cases of HFO1234yf ignition and combustion (as opposed to thermal decomposition) HF would have a high likelihood 

of being generated in the passenger or engine compartment above the AEGL-2 value. 

 Tests conducted at INERIS were not intended to simulate real-world conditions but were instead 

directed at understanding the impact of various variables on HF generation. The INERIS data were 

therefore used in a supporting role in the risk assessment. For example, the data from INERIS indicated 

that generation of HF from HFO-1234yf via thermal decomposition is highly dependent on hot surface 

temperature and time of contact between the hot surface and the refrigerant. Thus, in the fault trees 

related to engine compartment releases, cases where the vehicle fan is operating were considered to not 

pose a risk since the refrigerant would not be in contact with the hot surface long enough to produce a 

measureable amount of HF. Likewise, the INERIS data suggested that hot surface temperatures below 

550°C would produce negligible amounts of HF and therefore the fault tree analysis assumed that the 

potential for HF generation above the AEGL-2 value would be negligible below this temperature. 

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Table 3-1. HFO-1234yf concentrations measured in vehicle compartments by different OEMs after simulated refrigerant release 

Leak diameter (mm)§ Leak rate (g/s)§ HVAC mode 

setting

Blower 

speed

Mode Maximum Concentration (%) 

Driver Face Driver Lap Foot Area 

OEM1 (charge = 350 g, compartment volume = 1.7 m3)

0.5 vent on Recirc 1.1 1.3 1.9

0.5 foot on Recirc 0.8 1.2 2.3

0.5 foot off Recirc <0.3 <0.3 1.3

6 vent on Recirc 1.9 3.2 6.7 

6 foot on Recirc 1.5 3 11

6 foot off Recirc <0.3 <0.3 9.2 

OEM2(charge = 450 g, compartment volume = 1.5 m3)

0.5 floor 2 Recirc 5.8, 4.8, 4.5

0.5 floor 4 Recirc 4.6, 3.9, 3.8

0.5 floor 4 Recirc 

0.5 floor 1 Recirc 

0.5 floor off Recirc 10.8, 8.1, 0.3 

0.5 defrost 2 Recirc 4.5, 4.2, 3.1

OEM3 (charge = 590 g, compartment volume = 2.1 m3) 

 1.14* vent 1 Recirc 5.02 4.79 5.18 

 1.14* vent 4 Recirc 4.53 4.39 4.62 

 1.14* floor 1 Recirc 4.06 6.21 6.57 

 1.14* floor 4 Recirc 4.44 4.47 4.64 

 1.14* defrost 1 Recirc 4.87 4.78 5.19 

 12.4 vent 1 Recirc 5.03 4.98 7.52 

 12.4 vent 4 Recirc 5.0 4.89 5.28 

 12.4 floor 1 Recirc 3.14 6.56 7.53 

 12.4 floor 4 Recirc 3.91 6.2 7.05 

 12.4 defrost 1 Recirc 4.47 4.69 8.29 

 12.4 defrost 4 Recirc 4.91 4.95 6.14 

OEM4 – Vehicle 1 (charge = 757 g, compartment volume = 2.56 ) 

 13 vent high Recirc 5 5 5 

 13 vent low Recirc 

 13 vent off Recirc 0 2.1 15.8 

 13 vent high OSA 1.6 1.7 

 13 vent low OSA 2.1 5.5 5.2 

 13 vent off OSA 

 13 floor high Recirc 3.7 3.7 

 13 floor low Recirc 

 13 floor off Recirc 

 13 floor high OSA 0.9 0.8 

 13 floor low OSA 2.6 3.8 6.3 

 13 floor off OSA 0 0.2 14.7 

OEM4 – Vehicle 2 (charge = 650 g, compartment volume = 1.12 ) 

 13 vent high Recirc 3.7 

 13 vent low OSA 1.5 6 4.5 

 13 vent off OSA 0.9 6 5.7 

 13 bi level off Recirc 11.8 

§ In prior risk assessments conducted by the CRP, a leak rate of 1.14 g/s was considered equivalent to a leak hole diameter of 0.5 mm while a leak rate of 13 g/s was 

considered equivalent to a leak hole diameter of 6.35 mm. 

* Average value from 2 tests 

 The three values indicate concentrations at increasing distance from the vehicle floor 

Data shown in bold indicate values above the LFL.

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Table 3-2 Results of Modeling Conducted for HFO-1234yf Concentration in Passenger Compartment, 

Lower 1/3 of Passenger Compartment (Uniform Mixing Model) 

Conditions

Maximum 

Concentration 

(ppm)

Peak 5-

minute TWA*

Peak 30-

minute 

TWA*

Small diameter leak (0.01 mm), blower speed low (1.7 ACH) 16 16 16 

Medium diameter leak (0.1 mm), blower speed low (1.7 

ACH) 3,800 3,800 3,700 

Large diameter leak (0.5 mm), blower speed low (1.7 ACH) 64,700 63,500 58,400 

Very large leak (6.5 mm orifice associated with tube 

rupture), blower speed low (1.7 ACH) 74,600 70,600 64,800 

Table 3-3. Results of Air Modeling for Certified AC Service Technician Scenarios, HFO-1234yf 

Scenario Predicted HFO1234yf 

Concentration 

(Maximum, ppm)

Predicted HFO1234yf 

Concentration (5-

minute TWA, 

ppm)

Predicted HFO1234yf 

Concentration 

(30-minute TWA, 

ppm)

Hose leak during system drain and recharge 146 141 122 

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Table 3-4. HF Concentration Measurements Obtained by INERIS in Bench Scale Tests 

Test Refrigerant charge 

(g) Oil (g) 

Hot Body 

Temperature 

(°C)

Orientation of 

Discharge to 

Hot Body

HF Concentration 

(ppm) 

4 600 (HFO-1234yf) 0 700 direct 1,126 

5 600 (HFO-1234yf) 24 700 direct 3,000 

8 600 (HFO-1234yf) 24 700 direct 6,001 

9 600 (R-134a) 24 700 direct 2,915 

11 600 (R-134a) 0 700 direct 626 

13 600 (HFO-1234yf) 24 700 direct 6,939 

14 600 (HFO-1234yf) 24 700 indirect* 5,861 

15 300 (HFO-1234yf) 24 700 direct 648 

16 600 (HFO-1234yf) 12 550 direct 179 

17 300 (R-134a) 12 700 direct 280 

18 600 (HFO-1234yf) 24 630 direct 3,196 

19 600 (R-134a) 24 700 direct 1,861 

20 600 (HFO-1234yf) 24 700 – small surface indirect* 146 

 Values shown in bold exceed the AEGL-2 value of 95 ppm. 

 * Discharge was sprayed towards the chamber ceiling near the hot surface rather than directly onto it. 

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Table 3-5. Results of Passenger Cabin HF Concentration Measurements Obtained by Hughes Associates 

Test Refrigerant charge 

(g) Test condition 

 HFO-1234yf 

concentration at 

lighter ignition 

(%)

0 to 3 minute HF 

concentration in 

driver's breathing 

zone (ppm)

Wipe sample 

concentration 

(g/cm2) 

SAE1 575 (R-134a) quick discharge 1.7 0.98* 0.13-0.17 

SAE2 575 (HFO-1234yf) quick discharge 1.5 1.69* 0.06-0.10 

SAE3 575 (HFO-1234yf) quick discharge 1.6 0.79* 0.07-0.10 

SAE4 575 (HFO-1234yf) quick discharge 1.08 5.42 0.05-0.10 

SAE5 575 (HFO-1234yf) quick discharge 0.71 1.86 0.11-0.16 

SAE6 900 (HFO-1234yf) quick discharge 0.90 1.28 0.11-0.16 

SAE7 900 (HFO-1234yf) quick discharge 1.3 1.59 not collected 

SAE8 575 (HFO-1234yf) slow discharge 0.19 0.64 not collected 

SAE9 575 (HFO-1234yf) slow discharge 1.96 3.10 not collected 

SAE10 1000 (HFO-1234yf) slow discharge 3.6 5.08 not collected 

SAE11 900 (HFO-1234yf) quick discharge 1.03 4.06 not collected 

SAE12 900 (HFO-1234yf) spark augmented 

quick discharge 3.2 14.3 not collected 

SAE13 900 (HFO-1234yf) spark augmented 

quick discharge 3.5 21.2 not collected 

SAE14 575 (HFO-1234yf) spark augmented 

quick discharge 2.6 10.3 not collected 

SAE15 575 (R-134a) spark augmented 

quick discharge 2.4 4.0 not collected 

SAE16 825 (HFO-1234yf) spark augmented 

quick discharge 3.3 13.6 not collected 

SAE17 825 (R-134a) spark augmented 

quick discharge 3.3 5.2 not collected 

SAE18 850 (HFO-1234yf) spark augmented 

quick discharge 4.2 11.9 not collected 

SAE19 850 (HFO-1234yf) spark augmented 

quick discharge 4.0 23.5 not collected 

SAE20 1000 (HFO-1234yf) spark augmented 

quick discharge 4.2 34.6 not collected 

Wipe sample controls 0.03-0.15 

 * For the 1st three tests Hughes collected samples for a 10-minute timeframe only. 

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Table 3-6. Results of Engine Compartment HF Concentration Measurements Obtained by Hughes Associates 

Test Refrigerant 

charge (g) 

Hot Body 

Temperature 

(°C) 

Hood 

Position 

and Fan 

setting

Engine 

Compartment 

HF 

Concentration 

in air (ppm)§

Passenger 

Compartment 

e HF 

Concentration 

in air, Driver's 

Breathing 

Zone (ppm)

Deposition 

sample 

concentration 

(g/cm2) 

Test1U 575 (HFO1234yf) 

450 open/on 4.3 / 7.5 2.8 NA 

Test2U 575 (HFO1234yf) 

450 closed*/on 0.4 / 0.6 0.4 NA 

Test3U 575 (HFO1234yf) 

700 midway/on 

0.9 / 0.1 0.8 0.01- 0.04 

Test4U 575 (HFO1234yf) 

700 closed*/on 81.8 / 118.8 49.6 0.01-46.8 

Test5U 575 (R-134a) 700 closed*/on 36.1 / 4.4 7.5 0.04-6.02 

Test7U575 (HFO1234yf) 

700 closed*/off 47.3/80.4 2.3 0.07-25.2 

Test9U575 (HFO1234yf) 

700** closed*/on 7.8-53.3 12.0 0.08-0.24 

Shading indicates values above the AEGL-2 value for HF of 95 ppm. 

*

 In these tests the engine compartment seals were intentionally compromised 

** In this test the refrigerant was not discharged directly on to the hot surface but from a distance of approximately 12.5 inches away.

§First number indicated the HF concentration measured in the engine compartment below the hot surface, the second number is the concentration measured above 

the hot surface. 

Tests conducted by Hughes Associates under contract to GM and Ford Motor Company. Results were provided to CRP1234.  Tests were conducted with the AC fan running to evaluate whether HF generated in the engine compartment will be aspirated into the passenger 

compartment through the air intake vents located beneath the windshield.

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4 FTA for HFO-1234yf 

The fault tree analysis for HFO-1234yf considered both the exposure of individuals to an open 

flame (due to refrigerant ignition) as well as potential exposures to hydrogen fluoride (HF) above a healthbased limit. The scenarios can be further stratified into those involving exposures in the passenger 

compartment and those involving exposure outside in, or adjacent to the engine compartment. These 

scenario categories are described in further detail in the following sections. 

4.1 Refrigerant Ignition, Passenger Compartment Scenario 

The CRP1234 members developed a fault tree that describes the probability of exposure of the 

vehicle occupant to an open flame due to refrigerant release and ignition in the passenger compartment as a 

result of vehicle collision (Fault Tree I3). This scenario contains a number of collision speed and location 

dependent elements because the speed and location (frontal, side) at which the collision occurs influences a 

number of factors, including the likelihood of refrigerant release, the likelihood that the vehicle windows or 

other structures will be broken introducing outside air into the passenger compartment, the probability that 

the passenger will be able to leave the vehicle after the collision, and the probability of sparks or other 

ignition sources being created. The tree also requires that an ignition source of sufficient energy be present 

at the same location and time as that where the concentration of HFO-1234yf is in the flammable range. 

After considerable discussion, the CRP1234 members determined that this scenario was only relevant to 

high speed collisions, those where the vehicle was traveling at relative speeds greater than 50 km/hr 

because collisions at lower speeds would be unlikely to damage the evaporator producing a leak and also 

unlikely to produce sparks or shorts in the passenger compartment with sufficient energy to ignite the 

refrigerant. This fault tree also contains use of a butane lighter or a match as a potential ignition source 

although most individuals would not be expected to light a cigarette in the passenger compartment after a 

high speed collision. Refer to Fault Tree I3 in Attachment A for further details. The basis for values used 

as inputs in the fault tree are provided in tabular form in Attachment A. Many of the speed dependent 

probabilities that apply to the collision-related fault trees were obtained through an analysis of the GIDAS 

database, a German registry which provides a detailed analysis of the effects of vehicle collision at various 

speeds on vehicle integrity. A summary of the data extracted from the GIDAS database as well as other 

sources is provided as Attachment S. 

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4.2 Refrigerant Ignition, Engine Compartment Scenarios 

The CRP1234 members developed two fault trees to address the probability of exposure to an open 

flame due to refrigerant release and ignition in the engine compartment. The first fault tree (Fault Tree I4) 

describes the risk associated with a non-collision related failure of an AC system component (for example 

an abraded hose or fractured tube) which releases refrigerant into the engine compartment. The refrigerant 

then could be ignited if it comes in contact with a sufficiently hot surface such as the exhaust manifold. 

This scenario also requires that air movement in the engine compartment (either due to wind or vehicle 

movement) is not sufficient to disperse the refrigerant before it accumulates above the LFL, that a vehicle 

occupant stops the vehicle and looks in the engine compartment to investigate the cause of the AC failure, 

and that the former occupant performs this investigation at the same location and time as the refrigerant 

release and ignition. In terms of the temperature required for the hot surface to ignite the HFO-1234yf/oil 

mixture, the CRP decided to conservatively assume that temperatures of 550°C would be sufficient to cause 

ignition, although testing conducted at INERIS indicated that the temperature at which the HFO1234yf/PAG oil mixture ignites in air is approximately 750°C and one test conducted at Hughes Associates 

indicated refrigerant/oil ignition at 700°C. It should be noted that the boundary conditions employed in this 

fault tree represent extreme use conditions (e.g., vehicle engine surface temperatures would only exceed 

550°C during times of extreme use and are not indicative of normal city driving). Refer to Fault Tree I4 in 

Attachment A for further details. The basis for values used as inputs in the fault tree are tabulated in 

Attachment A. 

The CRP1234 members also developed a second engine compartment fault tree (Fault Tree I5) 

which considered the risk associated with refrigerant ignition associated with vehicle collision. This 

scenario assumed that collisions of sufficient severity to damage the AC system (e.g., at the condenser) 

would release refrigerant that could come into contact with a hot surface and be ignited. A vehicle 

occupant may decide to look at the engine compartment after a collision (e.g., to determine the level of 

damage and whether the vehicle can be driven away) and may be exposed to an open flame if the 

refrigerant is released and ignited while this former occupant is investigating. As with Fault Tree I3, this 

scenario also requires a number of collision speed and location dependent factors such as the probability of 

refrigerant release, the likelihood that the occupant is able to leave the passenger compartment and 

investigate in the engine compartment, and the probability that the engine fan is no longer operating 

leading to accumulation of refrigerant. In this scenario the CRP1234 members judged that low, medium 

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and high speed collisions all could contribute to differing degrees to the overall risk, because the engine 

compartment side of the AC system could possibly be damaged even in lower speed collision. The tree 

also requires that the former occupant investigates the engine compartment within 3 minutes of the 

collision because this is the expected time frame over which a collision-related release will occur and also 

when engine components are still hot enough to ignite the refrigerant. Refer to Fault Tree I5 in 

Attachment A for further details. The basis for values used as inputs in the fault tree are tabulated in 

Attachment A. As previously noted, many of the speed dependent probabilities were obtained through an 

analysis of the GIDAS database. 

4.3 Refrigerant Ignition, AC Service Scenarios 

CRP1234 members also evaluated the probability of exposure to an open flame by certified AC 

service technicians (Fault Tree I6). Two possible scenarios were considered; rupture of the AC system in 

the vehicle during service and leaks from service equipment connected to the vehicle AC system during 

system drain and recharge. In addition to the frequency of these refrigerant releases, the other key 

components of this fault tree are the probability of inadequate ventilation in a professional shop and the 

probability that service workers either use lighters or high energy service equipment (welding arcs, grinders 

in the vicinity of the refrigerant release. Earlier analysis for Phase II indicated that HFO-1234yf 

concentrations would not exceed the LFL assuming uniform mixing of refrigerant in a small (1,000 square 

feet or 93 m2

) service shop. As part of Phase III, the CRP1234 members also wished to explore the 

possible risk of non-uniform localized HFO-1234yf concentrations in the event of a release. CFD 

modeling conducted by DuPont as part of Phase III, indicated that HFO-1234yf concentrations would only 

exceed the LFL in a narrow plume extending approximately 10 cm from the origin of the release. The 

CRP1234 members judged that there was no reason to expect a trained/educated technician to use a butane 

lighter this close to the AC system (e.g., electric lights are available and far brighter). They also believed it 

was implausible that certified service technicians, who have received training about the hazards of various 

flammable fluids present in automobiles (gasoline, oil, brake fluid) would perform such activities as 

welding or grinding directly in the engine compartment rather than on disconnected and appropriately 

drained parts. Thus for this tree, the probability that a sufficient ignition source is present is zero and the 

resulting probability is also zero. 

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4.4 HF Generation, Passenger Compartment Scenarios 

The CRP1234 members developed a series of fault trees to quantify the probabilities of exposure of 

occupants of the passenger compartment to HF. The first fault tree describes the probability of exposure 

of the vehicle occupant to HF generated by HFO-1234yf release and ignition in the passenger compartment 

as a result of vehicle collision (Fault Tree HF3). This fault tree is analogous to Fault Tree I3 which also 

addresses the effects of a collision on the occupants of the passenger compartment. It therefore shares with 

Fault Tree I3 the same speed and location dependent collision factors. As with Fault Tree I3, the CRP1234 

members determined that this HF exposure scenario was only relevant to high speed collisions (i.e., those at 

> 50 km/hr) because only those collisions would produce conditions where refrigerant ignition and HF 

generation are possible. In addition to the factors included in Fault Tree I3, this tree also includes the 

probabilities that HF is produced in the passenger compartment above the HBL (necessary since some large 

passenger compartments may allow sufficient dilution) and the probability that the individual does not exit 

the vehicle when HF is initially being produced due to the irritating nature of this chemical at 

concentrations that are well below the HBL. Refer to Fault Tree HF3 in Attachment A for further details. 

The basis for values used as inputs in the fault tree are provided in Attachment A. 

Fault Tree HF4.2 involves a situation where a collision causes release and ignition of refrigerant in 

the engine compartment and HF that is generated is subsequently aspirated into the passenger compartment 

via the air intake located at the base of the windshield. This fault tree also contains collision speed and 

location specific inputs, although in this case the CRP1234 members judged that low, medium and high 

speed collisions all could contribute to differing degrees to the overall risk, because the engine 

compartment side of the AC system could possibly be damaged even in lower speed collision. This tree 

also requires that the hood is buckled during the collision in a manner which creates a flow path for HF 

from the engine compartment to the air intakes. Vehicles have hood seals which normally prevent such a 

flow but hood buckling in the case of a collision could negate this protective design. This fault tree also 

requires the probability that HF is present at the air intake vents above the HBL and is not dispersed by 

wind or rain16. It also requires that the vehicle is switched to outside air mode and that HF is not 

sufficiently dissolved in moisture present on the surface of the evaporator such that the HBL is not reached 

 

16 Because HF is water soluble, rain or even fog may act to attenuate or "scrub" the concentration of HF in air. If air containing HF 

is drawn through a region saturated with water vapor, some of the HF will dissolve in the water particles and either deposit on

surfaces prior to being aspirated into the vehicle cabin or deposit on the vehicle's air filters (note that the potential for air filters to 

absorb HF in dry air was not considered as a potential attenuating factor in the risk assessment). The extent to which this occurs is 

not well understood and the impact of this particular parameter is explored further via sensitivity analysis (Section 4.8.1). 

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in the passenger compartment. One mitigating factor that is not accounted for in this tree is the possibility 

that a substantial fraction of the HF entering passenger cabin air may be absorbed by activated charcoal 

filters in vehicles which have this feature. Refer to Fault Tree HF4.2 in Attachment A for further details. 

The basis for values used as inputs in the fault tree are provided in Attachment A. 

Fault Tree HF7.1 concerns a scenario where a vehicle fire occurs as a result of a fault in non-AC 

system components (e.g., a fire due to an electrical short or overheating of the vehicle brakes). Such a fire 

could progress to the point where it impacts the AC system, causes release and ignition of refrigerant and 

generates HF which can then be aspirated into the passenger compartment. This scenario was not evaluated 

with an ignition fault tree because the low burning velocity of HFO-1234yf indicates it would contribute 

minimally to a fire compared to other flammable vehicle substances. It is evaluated from the standpoint of 

HF generation because this is a risk specifically related to the refrigerant. This fault tree requires the 

probability of a non-AC system derived vehicle fire (obtained from U.S. National Fire Protection 

Association data), the probability the fire involves the AC system and ignites the refrigerant, the probability 

that any HF generated is not dispersed by wind, rain or thermal convection currents from the fire itself, and 

probabilities (analogous to those discussed above) that the HF is aspirated into the passenger compartment 

and exceeds HBL. It also requires that the occupants remain in the vehicle during the course of the vehicle 

fire. Refer to Fault Tree HF7.1 in Attachment A for further details. The basis for values used as inputs in 

the fault tree are provided in Attachment A. 

Fault Tree HF8.1 concerns a scenario where a vehicle fire occurs as a result of a non-collision 

related failure of an AC system component (e.g., a large release of HFO-1234yf due to hose fatigue). If it 

comes in contact with a hot surface, the refrigerant could ignite or undergo thermal decomposition (at 

temperatures below those associated with ignition) to produce HF. This HF could then be aspirated into the 

passenger compartment and result in occupant exposure. Similar to the fault trees discussed earlier, this 

fault tree contains probabilities relating to potential dispersion of HF by wind, rain or air currents 

associated with vehicle movement and with aspiration of the HF into the passenger compartment. It also 

requires that the vehicle occupant does not leave the vehicle or open the window when the irritant nature of 

HF is first perceived. Refer to Fault Tree HF8.1 in Attachment A for further details. The basis for values 

used as inputs in the fault tree are provided in Attachment A. 

 

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4.5 HF Generation, Engine Compartment Scenarios 

With respect to scenarios involving exposure in the engine compartment, the CRP1234 members 

developed a fault tree (Fault Tree HF4) which considered the risk associated with refrigerant ignition and 

HF generation in the event of a vehicle collision. Like the scenario for Fault Tree I5, this scenario assumed 

that collisions of sufficient severity to damage the AC system (e.g., at the condenser) would release 

refrigerant that could come into contact with a hot surface and be ignited. This scenario further assumed 

that HF could be generated from either refrigerant ignition or thermal decomposition. A vehicle occupant 

who exits the vehicle to investigate the collision related damage in the engine compartment might therefore 

be exposed to HF if the refrigerant is released and ignited while the former vehicle occupant is 

investigating. Similar to Fault Tree I5 this scenario requires a number of collision speed and location 

dependent factors such as the probability of refrigerant release, the likelihood that the occupant is able to 

leave the passenger compartment and investigate in the engine compartment, and the probability that the 

engine fan is no longer operational, allowing for the accumulation of refrigerant. All collision speeds and 

locations are considered in this fault tree although the contribution of each to the overall risk differs. The 

tree also requires that the former occupant investigates the engine compartment within 3 minutes of the 

collision because this is the expected time frame over which HF from a release will be generated and will 

not have been dispersed by convection or simple dilution. Refer to Fault Tree HF4 in Attachment A for 

further details. The basis for values used as inputs in the fault tree are provided in Attachment A. 

Fault Tree HF5 is quite similar to Fault Tree HF4 but describes the risk for a good Samaritan rather 

than a former vehicle occupant. The good Samaritan is an individual who observes that an individual is 

trapped in a vehicle after a collision, attempts to free them, and is present near the engine compartment 

while doing so. Like Fault Tree HF4, this tree assumes that collisions of sufficient severity to damage the 

AC system (e.g., at the condenser) would release refrigerant that could come into contact with a hot surface 

and produce HF via ignition or thermal decomposition. In this case, the CRP1234 members judged that 

only high and medium speed (>16 km/hr) collisions would be sufficient to attract the attention of good 

Samaritans. They also decided that good Samaritans would be most likely to stop their own vehicle and 

help in the medium speed collision case, since high speed collisions may occur on highways where 

individuals are moving at too high a speed to consider lending assistance in the relevant time frame. Two 

unique components of this fault tree are the probability that the good Samaritan chooses to stay and assist 

the trapped occupant despite the irritancy of HF and the probability that the good Samaritan arrives before 

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HF has been dispersed due to convection or simple dilution. Refer to Fault Tree HF5 in Attachment A for 

further details. The basis for values used as inputs in the fault tree are provided in Attachment A. 

The last fault tree developed by the CRP1234 members involving engine compartment exposures 

relates to vandalism (HF6). Owners or other individuals may attempt to extinguish a vehicle fire caused 

by vandalism and may be exposed to HF if they are present at the time when the fire reaches the AC system 

and causes refrigerant release and ignition. In addition to the frequency of vandalism related car fires, the 

fault tree requires the probabilities that the fire reaches and ruptures the AC system, that the resulting HF is 

not dispersed by wind, rain or convection currents related to the fire, that the person is present when HF is 

present above HBL and does not choose to abandon the vehicle once they perceive the irritant nature of 

HF. Because car fires also result in the release of many other toxic gases, this scenario also requires that 

the individual takes action before exposure to these other combustion products would result in more 

significant exposures relative to HF. Refer to Fault Tree HF6 in Attachment A for further details. The 

basis for values used as inputs in the fault tree are provided in Attachment A. 

4.6 High Pressure Equipment Risks in Vehicle Service Areas 

CRP1234 members also developed a fault tree to assess the physical risks to AC service 

technicians working with high pressure refrigerant hoses. Although equipment for servicing AC systems is 

designed with various safety devices (e.g., automatic shut off coupling on equipment hoses), it is possible 

that high pressure hoses could rupture or break during service and could possibly strike a service technician 

(a risk which also would exist with current R-134a systems). This fault tree therefore contains the 

frequency that vehicles are serviced and the probabilities that high pressure hoses fail or rupture, that the 

hose strikes the technician before they can move away, and that the hose strikes the technician with 

sufficient force to create a reportable injury. Refer to Fault Tree P1_1234 in Attachment A for detail about 

this organization of this fault tree. The basis for values used as inputs in the fault tree are provided in 

Attachment A. 

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4.7 Risks for Other Receptors 

4.7.1 Potential Risks to Other Human Receptors 

 Risks for automotive assembly workers are anticipated to be insignificant. Data from GM (Bureau 

Veritas, 2008) indicate that R-134a concentrations measured in assembly plants were consistently below 10 

ppm (i.e., 100 times less than the Occupational Exposure Limit of 1,000 ppm). Although there is some 

uncertainty in extrapolating results for R-134a to HFO-1234yf, it would be reasonable to expect that HFO1234yf concentrations in assembly plant air would be generally similar to levels of R-134a. The AIHA 

WEEL value for HFO-1234yf is 500 ppm (AIHA, 2008). This value is more than 50 times higher than the 

typical R-134a concentration measured in the GM assembly plants, which provides a sufficient margin of 

safety for assembly workers. 

4.7.2 Ecological Risks 

Honeywell has completed a set of ecological aquatic toxicity tests for HFO-1234yf. All test results 

appear to be at levels which would preclude a designation of "very toxic", "toxic" or "harmful" to aquatic 

organisms under the European Union Directive (European Union, 2006). Although EC5017 results for the 

algal and Daphnia tests (i.e., greater than 75 mg/L and greater than 83 mg/L, respectively, the maximum 

concentrations of HFO-1234yf that could be achieved in water for each test) were not clearly above 

compared to the 100 mg/L threshold below which chemicals are designated as "harmful to aquatic 

organisms", the testing laboratory's opinion, in which we concur, is that because no effects at all were 

observed at these maximum tested concentrations, the actual EC50s must be above the 100 mg/L criterion 

and HFO-1234yf would not be considered harmful to aquatic organisms. A comparison of toxicity values 

is provided in Table 4-1. 

 Fluorocarbon gases such as HFO-1234yf may also be oxidized in the atmosphere to produce 

trifluoroacetic acid (TFA). The potential for TFA formation in the atmosphere would not be a concern in 

terms of potential human exposure because the levels of TFA would be minute. An analysis by U.S. EPA 

has also indicated that TFA formation and deposition from HFO-1234yf would not pose a concern for 

ecological receptors, although the Agency did express concern over deposition in seasonal (i.e., vernal) 

water pools which are created by snow melt or rainwater and have limited potential for dilution (U.S. EPA, 

 

17 EC50 is the concentration of the test chemical in water at which half of the test organisms are adversely affected. 

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2009). However, studies of TFA deposition into such water bodies (Benesch et al., 2002) indicated no 

adverse physiological responses on microbes or plants exposed to TFA concentrations as high as 10,000 

g/L, which are in turn much higher than could be expected to be produced by fugitive emissions or 

accidental releases of HFO-1234yf. For example, it has been estimated that the average concentration of 

TFA in rainwater in the Northern Hemisphere resulting from the atmospheric oxidation of HFO-1234yf 

will be below 0.2 μg/L (SAE, 11/10/09). In addition, Luecken et al. (2009) have conducted modeling 

studies and have shown that the oxidation of HFO-1234yf will result in a peak concentration of TFA in 

rainfall of 1.3 μg/L (a level that is 80 times lower than the lowest level considered safe for the most 

sensitive aquatic organisms). 

The results of studies of the atmospheric chemistry and environmental impact of R-1234yf have 

been published recently (Hurley et al., 2008, Nielsen et al., 2007) The conclusion of the studies was that 

the products of the atmospheric oxidation of HFO-1234yf will have negligible environmental impact and 

will form the same atmospheric breakdown products as HFC-134a. 

4.8 Sensitivity analysis and description of uncertainties 

As with any risk assessment, the present work contains a number of sources of uncertainty that 

make the resulting risk estimates themselves uncertain. However, in each area of uncertainty, an effort was 

made to err on the side of generally over predicting rather than under predicting actual risks since the goal 

of this risk assessment is to ensure the chosen refrigerant does not pose a threat to human health or the 

environment. It should be noted that multiplying several highly conservative assumptions can lead to 

analysis results that may over predict risk by several orders of magnitude. 

4.8.1 Sensitivity Analysis 

 In order to better understand the impact of some key assumptions on the results of the fault tree 

analysis, CRP1234 members conducted a sensitivity analysis. Parameters that were felt to be uncertain 

were changed to other plausible values and the resulting frequency of the top event (i.e., either exposure to 

an open flame or exposure to HF above a health-based limit) was then compared to the value obtained 

using the original assumption. The parameters evaluated in the sensitivity analysis of the HFO-1234yf 

fault trees consisted of the following: 

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Secondary fires. In developing the fault trees, CRP1234 members considered the 

possibility that collisions could produce secondary fires (e.g., due to ignition of fuel or 

motor oil) that could then later cause ignition of refrigerant and HF generation. A key 

limitation in such a scenario was the factor of timing; if refrigerant were to be released due 

to a vehicle collision, the release would start and end quite soon after the collision. The 

refrigerant is likely to be dispersed to a degree that the refrigerant will no longer be in the 

flammable concentration range and any HF produced would be very limited. A relatively 

longer period of time would be required for a secondary fire to reach the stage where it 

could cause a leak of refrigerant from the AC system and the presence of refrigerant would 

aggravate the fire event. To assess the impact of the secondary fires, this source of ignition 

was eliminated from the fault trees, leaving hot surfaces and sparks as sources of 

refrigerant ignition and subsequent HF generation. This change had no impact on the 

estimated top event frequencies in any of the relevant collision trees (HF4, HF4.2, HF5) 

because the hot surfaces or collision related sparks are the most significant pathways of HF 

generation in these fault trees. 

HFO-1234yf ignition temperature. Testing by Honeywell, Exponent, and INERIS have all 

indicated that temperatures of approximately 750°C are required in order to ignite 

refrigerant/oil mixtures. Nonetheless, because the ignition test conditions did not 

encompass all possible situations, the FTA assumed that ignition could occur at 550° C. A 

key input to the FTA was therefore the probability that the hot surface would be at this 

temperature at the time of refrigerant release. Such an assumption was considered to be 

particularly conservative in the case of vehicle collisions because CRP1234 members have 

data indicating that vehicle engines cool rapidly when a vehicle is no longer operating (as 

would be expected after a collision, see Attachment T). The sensitivity analysis therefore 

looked at the impact of changing the “relevant temperature” for HFO-1234yf ignition to 

700°C. This change was made in fault trees I5, HF4, HF4.2, and HF5 (all related to 

collision scenarios involving an engine compartment release). This change decreased the 

frequency of top events in each of these trees. However, in the case of the exposure to an 

open flame, the overall FTA risk is unchanged because fault tree I3 (describing release and 

ignition in the passenger compartment) predominates and the engine compartment hot 

surface is not involved in this scenario. In the case of HF exposure above the HBL, overall 

risk decreases slightly because fault tree 4.2 contributes approximately equally with fault 

tree 7.2 (the non-AC component fire) to the top event. Since fault tree 7.2 is unaffected by 

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this change, the overall HF exposure risk remains at the same order of magnitude. 

Therefore, the impact of extreme engine events which might create these high temperatures 

is not considered a significant risk driver.

Collision Scenarios. In addressing the collision scenarios, the OEMs evaluated the 

scenario as a series of required events that must occur in order for a collision to lead to 

exposure to an open flame or HF above the HBL. For the sake of simplicity and clarity, 

these were treated as independent probabilities and data were extracted from the GIDAS 

database for inclusion in the fault trees (Appendix A). In reality these events are interrelated in that a collision which is severe enough to damage the fairly well protected 

evaporator is also highly likely to damage the other parts of the AC system, break the 

vehicle windows, etc.. Thus treating the contributing probabilities as independent is likely 

to overestimate the probability of the top event. Collision data obtained by Toyota 

(Attachment T) indicate no instances of a collision causing evaporator damage but not 

damaging other relevant parts of the vehicle and thus support this idea. If the probability 

of a collision-related HFO-1234yf release into the passenger compartment in the absence 

of other vehicle damage is set to zero, then fault trees I3, I4, and HF3 are eliminated and 

the top event probabilities estimated in fault trees I5, HF4, HF4.2, HF5 and HF8.1 are 

greatly reduced. The probability of exposure to an open flame is reduced to 4 x 10-19, or by 

a factor of approximately 23,000. With respect to the FTA relating to HF exposure above 

a HBL, the results are not changed substantially because fault tree 7.2 (HF generated by a 

fire caused by a non-AC component) remains a key risk driver. 

Effect of rain on HF aspiration. The HFO-1234yf fault trees included an assumption that 

in cases where HF was aspirated into the passenger compartment from the engine 

compartment, attenuation of the HF concentration by rain would occur if the engine 

compartment release and HF generation happen on days with precipitation. This assumed 

that the pathway from the engine compartment to the air intake vents at the base of the 

windshield would be open to the weather. This issue is relevant to three fault trees: 

fault tree 4.2 (aspiration of HF into the vehicle compartment from the engine 

compartment in the case of refrigerant decomposition related to a collision), 

fault tree 7.1 (aspiration of HF into the passenger compartment from the engine 

compartment as a result of a non-AC component fire), and 

fault tree 8.1 (aspiration of HF into the passenger compartment as a result of an AC 

component failure). 

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Note that this concern does not apply to fault trees HF5, HF6 and HF7.2, all of which include the rain 

parameter but which also involve exposures outside of, or around, the vehicle where environmental factors 

such as rain could be present. Discussions among CRP1234 members suggested that not all vehicle 

situations would be represented by the assumption. For example, in some collisions the pattern of hood 

buckling might cover the path between the engine compartment and the hood. In addition, data are lacking 

to quantify the actual ability of rain to "scrub" HF from air. In order to address the issue, the CRP1234 

members examined the effect of changing the value of the parameter "Probability HF not removed by rain" 

to 1.0 (i.e., assuming HF is never removed by rain to any degree in any situation). It was acknowledged 

that this was a highly conservative action, since some level of attenuation should be expected in at least 

some situations. Nonetheless, the impact of this change in parameter assumption was limited. The 

probability of the top event in HF4.2 increased by approximately 20%, from 2.6 x 10-12 to 3.0 x 10-12, the 

probability of the top event in HF7.1 increased from 1.2 x 10-15 to 1.5 x 10-15, and the probability of the top 

event in HF 8.1 increased from 1.2 x 10-18 to 1.5 x 10-18. The overall impact on the results of the risk 

assessment is minimal because only HF4.2 is a risk driving scenario and the change in the probability of the 

top event for this fault tree (an increase of 20%) is limited. 

The results of the sensitivity analysis are summarized in Table 4-2. 

4.8.2 Sources of Uncertainty in the Risk Assessment 

 Areas of uncertainty present in the risk assessment include the following: 

Modeling Uncertainty – Both the CFD and perfect mixing models require simplification of the real world 

situation. It is possible that details not included in the models could affect processes such as airflow 

patterns and cause inaccurate results. While such errors may be possible, the magnitude of their effect is 

likely to be small relative to the low risk levels calculated in the risk assessment. It should be noted that 

passenger compartment concentration modeling was conducted with a single vehicle type representative of 

average cabin size and charge; neither dual evaporator system vehicles nor refrigerant cooled hybrid 

vehicles were included in the modeling. The results of the modeling were intended to characterize the 

entire vehicle fleet, and as such needed to be based on a representative vehicle. Modeling uncertainty is 

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less of a significant concern for the Phase III risk assessment than for earlier efforts because substantial 

measurement data were obtained which supported the modeling results. 

Measurement Uncertainty – Measurement uncertainty concerns the ability of test instruments to accurately 

and precisely detect the parameter of interest and test protocols to effectively simulate real-world 

conditions (i.e., test validity). In terms of instrumental accuracy and precision, the various tests conducted 

or commissioned by CRP1234 (e.g., HF measurement, LFL/UFL measurement) relied on appropriate 

instrumentation and well-established protocols for sample collection and analysis. Concentration 

measurements by GM indicate that the concentration in the vehicle interior could be +/- 1.0%. One 

limitation with respect to the HF analyses is that HF samples (e.g., those collected by Hughes Associates) 

were collected using sampling pumps as time-weighted averages and the peak concentration within that 

timeframe is unknown. However, the 3-minute TWA HF data were compared to the AEGL-2 value 

established for 10-minute exposures. The uncertainty in peak HF concentrations may therefore be to some 

extent addressed by the use of an exposure limit determined to be acceptable for longer periods of 

cumulative exposure. 

 Uncertainty due to test validity was an important consideration in both bench scale and field 

(vehicle) testing of HFO-1234yf ignition potential and HF generation. In both cases, tests were designed in 

an attempt to encompass worst-case situations. Test conditions were carefully developed by OEM 

engineers to simulate as much as possible plausible worst-case operating conditions that could be present in 

a failure situation. Thus, HF exposure in the vehicle cabin was measured with hood seals removed and the 

AC blower running at top speed to simulate worst-case HF aspiration into the vehicle cabin. Samples were 

collected in areas of greatest concern for potential human exposure. In such tests, vehicles were stationary 

and shielded from external airflow, both factors which minimized dilution of the released refrigerant or HF. 

Parameter Uncertainty – A large number of the parameters used in the FTA were based on expert judgment 

and only limited data. In many cases, the values reflect conservative assumptions that are likely to result in 

overestimates of the actual risks. For example, air exchange rates used in the AC service scenario modeling 

are likely unrealistically low for most facilities. Since the results of that analysis were used to represent 

risks associated with all vehicle AC service operations, the predicted risks are again likely to be overly 

high. 

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Toxicological Uncertainty – There are a number of uncertainties involved in applying toxicological data 

obtained with experimental animals in a risk assessment of human health risks. Studies typically involve a 

limited number of animals per dose group, the species involved may not be as sensitive as humans, and 

specially sensitive subpopulations may exist in the human population which are not addressed by the 

uniform strains of animals used for experimental testing. With respect to HF, one also must extrapolate 

across different exposure durations. These uncertainties are addressed by again being cautious in the 

application of the toxicological data. For example, endpoints based acute toxicity are obtained from animal 

studies which generally involve 4 hours of continuous exposure to the test compound whereas human 

exposures would be expected to be substantially shorter based on the modeling results. Furthermore, 

human health-based exposure criteria used in the risk assessment were based on either a No Observed 

Adverse Effect Level (NOAEL) from the animal study (e.g., for cardiac sensitization) or an effect level 

divided by a safety factor (e.g., for acute toxicity or anesthetic potency). These values are biased towards 

the conservative. For example, exposures somewhat above the NOAEL may not produce any adverse 

effects, the NOAEL is simply the highest dose tested where effects were not seen. 

 Based on the rationale stated above, it can be reasonably expected that the top event probabilities 

associated with refrigerant exposures developed in this risk assessment are more likely to overestimate 

rather than underestimate the likelihood of such events that might experienced by real world vehicle 

passengers or service personnel. This is appropriate if one goal of the risk assessment is to ensure health 

and safety. 

4.9 Conclusions Regarding HFO-1234yf 

HFO-1234yf offers equivalent or lower toxicity compared to R-134a in terms of both acute human 

health effects and ecological effects. It has been shown that HFO-1234yf is a non Ozone Depleting 

Substance and that is has a GWP of 4 which meets the European Union F-gas requirements. HFO-1234yf 

is flammable, although work conducted by DuPont, Honeywell, and INERIS indicates the ignition potential 

of this chemical is far less than that of other flammable refrigerants (e.g., R-152a). Nonetheless, due to the 

flammability of HFO-1234yf, the risk assessment addressed both health effects and potential ignition. 

Toxicity data for HFO-1234yf are available from a large number of studies conducted by 

independent testing laboratories commissioned by the manufacturers. These toxicity data, when combined 

with the results of air modeling to predict exposure concentrations, indicate that potential HFO-1234yf 

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exposures of vehicle passengers in the event of corrosion leaks or collision-related leaks would be below 

levels associated with potential health effects. Similarly, HFO-1234yf exposures of AC service technicians 

are expected to be below levels associated with potential health effects. Potential exposures of automotive 

assembly workers, which could occur due to small losses of refrigerant during vehicle assembly and/or 

service, were also judged to be below levels of health concern. Thus, potential health effects from 

exposures to HFO-1234yf were not judged to be a source of concern in the risk assessment. 

When the results for these potential ignition scenarios are combined, the fault tree analysis 

indicates that the probability of an occupant (or former occupant) being exposed to an open flame due to 

HFO-1234yf ignition will be extremely low, on the order of 9 x 10-14 events per hour of vehicle operation. 

This was essentially entirely attributable to the scenario involving release and ignition of refrigerant as a 

result of a vehicle collision. Note that not all ignition events leading to the presence of an open flame will 

lead to potential injury. The probability of exposure to an open flame in the case of AC service technicians 

were also found to be inconsequential because modeled air concentrations of HFO-1234yf in the service 

areas only reached the LFL at a distance of 10 cm or less from the source of the AC system leak (DuPont, 

2009). The CRP1234 members judged that ignition sources would not be located this close to the AC 

system given the presence of other flammable materials in the engine compartment. Injury to AC service 

technicians from rupture of high pressure refrigerant recharge hoses was also evaluated in the FTA and 

found to result in 3 x 10-8 reportable vents per vehicle service hour. 

CRP1234 also evaluated potential exposures to HF that might occur in the event of HFO-1234yf 

ignition or thermal decomposition. The risk assessment considered potential HF exposures in the passenger 

compartment, either from refrigerant entering the compartment after a collision and being ignited or HF 

entering the compartment after being produced in the engine compartment. The risk assessment also 

considered potential HF exposures in the under hood area for various receptors (former occupants, good 

Samaritans) and under different scenarios (i.e., collision, AC equipment failures, vehicle fires related to 

non-AC equipment failure, vehicle fires to due vandalism). When the results for these potential HF 

exposure scenarios are combined, the fault tree analysis indicates that the probability of an individual being 

exposed to HF above the relevant health-based limit will be extremely low , on the order of 5 x 10-12 events 

per hour of vehicle operation. This is almost entirely and equally attributable to two scenarios: HF 

generation by vehicle fires arising from failure of non-AC vehicle components and decomposition of 

refrigerant in the engine compartment after a vehicle collision with subsequent aspiration of HF into the 

vehicle cabin. 

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 Table 4-3 shows the probabilities of the top events associated with use of HFO-1234yf in mobile 

air conditioning systems compared to probabilities of other adverse events that individuals might 

encounter. This comparative risk analysis demonstrates that the estimated probabilities for vehicle 

passengers of adverse events associated with accidental refrigerant release of HFO-1234yf are less than the 

probabilities associated with other commonly accepted risks.. The probability of injury of vehicle service 

technicians (associated with the failure of high pressure equipment hoses) are also well below the rate of 

other workplace injuries. Thus the human health and flammability risks associated with use of HFO1234yf as a new refrigerant for MAC systems should be considered acceptable. 

 Finally, as part of the SNAP evaluation process, the U.S. EPA is required to evaluate the risks 

posed by any new automotive refrigerant against both R-12 (the original refrigerant addressed by SNAP) 

and R-134a (the R-12 replacement). The Agency's objective is to approve the use of new refrigerants 

which provide, overall, a lower or equivalent level of human and ecological risk compared to R-12 and R134a. A comparison of the overall risks of the three refrigerants is shown in Table 4-4. As indicated in this 

table, use of HFO-1234yf in MAC systems would entail slightly increased risks due to refrigerant 

flammability and a somewhat lower occupational exposure limit relative to R-12 and R-134a. The 

increased risk is minimal because the FTA shows that refrigerant ignition and HF exposure risks are still 

well below other risks associated with vehicle operation and workplace air concentrations of refrigerant 

would be expected to be well below the WEEL value (as described in Section 4.7.1). Against this slight 

increase in potential risk, HFO-1234yf offers decreased toxicity (in terms of cardiac sensitization)18 and a 

greatly decreased ecological impact (in terms of a lower GWP). Overall, the CRP1234 Phase I, II, III risk 

assessments demonstrate that HFO-1234yf is suitable for use in direct-expansion air-conditioning systems. 

 

18 Note that the CRP1234 Phase I risk assessment, exposures to R-12 were found to exceed the health-based limit based on cardiac 

sensitization potential in the case of large or very large leaks. Thus, although neither R-12 nor R-134a were evaluated in the Phase 

III risk assessment, the much higher NOAEL value for cardiac sensitization demonstrated for HFO-1234yf may be a significant 

advantage. 

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Table 4-1. Ecological Toxicity Data for Selected Chemicals Compared to HFO-1234yf 

Chemical Usage EC50 algal 

growth inhibition 

in 96-hr test 

(mg/L) 

EC50 

Daphnia in 

48-hr test 

(mg/L) 

LC50 fish 

in 96-hr test (mg/L) 

Source 

1234yf Potential 

refrigerant 

>75 >83 >197 (carp) Honeywell 

Comparison Chemicals 

Benzene Gasoline 

component 

> 1,360 356-390 5.3 (salmon) ESIS 

Toluene Gasoline 

component 

>433 11.5L 26 (minnow) 

24 (trout) 

ESIS 

Xylenes Gasoline 

component 

10* no data 26.7 (minnow) 

>780 (carp) 

ESIS 

Pentane Gasoline 

component 

no data 9.74 no data ESIS 

Gasoline Fuel 4,700 (72-hr test) no data no data ESIS 

Ethylene glycol Antifreeze 

component 

6,500-7,500 >10,000 17,800 (trout) ESIS 

Triethylene glycol 

monobutyl ether 

Brake fluid 

component 

>500** 1740 -2,802 2,400 (minnow) ESIS 

Chlorine Disinfectant 

gas 

no data 0.017-0.085 0.08 (minnow) 

0.014 (trout) 

ESIS 

Chloroform Water 

disinfection 

by-product 

950** 66-90 18 (trout) 

71 (minnow) 

ESIS 

Malathion Pesticide 4.06 (72-hr test) 0.0009 0.04 (minnow) ESIS 

2,4-D Herbicide 33.2 417 (72 hr 

test) 

128-217 (silverside) ESIS 

R-134a Current 

automotive 

refrigerant 

no data 980 450 (trout) DuPont 

ESIS – European Chemical Substances Information System, http://ecb.jrc.it/esis 

Unless otherwise noted, algal tests were conducted with Selenastrum capricornutum. Where more than one test result was 

available in the ESIS database, the most representative number was chosen. 

* Test conducted with Cricosphaera carterae

** Test conducted with Scenedesmus subspicatus 

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Table 4-2 

Results of Sensitivity Analysis for HFO-1234yf FTA 

Parameter Change Rationale Fault trees 

affected 

Effect on top event frequencies in relevant 

trees 

Overall effect on risk 

assessment 

Removal of secondary fires after collision as a potential source of HF generation 

By the time a fire secondary to a collision starts and becomes significant enough to ignite refrigerant, the refrigerant has probably already been released and dissipated to concentrations below the LFL. 

HF4 

HF4.2 

HF5 

No change, other sources of potential HF generation (ignition or thermal decomposition due to hot surface) are more significant. 

No change. Other sources of potential HF generation (ignition or thermal decomposition due to hot surface) are more significant 

Change in "relevant" hot surface temperature for refrigerant ignition from 550 to 700°C 

To be conservative, CRP1234 assumed temperatures of 550°C could ignite the refrigerant/oil mixture in some cases even though testing has indicated temperatures in excess of 700°C would be required. Exploring the effect of this assumption is particularly relevant for the collision case where engine temperatures decline very rapidly after a collision. 

I5 I5 decreased, 9.4 x 10-15 to 4.1 x 10-16 No change, risks from 

unaffected fault tree I3 (9 x 10-14) 

still predominate 

HF4 

HF4.2 

HF5 

HF4 decreased, 6.6 x 10-15 to 9.5 x 10-16

HF4.2 decreased, 2.6 x 10-12 to 1.8 x 10-13

HF5 decreased, 5.3 x 10-16 to 6.0 x 10-17

Minimal change. The top event probability for fault tree HF4.2 decreased (2.6 x 10-12 to 1.8 x 

10-13) but the top event 

probability for fault tree HF7.2 were unaffected (2.3 x 10-12) 

Change in overall probability of a collision related release that does not break other relevant vehicle components to zero. 

CRP1234 treated the contributing probabilities for the collision scenario as independent but they are correlated and data show the likelihood of a case where the evaporator is damaged but other relevant components are intact is essentially zero 

I3, HF3 I3 eliminated 

HF3 is eliminated 

Probability of open flame exposure changes from 9 x 10-15

to 4 x 10-19. Probability of 

exposure to HF above HBL is unaffected because HF7.2 is a risk driver and is unchanged. 

Change in probability that rain does not disperse HF prior to aspiration into vehicle from 0.85 (HF4.2 and HF8.1) or 0.24 (HF 7.1) to 1.0 (i.e. no 

attenuation in any case) 

Not all vehicles designs or collision situations will permit this attenuation. In addition there are no data which quantify the extent of the possible attenuation. 

HF4.2, HF7.1, 

HF8.1 

HF4.2 increased, 2.3 x 10-12 to 2.6 x 10-12

HF7.1 increased, 1.2 x 10-15 to 5.2 x 10-15HF8.1 increased, 1.7 x 10-18 to 1.9 x 10-18

Minimal change. The top event probability for fault tree HF4.2 increased by 20%. The top event probability for fault tree 7.2 (the other risk driver) were unaffected. 

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Table 4-4. Probabilities of Various Adverse Events Compared to Probabilities of Events 

Associated with Leaks of HFO-1234yf 

Event Probability per 

vehicle per 

operating hour* 

Citation 

Probability of being in a police reported vehicle collision1 5 x 10-5 NHTSA, 2007a 

Probability of dying during a 5000-mile plane trip2 4 x 10-7 Wilson and Crouch, 1987 

Probability of vehicle collision due to vehicle brake failure3 3 x 10-7 New York State DMV 

Probability of highway vehicle fire (any cause)4 1 x 10-7 Ahrens, 2008; BTS, 2004 

Probability of an airbag-related fatality associated with a vehicle collision5 2 x 10-10 NHTSA, 2007b

Probability of vehicle occupant/former occupant experiencing HF 

exposure above health based limits associated with an HFO-1234yf 

ignition event 

5 x 10-12 Current analysis 

Probability of vehicle occupant being exposed to an open flame due to 

HFO-1234yf ignition 9 x 10-14 Current analysis 

Event Probability per 

working hour* 

Citation 

Probability of non-fatal recordable Injury at work (all occupations, per working 

hour)6 2 x 10-5 NSC, 2004

Probability of an automotive AC service technician being struck by object 

resulting in lost work days (per working hour)7 2 x 10-6 BLS, 2007 

Probability of recordable incident involving AC service technicians 

being struck with HFO-1234yf equipment hose (per hour of vehicle 

service)

3 x 10-8 Current analysis

Probability of service technician exposure to an open flame due to 

HFO-1234yf ignition (per hour of vehicle service) 1 x 10-26 Current analysis 

1 There were 6,024,000 police reported vehicle collisions in the U.S. in 2007. This is divided by the number of registered vehicles (255,748,000, 

NHTSA 2009) and the average number of hours each vehicle is operated (approximately 500 hours based on SAE J2766, Table 6). 

2 Frequency divided by 8 hours assuming plane travels between 600 and 650 mph. 3 The New York State Department of Motor Vehicles reports that there were 1753 accidents in 2008 attributed to brake failure (NYSDMV, 2008) 

and that there were roughly 11 million registered vehicles in New York State in 2008. Combining these data with the "operating hours per year" 

suggested in J2766 (~500 as an average) yields the an accident frequency per vehicle operating hour. 4 Ahrens reports that 33 highway vehicle fires are reported per hour. This is divided by the number of registered vehicles in the U.S. 

(255,748,000, NHTSA 2009) . 

5 Based on NHTSA statistics for 1990 to 2001, the average number of confirmed air-bag related fatalities per year is 16. This is divided by the 

number of hours of vehicle operation per year ((approximately 500 hours based on SAE J2766, Table 6) and the number of vehicle drivers in 

the U.S. (200 million) to arrive at an average hourly risk rate. 

6 The rate (3.7 per 100 workers) for non-fatal workplace injuries reported for 2008 by the U.S. Bureau of Labor Statistics (BLS, 2008) divided by 

the number of working hours per year in the U.S. (i.e., 2080). 7 The U.S. Bureau of Labor Statistics (2007) reports a total of 3450 cases in 2005 of automotive repair technicians and mechanics being struck 

by an object at work resulting in an injury that required time off from work. This can be divided by the number of technicians and mechanics 

working in the U.S. (954,000, BLS, 2007) and 2,080 working hours per year to yield a risk per working hour. 

* Probabilities are given in these units to be consistent with international performance standards used by the automotive industry. Results of the 

Phase I and Phase II risk assessment were given in units of "per year". Conversion between these units is possible by accounting for the 

number of vehicle operating hours per year (roughly 500 hours) and the size of the relevant vehicle fleet (e.g., roughly 250 million vehicles in the 

U.S.). Note that the outcomes of individual fault trees should not all be combined because some have different denominators (events per vehicle 

operating hour, events per year). The values shown here represent the highest estimates when combining individual fault trees with common 

units of the top level events. 

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Table 4-5 

Overall Comparison of HFO-1234yf to R-12 and R-134a 

Property R-12 R-134a HFO-1234yf Comparison among 

refrigerants

Toxicity 

Acute toxicity (LC50, ppm) 600,000 >500,000 >405,000 equivalent 

Anesthetic potency (ppm) 25,000 50,000 120,000 more favorable to HFO-1234yf 

Genotoxicity/ 

Carcinogenicity 

negative negative negative equivalent 

ASHRAE ATEL (ppm, 

acute health effects value) 

18,000 50,000 101,000 more favorable to HFO-1234yf 

ASHRAE Safety 

Classification (ppm, 

chronic health effects 

value) 

A A A equivalent 

Ecological Toxicity low low low equivalent 

Flammability not flammable not flammable weakly 

flammable 

less favorable to HFO-1234yf 

but FTA indicates risk remains 

quite low 

ODP 1.0 0 0 HFO-1234yf and R-134a 

equally superior to R-12 

100 year GWP (CO2=1) 10,600 1,430 4 much more favorable to HFO1234yf 

Estimated total lifetime 

annual refrigerant loss rate 

(grams/year) 

75 65 50 equivalent 

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5 R-744 

5.1 General Properties 

Carbon dioxide (CO2, R-744)19 has been proposed as an alternative automotive refrigerant to 

replace R-134a. At low concentrations CO2 is a normal component of both the Earth’s atmosphere 

(approximately 400 ppm) and exhaled breath (approximately 35,000 ppm). Although CO2 is a gas which 

contributes to global warming, it is a fairly weak greenhouse gas on a per kilogram basis and is proposed 

for use in automotive air conditioning systems as a replacement for R-134a which has a global warming 

potential estimated to be 1,430 times greater (IPCC, 2007). CO2 is not flammable and therefore the 

potential effects evaluated in this assessment are limited to human toxicity. 

 Occupational exposure limits have been established for CO2 by the U.S. Occupational Health and 

Safety Administration (OSHA), the U.S. National Institute for Occupational Safety and Health (NIOSH), 

the American Council of Governmental Industrial Hygienists (ACGIH) and the Deutsche 

Forschungsgemeinschaft (DFG), each of which stipulates an 8-hour time-weighted exposure limit (TWA) 

of 5,000 ppm. Occupational exposure limits for other time frames are discussed later in this report. The 

ODP of CO2 is zero while the GWP is 1.0 (CO2 being the standard to which all other greenhouse gases 

are compared). Thus, use of CO2 as an automotive refrigerant will satisfy the European Union F-gas 

directive. 

5.2 Toxicity Assessment for CO2

5.2.1 Summary of Relevant Data 

New toxicological testing has not been conducted for CO2 as part of the evaluation process. 

Toxicity data have been published previously in the scientific literature, as early as the 1930s. This 

literature has previously been reviewed by a number of authors and authoritative bodies to establish 

health guidance values (NRC, 1996; NIOSH/CDC, 1996; U.S. EPA, 2006; Scott et al. 2008). These 

 

19 We will use the term CO2 when referring to general properties of the chemical and the term R-744 when referring specifically 

to its application to air conditioning. 

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reviews constituted the primary input for the toxicity assessment conducted here. A detailed review of the 

primary literature was not conducted as part of this assessment. 

 The available data concerning the toxicity of CO2 are summarized in Table 5-1. The key health 

concern with CO2 is central nervous system depression, resulting in anesthetic-type effects. These effects 

can range from headache and dizziness to loss of consciousness, coma and death at very high exposures 

(Blackwell et al., 2006). As with other chemicals, there is a time-concentration dependence such that the 

effects seen with prolonged exposures at low concentrations (e.g., 20,000 ppm for several hours) can be 

observed at higher exposure concentrations in much shorter time frames (e.g., several minutes at 60,000 

ppm). Thus the time frame of exposure and the exposure concentration are equally important 

determinants of health effect. Data summarized by U.S. EPA concerning the concentration-time 

relationship of CO2 as regards central nervous system depression are provided in Table 5-2. 

 Besides the leakage into the passenger compartment from the AC system, one must also consider 

the contribution to CO2 levels in the passenger compartment from passenger respiration. A vehicle 

occupied by 1.59 passengers (the default assumption for the U.S.) for a typical driving time of 20 minutes 

could have a baseline CO2 concentration in air of approximately 1,800 ppm (0.18%).20

Data on anesthetic effects are based primarily on human exposure studies. While such studies 

have the advantage of being clearly relevant to humans (as opposed to studies in rodents), human studies 

are often limited by the number of subjects and the methods employed. This is particularly true when 

assessing anesthetic effects which are typically evaluating using subjective reports rather than objective 

measurements. The exposure concentrations associated with specific health effects should therefore be 

viewed as general approximations. 

 In this assessment, the primary concern is the potential for vehicle occupants to become 

disoriented or otherwise incapable of exiting the vehicle in the event of a CO2 release. Since the 

occupant’s exposure will be of short duration (i.e., < 1 hour), standard occupational exposure limits (i.e., 

 

20 Specifically, we assume no loss of CO2 to air outside the cabin and no change in passenger CO2 output as concentrations rise. 

A typical ambient background concentration of CO2 in urban areas is 450 ppm or 810 mg/m3

. The typical individual respiration 

rate is 15 breaths per minute and the corresponding breath volume is 500 cm3

. The CO2 concentration in exhaled air when at rest 

will be approximately 3.5% or 63,000 mg/m3

. Assuming the vehicle is occupied by an average of 1.59 passengers for an average 

trip time of 20 minutes and an air exchange rate in the moving vehicle of 1.7 air changes per hour (ACH), the baseline CO2

concentration in the passenger cabin can be calculated as: 

810 mg/m3 + (63,000 mg/m3 x 500cm3 per breath x 15 breaths/minute x 1.59 passengers x 20 minutes) x (1 - 1.7 ACH x 0.33 hour) divided 

by a 2.5 x 106

 cm3 cabin volume 3,100 mg/m3 or roughly 1,800 ppm 

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8-hour TWAs) would be inappropriate comparison values. The U.S. National Institutes for Occupational 

Safety and Health (NIOSH) has established an Immediately Dangerous to Life and Health (IDLH) value 

for CO2 of 40,000 ppm, which is defined as “[t]he maximum level to which a healthy individual can be 

exposed to a chemical for 30 minutes and escape without suffering irreversible health effects or impairing 

symptoms.”21 (U.S. EPA, 2006) We have used this value as the health-based criteria for assessing 30-

minute CO2 exposures. It is also likely that shorter term exposures may be more critical given the rapidly 

changing refrigerant concentration profile after a release. Based on a review of the scientific literature, 

we therefore used 5.5% (55,000 ppm) as a limit for 5-minute exposures and 9% (90,000 ppm) as a limit 

for 10 second exposures. The 5-minute value is based on an interpretation of several studies. In the study 

by Sayers et al. (1987), subjects exposed to 6.5% CO2 for 5 minutes demonstrated significant 

performance decrements on a number of logic-based tasks. The relevance to these types of tasks to 

vehicle operation or escape in the event of a collision is uncertain and it is worth noting that subjects 

exposed to this level did not self rate their alertness any differently than those exposed to air alone. In the 

study by Gellhorn (1936), 1 to 2 minutes of exposure to 6% CO2 lead to "decreased visual intensity 

discrimination". We are inclined to give this study limited credence due to its date and the general 

difficulty in accurately measuring gas concentrations in that era. In addition, the significance of the 

reported results in relation to risk avoidance behavior is uncertain. More relevant is a third study, by 

Schulte (1964) which indicated CNS depression at 5% CO2 only with exposures of "several hours". 

Taken together, these studies suggests that exposures to 5.5% CO2 for 5 minutes would be unlikely to 

produce effects that would impair an individual’s ability to escape from a hazardous situation. A study by 

Lambertsen (1971, as reported by NRC, 1996) reported CNS effects at CO2 exposure concentrations of 

100,000 ppm for exposures as short as 1.5 minutes. Thus, keeping the peak concentration below 90,000 

ppm should also be protective. Note that these levels are similar to those employed by Blackwell et al.

(2006) in their earlier assessment of CO2 as an alternative refrigerant. 

We are aware that the U.S. EPA has proposed a CO2 exposure limit of 30,000 ppm for 15 minutes 

as well as a peak exposure limit of 40,000 ppm.. Although the basis for the 30,000 ppm value is not 

entirely clear, it may have the same basis as the ACGIH short term exposure limit (STEL) of 30,000 ppm. 

A STEL is a value allowing for several 15-minute exposures during the course of the day with the 

exposure pattern also recurring over the worker’s full job tenure. Such a value is not appropriate for the 

situation being considered here where exposure more than once per lifetime will be extremely unlikely. 

 

21 Although the NIOSH IDLH definition (applicable to all substances for which an IDLH has been established) references a 

"healthy individual", USEPA's assessment of CO2 Toxicity (Blackwell et al., 2006) indicated no particular added susceptibility 

for population subgroups such as children, the elderly or the infirm. Thus the 40,000 ppm 30-minute exposure limit should be 

applicable to all members of the population. 

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The NIOSH IDLH value is specifically intended to protect against short-term exposures (30-minute 

exposures) that impair the individual’s ability to escape from a dangerous situation. This value is 

therefore directly relevant to the exposure scenarios being considered in this risk assessment. Although 

the NIOSH value is intended primarily for working populations, a review of several specially sensitive 

subgroups (children, the elderly, individuals with particular medical conditions) conducted by the U.S. 

EPA (Blackwell et al., 2006) suggested that non-working groups in the U.S. population would not be 

significantly more sensitive to the anesthetic effects of CO2 relative to workers. With respect to the U.S. 

EPA's proposed peak exposure value of 40,000 ppm, we also believe this value is inappropriate since it is 

equivalent to the NIOSH IDLH, a value which is based on 30-minute exposures. Tables contained in the 

U.S. EPA's review of CO2 toxicity studies (ERG, 2009) suggests that peak (i.e., 10 seconds or less) 

exposures would need to be in the range of 100,000 to 120,000 ppm before they presented a serious risk 

of an individual not escaping from a hazardous situation (also suggesting our use of 90,000 ppm as a peak 

concentration is appropriate). 

The CRP1234 risk assessment also considered the CO2 exposure scenario which involved 

individuals who may be sleeping in a vehicle when a release occurs. Such individuals might include 

parents waiting for children to finish extracurricular activities or salesmen taking naps in between on-theroad meetings. In such a case, levels of exposure associated with mild disorientation, headache or 

dyspnea would not be of concern because the individual is not involved in a situation where the ability to 

take quick action is critical. However, because CO2 is odorless, the risk for such individuals is that they 

will not regain consciousness in time to prevent asphyxiation in the event of a large CO2 release. A 

review of the literature suggests that an exposure limit of 60,000 ppm (6%) for 30 minutes is appropriate 

for evaluating this scenario. A 30-minute time frame was chosen because it represents both the duration 

of a typical nap and the time that CO2 concentrations in the passenger compartment would not be 

significantly reduced by passive leakage through the vehicle to outside air. Several studies which 

investigated CO2 exposures in the 6% range (Gellhorn, 1936; White et al., 1952; Schulte, 1964) did not 

report unconsciousness or CNS depression even with exposures up to several hours. Unconsciousness 

was reported in studies conducted at 7-10 percent CO2 for a “few minutes” (Aero Medical Association, 

1953; Flury and Zemik, 1931; Hunter 1975, and Schaefer, 1951) although unconsciousness was not 

reported in the Dripps and Comroe (1947) study which evaluated 7.6% CO2 exposures for 2.5 to 10 

minutes. Taken as a whole, these studies suggest that 30 minute exposures to 6% CO2 would not pose a 

significant risk for sleeping individuals. 

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We are also aware of an analysis contracted by U.S. EPA (Rice, 2006) which suggests that shortterm exposures to CO2 could result in panic attacks (Beck et al., 1999) or QT prolongation and cardiac 

arrhythmogenicity (Roden, 1996; Kiely et al., 1996). Regarding the former, Beck et al. conducted a 

study examining panic and anxiety-related symptoms in individuals exposed to 5% CO2 (50,000 ppm) for 

5-minutes. Severity of panic symptoms reported by “normal” members of the subject group appeared to 

be mild (never more than 20 on a 0 to 100 scale). Among subjects diagnosed with pre-existing panic 

disorder, the average anxiety level reached a peak of roughly 45 on a 100 point scale. It is not clear if this 

level of anxiety would pre-empt taking appropriate action in the case of an emergency. Presumably, 

individuals suffering from panic disorder must have experience in maintaining their driving ability in the 

event of panic attacks which may be precipitated by various causes. The implications of the study design 

for assessing a “real world” event is also unclear. Subjects were wearing exposure masks and were aware 

they were being exposed to various gases, a situation which would itself tend to promote low level 

anxiety. We therefore do not believe this study requires revision of the 5-minute exposure limit of 60,000 

ppm discussed previously. 

Regarding QT prolongation and arrhythmogenicity, the two studies cited in the U.S. EPA report 

involved a 30-minute exposure to 6% CO2 and a 24-minute exposure to 7% CO2 (Roden, 1996; Kiely et

al., 1996). The results of these studies are consistent with our selection of a 4% CO2 level as a 30-minute 

threshold and a 6% level as a 5-minute threshold. 

There are no data to suggest that CO2 causes cardiac sensitization, an effect significant for other 

refrigerant. CO2 is structurally different from chemicals known to cause this effect (i.e., CO2 lacks 

halogenation and carbon-carbon bonds). Data are limited for evaluating the potential of CO2 to cause 

chronic health effects such as developmental toxicity, liver toxicity or cancer. An extensive review 

conducted by the U.S. NRC (1996) reported that in some studies, multi-week exposures to CO2 in the 

10,000 to 30,000 ppm range had effects on acid-base balance, breathing and heart rates, and calcium 

metabolism. However other studies using similar concentrations and timeframes reported no such effects. 

Effects that were reported would generally be expected to be reversible on cessation of exposure. None of 

the studies evaluated followed the standard protocols currently used for testing of new chemicals in the 

U.S. and Europe. No reliable data on carcinogenicity or genotoxicity were reported. 

Based on the toxicological data and lack of flammability, CO2 has been placed in the "A1" safety 

group under both the ANSI/ASHRAE and ISO 817 refrigerant safety classifications. According to the 

U.S. EPA SNAP regulations, each new refrigerant will also be evaluated in terms of potential health risk 

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relative to the current MAC refrigerant, R-134a, a comparison of key health indices for R-744 relative to 

R-134a is included in Table 5-3. 

5.2.2 Identification of Health-Based Limits for R-744 

 Unlike the case with other potential refrigerants, cardiac sensitization is not a potential endpoint 

of concern for CO2. However, the narcotic effects of CO2 may be significant and appear to occur fairly 

quickly at sufficient concentrations. For scenarios where the vehicle occupant or other involved 

individuals were assumed to be awake at the time of release, we evaluated CO2 exposures based on three 

time measures: peak (maximum) concentration, 5-minute TWA, and 30-minute TWA using the following 

limits: 

Peak concentrations = 90,000 ppm (9%) 

Peak 5-minute TWA = 55,000 ppm (5.5%) 

Peak 30-minute TWA = 40,000 ppm (4%) 

For the scenario involving an occupant who is asleep in the vehicle during a refrigerant release event, 

passenger compartment CO2 concentrations were evaluated using an exposure limit of 6% averaged over 

30 minutes. 

5.3 Hazards to be Quantitatively Addressed by FTA 

For R-744, the critical hazard is exposure to the refrigerant at levels above a health-based limit. 

R-744 is not flammable nor does it have decomposition products. In developing the exposure scenarios 

for R744, the CRP members considered the various triggering events which could cause R-744 to be 

released, the location of the release, and the specific type of individual (e.g., vehicle occupant, bystander, 

service worker) that might be involved. Based on detailed discussions among the CRP members, each 

scenario was evaluated to determine whether evaluation via FTA was required or whether the scenario 

could be disregarded based on already existing knowledge about the refrigerant or vehicle operation. The 

results of this deliberation are shown in Table 5-4. 

 As was the case with HFO-1234yf, the earlier Phase II risk assessment of R-744 included 

evaluation of small, medium and large corrosion leaks of the evaporator. Based on the tendency of 

corrosion leaks to release refrigerant incrementally as they increase over time, it does not appear likely 

that a sudden release of a full charge of refrigerant due to a corrosion leak represents a credible exposure 

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scenario. However, a gradual leak could present a risk for an individual sleeping in a vehicle for an 

extended period of time. Therefore this scenario is evaluated in the FTA but only for individuals who are 

sleeping in the vehicle during the time when the gradual release occurs. 

 The vehicle operation scenarios that are evaluated in the FTA can be summarized as follows: 

A individual sleeping in a stationary vehicle during which time a release of R-744 

refrigerant occurs due to an evaporator leak. 

A collision causing release of R-744 into the passenger compartment with the vehicle 

occupants present (also evaluated in Phase I and Phase II). 

A collision causing release of refrigerant into the engine compartment and a trapped 

occupant being exposed to R-744 which was released into the engine compartment and 

then aspirated into the passenger compartment via the air intakes located just above the 

hood. 

An AC system leak in an operated vehicle causing R-744 to be released into the engine 

compartment and then aspirated into the passenger compartment via the air intakes 

located just above the hood. 

The boundary conditions (i.e., operating conditions, environmental conditions, individual actions) used in 

the FTA of the vehicle operation and AC service scenarios were similar to those provided in Tables 2-5 

and 2-6, aside from those relating to refrigerant flammability. 

The CRP1234 members considered potential effects on bystanders who might be present near a 

vehicle when a R-744 release occurs. Because R-744 is heavier than air and would settle towards the 

ground rather than move laterally from the vehicle it was decided that such a release would not be a risk 

for bystanders. 

 In addition to the vehicle operation scenarios, the CRP1234 members also decided to evaluate 

potential risks for certified AC service technicians. Uniform mixing models used as part of the CRP1234 

Phase I and Phase II efforts indicated that R-744 concentrations in service areas would not exceed healthbased limits for CO2. The FTA also considered risks for AC service workers related to physical injury 

from a rupture of high pressure service lines. 

 As noted in Section 2, risks for DIYers are not being addressed via FTA in the Phase III risk 

assessment because the new vehicle service standards (J639, J2845, and J2911) will require that servicing 

be performed by certified AC service technicians. Warning labels will be included on the AC unit 

indicating that only service by certified individuals is acceptable. By designing the standards this way it is 

the expressed intention of the OEMs to prevent service to the vehicle by anyone but a properly training 

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technician from servicing the AC system. Therefore it is not appropriate to evaluate the risks of 

individuals who intentionally disregard the manufacturer's warnings and instructions. 

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Table 5-1. Summary of Available Toxicity Data for CO2

Endpoint CO2 Concentration 

Acute (LC50) (ppm) Approx. 400,000 (Mitsuda et al., 1967)* 

Anesthetic effect (ppm) 40,000 (30-minutes) 

Cardiac sens. No effect level (ppm) Not tested/not expected 

Worker exposure limit (ppm) 5,000 ppm (8-hour TWA) 

40,000 (IDLH) 

28-day NOAEL (ppm) No data#

90-day NOAEL (ppm) No data#

Developmental toxicity NOAEL (ppm) No data#

Genotoxicity No data#

Carcinogenicity No data#

Health-based limit (ppm) 

90,000 (peak) 

55,000 (5-min TWA) 

40,000 (30-min TWA)¤

60,000 (30-min TWA) ¤

ODP 0 

GWP 1 

* Study exposure was for 3 hours rather than 4 hours. Animal mortality was 21%. 

IDLH – concentration assumed to be immediately dangerous to life and health due to escape impairing effects 

(narcosis, anesthesia) 

# Standard tests for these endpoints have not been conducted. In subchronic studies that have been conducted, 

reports of effects are variable and not consistent. ¤ The 4% value indicates slight disorientation, the 6% value indicates a threshold for loss of consciousness.

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Table 5-2. Toxicity Effects Associated with Carbon Dioxide Exposures for Different Time/Duration 

Combinations 

CO2

Concentration 

(%) 

Time Adverse Effects References 

17-30 0-60 

seconds 

Loss of controlled activity, 

unconsciousness, death 

OSHA 1989; CCOHS 1990, 

Dalgaard et al. 1972; CATAMA 

1953, cited in USEPA 2000; 

Lambertsen 1971 

> 10-15 1-3 minutes Dizziness, drowsiness, muscle 

twitching, 

unconsciousness 

Wong 1992; CATAMA 1953; 

Sechzer et al. 1960, cited in 

USEPA 2000 

7-10 1.5-60 

minutes 

Headache, increased heart rate, 

shortness of 

breath, dizziness, sweating, rapid 

breathing 

Wong 1992; Sechzer et al. 1960 

and OSHA 1989, cited in USEPA 

2000 

7.5 5 minutes Significant performance decrement Sayers 1987 

6 1-2 minutes Hearing and visual disturbances White et al. 1952, cited in Wong 

1992 

 Several 

hours 

Tremors Schulte 1964, cited in Wong 1992 

5 Not stated Significant degradation in pilot 

performance during landing; 

unacceptable increase in touch 

down sink 

rates 

Wamsley et al. 1975, cited in 

Wong 1992 

4-5 4 hours Drop in body temperature (one 

degree); no deficit in performance 

on Army Intelligence Test 

Brown 1930, cited in Wong 1992 

 A few 

minutes 

Headache, dizziness, increased 

blood pressure, shortness of breath 

Schulte 1964, Schneider and 

Truesdale 1922, Patterson et al.

1955, cited in USEPA 2000 

3 1 hour Mild headache, sweating, dyspnea 

at rest 

Schulte 1964, cited in USEPA 

2000 

2 Several 

hours 

Headache, dyspnea upon mild 

exertion 

Schulte 1964, cited in Wong 1992 

Taken from Blackwell et al., 2006. Refer to that document for details on citations. 

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Table 5-3. Comparison of Toxicity of CO2 to the Toxicity of R-134a 

Endpoint CO2 R-134a

Acute (LC50) (ppm) Approx. 400,000 >500,000 

Anesthetic effect (ppm) 40,000 (30-minutes)* 81,000* 

Cardiac sensitization no effect level (ppm) Not tested/not expected 50,000 

Worker exposure limit (ppm) 5,000 ppm (8-hour TWA) 

40,000 (IDLH) 

1,000 (PEL) 

1,000 (MAK) 

28-day NOAEC (ppm) No data† 50,000 

90-day NOAEL (ppm) No data† 50,000 

Developmental toxicity NOAEL (ppm) No data† 10,000 [rat study#] 

40,000 [rabbit study##] 

Genotoxicity No data† Negative 

Carcinogenicity No data† Negative 

Health-based limit (ppm) 

(all based on anesthetic effects) 

90,000 (peak) 

55,000 (5-min TWA) 

40,000 (30-min TWA)¤

60,000 (30-min TWA) ¤

50,000 (cardiac sensitization – 5 min 

TWA) 

141,000 (acute toxicity) 

64,800 (anesthetic effects, 30 min) 

ODP 0 0 

GWP 1 1,430** 

* CO2 data are for humans (NIOSH IDLH value) whereas R-134a data are for rats. 

**Based on a 100-year time horizon and relative to the global warming potential of CO2. IPCC, 2007 

Shading highlights values indicating greater toxicity/environmental hazard compared to R-134a. 

† Standard tests for these endpoints have not been conducted. In subchronic studies that have been conducted, reports of effects are 

variable and not consistent. 

# Hodge et al., 1980 ## Collins et al, 1995 

 ¤ The 4% value indicates slight disorientation, the 6% value indicates a threshold for loss of consciousness.

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Table 5-4. Possible R-744 Hazard Scenarios Considered for Evaluation via FTA in CRP Phase III 

Triggering Event Location of Potential Exposure Exposed Individual

Type of Hazard Fault Tree 

ID Refrigerant exposure 

(toxicity) 

High Pressure 

Equipment 

Injury 

Collision, refrigerant released into passenger compartment Passenger compartment Vehicle occupant Evaluate via FTA NA CO2_3 Collision, refrigerant released into engine compartment 

Passenger compartment (aspiration from damaged engine compartment via

air intakes) 

Vehicle occupant Evaluate via FTA NA 

CO2_9 

Collision, refrigerant released into engine compartment Engine compartment 

Former occupant investigating engine or good Samaritan assisting trapped occupants 

No evaluation via FTA. 

Refrigerant is heavier than air 

and would present no risk 

near top of engine 

compartment. 

NA 

NA 

AC system leak (non-collision) in engine compartment 

Passenger compartment (aspiration from damaged engine compartment via

air intakes) 

Vehicle occupant Evaluate via FTA NA 

CO2_3 

AC system leak (non-collision) in engine compartment Engine compartment Bystander 

No evaluation via FTA. 

Concentrations outside of 

engine bay would not exceed 

HBL. 

NA 

NA 

Evaporator leaks due to corrosion, refrigerant released into passenger compartment 

Passenger compartment Sleeping vehicle occupant Evaluate via FTA NA CO2_ Sleeping in Car 

AC service technicians Service shop Worker Evaluate via FTA Evaluate via FTA CO2_6, P1_R744Do-It-Yourselfer (DIY) Home garage Owner 

No evaluation via FTA. New SAE equipment 

service standards will mandate service by certified 

individuals only.

NA

NA – Hazard is not relevant to the triggering event/exposure scenario described

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6 R-744 Data Collection 

6.1 Refrigerant Concentration Measurements – Passenger Compartment 

Data on CO2 concentrations in the event of a release into the passenger compartment have been 

obtained by two different OEMs. One OEM (OEM1) measured R-744 concentrations in a mid-size 

vehicle (compartment volume 2.7 m3

). When the vehicle was operating in recirculation mode, CO2

concentrations in the floor and occupant lap regions exceeded the 9% peak exposure limit for CO2; in a 

number of cases the 5.5% 5-minute exposure limit for CO2 was also exceeded. Another OEM (OEM2) 

measured R-744 concentrations in a very small passenger compartment (1.3 m3 total compartment 

volume, 1.1 m3 with two passengers included) as well as a mid-sized sedan (2.4 m3 compartment volume) 

and included both 0.5 mm and 6.5 mm leaks. In the smaller vehicle (R-744 charge - 400 g), it was found 

that at low blower speeds R-744 concentrations in the passenger compartment reached peaks of 10-12 

percent for both the 0.5 mm and 6.5 mm diameter leaks (the larger leak reaching the peak sooner). 

Concentrations appeared to decline only slowly with time (on the order of minutes). This OEM did find 

that switching to high blower rapidly after the release reduced R-744 concentrations in the compartment 

to a peak of approximately 3%, suggesting a possible mitigation strategy. In the large vehicle (R-744 

charge - 425 g) the OEM found a similar concentration-time profile although the maximum concentration 

reached was between 7 and 8% depending on sample location. This OEM also reported that in a number 

of instances the 5.5% 5-minute exposure limit was also exceeded. The measurement data collected by the 

OEMs are summarized in Table 6-1. 

6.2 Refrigerant Concentration Modeling – Passenger Compartment 

The measurement data noted above are limited in that the sampling duration was only 

approximately 100 seconds in the critical area of the occupants’ breathing zone. To obtain longer term 

concentration data with which to calculate relevant time weighted average exposure values, CRP1234 

members undertook modeling. This modeling was conducted as part of CRP1234 Phase II. 

R-744 exposure concentrations were estimated via modeling for each of the four leak types (i.e., 

the 0.01, 0.1, and 0.5 mm diameter holes associated with evaporator corrosion and the 1⁄4 inch [6.35mm] 

diameter orifice associated with rupture of the evaporator tubing). For each simulation, the vehicle was 

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assumed to be idling and the fan was operated at low speed. Previous modeling for CRP150 indicated 

that these were the most conservative options; i.e., moving vehicles and vehicles with the fan operating at 

high speed yielded lower concentrations in the passenger compartment. Modeling was done assuming 

complete uniform mixing in a 2.5 m3

 passenger compartment. The simulation conditions and leak rates 

for each leak scenario are shown in Table 6-2. 

Modeling results developed by CRP1234 provided predicted R-744 concentrations in the 

passenger compartment from the start of release until concentrations declined toward asymptotic levels. 

Using the modeling data, we calculated maximum concentration as well as 5- and 30-minute TWAs for 

each of the modeled scenarios. The estimated R-744 exposure values are shown in Table 6-2. When the 

vehicle blower was operated at low speed or was switched off, R-744 concentrations resulting from the 

0.5 mm leak exceeded the health-based limits. Health-based limits were also exceeded in the case of the 

very large 6.35 mm leak resulting from vehicle collision. 

The results of the modeling are consistent with the passenger compartment sampling conducted 

by the OEMs – both the 0.5 and 6.35 mm leak situations were found to exceed health-based limits. 

6.3 Modeling Refrigerant Concentrations for AC Service Scenarios

 Air modeling for AC service technicians who may be exposed to R-744 was carried out as 

described in Section 3.1.4. Results of the air modeling simulations are summarized in Table 6-3. In this 

table, the R-744 contribution of existing ambient air has been added into the modeling results. As shown 

in this table, estimated R-744 concentrations in service areas never exceeded health-based limits. A 

series of graphs, indicating the predicted air concentrations over time which are associated with each AC 

service scenario are included as Attachment U. 

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Table 6-1. CO2 Concentrations Measured in Passenger Compartment by Different OEMs After Simulated Refrigerant 

Release 

Leak 

diameter 

(mm)§

Leak rate (g/s)§ HVAC mode 

setting

Blower 

speed

Mode Maximum Concentration (%) 

Driver 

Face 

Driver 

Lap 

Foot Area 

OEM1 (charge =400 g, compartment volume = 2.7m3) 

 5.0 vent 2 OSA/ standstill 4 5.5 11 

 5.0 floor 2 OSA/ standstill 4.2 7.2 7.2 

 2.0 floor 2 OSA / standstill 2.2 3.5 6.5 

 2.0* vent 2 OSA / standstill 1 1.5 1.5 

 2.0* vent 4 OSA / standstill 1.8 2 7 

 2.0* vent 7 OSA / standstill 3 3.8 9 

 2.0* vent 7 OSA / 50 km/h 2 2 6 

 2.0* defrost 7 OSA / standstill 1.5 1.8 9 

 0.5* defrost 7 OSA / standstill 0.3 0.5 1 

 50.0 vent 2 Recirc / standst 5 9 12 

 5.0 vent 2 Recirc / standst. 6.5 9.5 12 

 0.5 vent 2 Recirc / standst 5 6 7 

 2.0 vent 2 Recirc / standst 5.5 8 8 

 2.0 vent 2 Recirc / 50 km/h 4 5.5 7.2 

 2.0 vent off Recirc / standst 2.5 5 18 

OEM 2 (charge = 425g, compartment volume = 2.56m3) - Sedan 

0.5 4.2 Vent Low Recirc 7.1 N/A 7.9 

0.5 4.2 Vent Low Recirc 8.3 N/A 8.5 

6.5 ~75 Floor Low Recirc 6.5 8.4 10.9 

6.5 ~75 Floor Low Recirc 8.4(1) 9.2(1) 10.3 

OEM 2 (charge = 400g, compartment volume = 1.12m3) – Roadster

6.5 ~75 Vent Low Recirc 12.0 11.8 12.2 

0..5 3.8 Vent Low Recirc 11.8 10.8 11.1 

0.5 4.3 Vent Low-High (2) OSA 2.8 2.7 2.8 

6.5 ~75 Vent Low-High(2) OSA 2.9 4.9 3.7 

6.5 ~75 Vent Low-Off(2) Recirc-OSA N/A 13.8 N/A 

6.5 ~75 Vent Off Recirc 4.0 12.2 16.6 

6.5 ~75 Vent Off OSA 2.8 6.6 10.8 

6.5 ~75 Vent Off Recirc +1/2” 

gap 

3.6 10.8 13.5 

6.5 ~75 Vent Low Recirc +1/2” 

gap 

17.2(3) 18.8(3) 18.4(3)

N/A 2.1 Vent Low Recirc 10.3 10.0 9.9 

0.5 5.1 Vent High Recirc 10.7 10.4 10.6 

(1) Rear seat measurement

(2) Changed blower speed after 10 seconds

(3) Double charge (800g) 

Numbers shown in bold indicate values above the HBL for CO2.

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Table 6-2. Results of Air Modeling Simulations for Passenger Scenarios, R-744

[Uniform Mixing Model]

Conditions Maximum Concentration (ppm)* 5-minute TWA 

(ppm)*

30-minute TWA 

(ppm)*

small diameter leak (0.01 mm orifice), vehicle 

idling, low fan, recirc mode 

1,867 1,867 1,857 

medium diameter leak (0.1 mm orifice), vehicle 

idling, low fan, recirc mode 

13,700 13,700 12,900 

large diameter leak (0.5 mm orifice), vehicle 

idling, low fan, recirc mode 

99,500 95,000 75,400 

rupture of evaporator hose, vehicle idling, low 

fan, recirc mode 

119,800 107,800 85,700 

Shading indicates an exposure scenario where the health-based limit is exceeded. 

* Includes the contribution of 1,800 ppm CO2 due to passenger respiration in addition to the CO2 contributed by the refrigerant leak 

Health-based limits for R-744, Peak – 90,000 ppm, 5-minute TWA – 60,000 ppm, 30-minute TWA – 40,000 ppm.

Table 6-3. Results of Air Modeling for AC Service Technician Scenarios, R-744 

Scenario Predicted CO2

Concentration 

(Maximum, ppm)*

Predicted CO2

Concentration (5-

minute TWA, ppm)*

Predicted CO2

Concentration (30-

minute TWA, ppm)*

AC service – hose leak during system drain and 

recharge 669 661 634 

* Includes the contribution of 450 ppm from ambient air. 

Shading indicates an exposure scenario where the health-based limit is exceeded. 

Health-based limits for R-744, Peak – 90,000 ppm, 5-minute TWA – 60,000 ppm, 30-minute TWA – 40,000 ppm.

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7 FTA for R-744 

7.1 CO2 Exposure, Passenger Compartment Scenarios 

The CRP1234 members developed a fault tree that describes the probability of exposure of the 

vehicle occupant to CO2 above the HBL that is released into the passenger compartment as a result of 

vehicle collision (Fault Tree CO2_3). This scenario contains a number of collision speed and location 

dependent elements because the speed and location (frontal, side) at which the collision occurs influences 

a number of factors, including the likelihood of refrigerant release, the likelihood that the vehicle 

windows or other structures will be broken introducing outside air into the passenger compartment, and 

the probability that the passenger will be able to leave the vehicle after the collision. As with the 

assessment of HFO-1234yf, the CRP1234 members determined that medium and high speed collisions 

would be relevant to this scenario whereas low speed collisions presented minimal likelihood of AC 

system damage. As with the HFO-1234yf fault trees, many of the speed dependent probabilities were 

obtained through an analysis of the GIDAS database, a German registry which provides a detailed 

analysis of the effects of vehicle collision at various speeds on vehicle integrity (Attachment S). This 

scenario also requires that the AC system fan be switched to recirculation (recirc) mode because 

measurements collected by the vehicle manufacturers have indicated that CO2 concentrations will not 

exceed the HBL in the occupant’s breathing zone when the vehicle is operated in outside air mode (refer 

to Section 6.1). Refer to Fault Tree CO2_3 in Attachment B for further details. The basis for values 

used as inputs in the fault tree are provided in tabular form in Attachment B. 

The CRP1234 members developed a second fault tree (CO2_9) related to release of CO2 in the 

engine compartment in the event of a collision which is then aspirated into the passenger compartment 

through the air intake vents located beneath the windshield. This fault tree also contains collision speed 

and location specific inputs and, as with the analogous HFO-1234yf fault trees, CRP1234 members 

judged that low, medium and high speed collisions all could contribute to differing degrees to the overall 

risk, because the engine compartment side of the AC system could possibly be damaged even in lower 

speed collision. This tree also requires that the hood is buckled during the collision in a manner which 

negates the effect of the hood seals and creates a flow path for CO2 from the engine compartment to the 

air intakes. This fault tree also requires the probability that CO2 is present at the air intake vents above 

the HBL and is not dispersed by wind or rain. It also requires that the vehicle is switched to outside air 

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mode and that CO2 is not sufficiently diluted at the air intake so that it is below the HBL. Refer to Fault 

Tree CO2_9 in Attachment B for further details. The basis for values used as inputs in the fault tree are 

provided in Attachment B. 

7.2 CO2 Exposure, AC Service Scenarios 

CRP1234 members also evaluated the probability of exposure to CO2 above the HBL by certified 

AC service technicians (Fault Tree CO2_6). Work done in Phase II using uniform mixing models 

indicated that CO2 concentrations resulting from a release of service equipment or a AC system rupture 

never exceeded health-based limits. As a result, the probabilities assigned to this fault tree are zero and 

the probability of exposure is zero. 

7.3 High Pressure Equipment Risks in Vehicle Service Areas 

CRP1234 members also developed a fault tree to assess the physical risks to certified AC service 

technicians working with high pressure refrigerant hoses. Although equipment for servicing AC systems 

is designed with various safety devices (e.g., automatic shut off coupling on equipment hoses), it is 

possible that high pressure hoses could rupture or break during service and could possibly strike a service 

technician. This fault tree therefore contains the frequency that vehicles are serviced and the probabilities 

that high pressure hoses fail or rupture, that the hose strikes the technician before they can move away, 

and that the hose strikes the technician with sufficient force to create a reportable injury. Note that 

because R-744 systems will operate at higher pressure than systems employing HFO-1234yf, the values 

assigned for some of the probabilities are higher for R-744 than the corresponding values for HFO1234yf. Refer to Fault Tree P1_R744 in Attachment B for detail about this organization of this fault tree. 

The basis for values used as inputs in the fault tree are provided in Attachment B. 

7.4 CO2 Exposure for Individuals Sleeping in Vehicles During a Refrigerant 

Leak

CRP1234 members also considered an exposure scenario which involves individuals who may be 

sleeping in a vehicle when a CO2 release occurs. Such a scenario is not relevant to R-1234 since even a 

very large leak would not exceed the threshold for anesthetic effects (201,600 ppm) but modeling and 

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measurement data suggest that in the event of an evaporator leak a sleeping individual could continue to 

be exposed to high levels of CO2 and potentially experience severe CNS depression. Individuals who 

might sleep in a vehicle include parents waiting for children to finish extracurricular activities or 

salesmen taking naps in between on-the-road meetings. As noted earlier, an exposure limit of 60,000 

ppm (6%) for 30 minutes was used in evaluating this scenario. The fault tree developed for this scenario 

(Fault Tree CO2_Sleeping_In_Car) is split into two branches, one for individuals sleeping in a sitting 

position and one for individuals lying down on the seats. The two subscenarios involve different 

probabilities that an individual would sleep in each position and also different probabilities that the HBL 

would be exceeded in the event of a leak. Both branches contain the probability that the leak is sufficient 

to produce CO2 concentrations above the HBL, the probability that the individual is asleep in the relevant 

position, the probability they do not awaken before or during the time that CO2 concentration exceed the 

HBL, and the probability that the vehicle windows are closed (thus preventing dilution with outside air). 

Refer to Fault Tree CO2_Sleeping_In_Car in Attachment B for detail about this organization of this fault 

tree. The basis for values used as inputs in the fault tree are provided in Attachment B. 

7.5 Sensitivity analysis and description of uncertainties 

 One source of uncertainty related to the FTA for CO2 relates to the potential for evaporator leaks 

in a high pressure CO2 system. Because current MAC AC systems operate at lower pressure than will be 

required for a CO2 system, CRP1234 members felt that the assumption for the frequency of evaporator 

leaks, 2.3 x 10-8 events per year based on OEM data, could be too low. As described previously, 

evaporator leaks were not considered to be credible sources of refrigerant discharge when vehicles were 

operating. However, an evaporator leak was considered in the case of an occupant sleeping in the vehicle 

(Fault Tree CO2_Sleeping in Car). To assess the potential impact of a high pressure CO2 system on this 

fault tree, the frequency of evaporator leaks was increased 10-fold (to 2.3 x 10-7 events per year). This 

increased the overall risk of the top event (i.e., exposure to CO2 above the relevant health-based limit) 

from 2.4 x 10-12 to 2.4 x 10-11. However, this had a minimal impact on overall assessment of R-744 

because the collision scenario (fault tree CO2_3) remains the risk driver (1 x 10-10 events per vehicle 

operating hour).

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7.6 Conclusions Regarding R-744 

The available toxicology data for CO2 indicate that acute adverse health effects occur at lower exposure 

concentrations in air than the acute adverse effects of R-134a exposures. For example, animal studies 

indicate an absence of anesthetic effects with R-134a air concentrations as high as 81,000 ppm (based on 

exposures for 6 hours per day). For CO2, the threshold effect level for anesthetic effects in studies of 

human populations was 50,000 ppm (based on exposures for 30 minutes). Although R-134a, unlike CO2, 

exhibits the property of cardiac sensitization, the threshold concentration for this endpoint was 75,000 

ppm in air, which is higher than the CO2 concentration associated with anesthesia and central nervous 

system (CNS) depression. CO2 also has an ODP of 0 and a GWP of 1 which meets the European Union 

F-gas requirements. Based on a review of potential toxicological effects of CO2, the risk assessment was 

focused on exposures that might exceed the threshold for anesthesia/CNS depression. Passengers whose 

exposures exceed this level may not be able to take appropriate action to promptly exit the vehicle and 

limit further exposure. R-744 concentrations in the air of the passenger compartment in the event of an 

evaporator or collision-related leak were assessed using both modeling and field measurement. Results in 

both cases indicated that when the vehicle blower is operating at low speed (less than 10 liters/second) or 

is switched off, the threshold for anesthetic effects would be exceeded in the vehicle collision scenario. 

R-744 concentrations in air were below health-based limits when the vehicle blower was operating in 

medium or high speed (greater than 10 liters/second) in outside air mode. Using these conclusions as 

inputs, the fault tree analysis estimated that the number of CO2 exposure events above the health based 

limits would be on the order of 2 x 10-10 events per hour of vehicle operation. This risk was largely 

attributable to refrigerant release associated with a vehicle collision. Exposures of certified AC service 

technicians were found to be in the range of 3 x 10-17 per vehicle service hour. Injury to AC service 

technicians from rupture of high pressure refrigerant recharge hoses was also evaluated in the FTA and 

found to result in 3 x 10-7 reportable vents per vehicle service hour. 

 Table 7-1 shows the risks estimated for use of R-744 in mobile air conditioning systems 

compared to other risks that individuals might encounter. This comparative risk analysis demonstrates 

that the estimated risks for vehicle passengers of adverse events associated with accidental refrigerant 

release of this refrigerant are less than the risks associated with many other commonly encountered and 

accepted risks. The risk for vehicle passengers is similar to the risk associated with accidental 

deployment and injury from an airbag. Risks for injury of AC service technicians (associated with the 

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failure of high pressure equipment hoses) are also below the rate of other workplace injuries. Thus R-744 

should be considered acceptable for use as a new refrigerant for MAC systems. 

 Finally, as noted earlier, the SNAP program rules require an evaluation of the risks posed by any 

new automotive refrigerant compared to both R-12 and R-134a to ensure that any new refrigerant 

provides a lower or equivalent impact on human health and the environment. A comparison of R-744 to 

the two earlier refrigerants is shown in Table 7-2. As indicated in this table, use of R-744 in MAC 

systems would entail potentially increased risk associated with refrigerant toxicity (i.e., anesthetic 

potency) but would eliminate the risk associated with fluorocarbon refrigerants due to potential cardiac 

sensitization. R-744 also offers a higher workplace exposure limit (of limited benefit given the low 

refrigerant air concentrations expected in vehicle assembly plants) and a substantially decreased 

ecological impact (in terms of a lower GWP). Overall, R-744 meets the SNAP requirement of less or 

equivalent risk and could be approved as a replacement for R-12 and R-134a. 

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Table 7-2. Probabilities of Various Adverse Events Compared to Probabilities of Events 

Associated with Leaks of R-744 

Event Probability per 

vehicle per 

operating hour* 

Citation 

Probability of being in a police reported vehicle collision1 5 x 10-5 NHTSA, 2007a 

Probability of dying during a 5000-mile plane trip2 4 x 10-7 Wilson and Crouch, 1987 

Probability of vehicle collision due to vehicle brake failure3 3 x 10-7 New York State DMV 

Probability of highway vehicle fire (any cause)4 1 x 10-7 Ahrens, 2008; BTS, 2004 

Probability of an airbag-related fatality associated with a vehicle collision5 2 x 10-10 NHTSA, 2007b

Probability of vehicle occupant exposure to R-744 above health based 

limits 2 x 10-10 Current analysis 

Event Probability* Citation 

Probability of non-fatal recordable injury at work (all occupations, per 

working hour)6 2 x 10-5 NSC, 2004

Probability of an AC service technician being struck by object resulting in 

lost work days (per working hour)7 2 x 10-6 BLS, 2007 

Probability of recordable incident involving service technicians being 

struck with high pressure CO2 equipment hose (per hour of vehicle 

service) 

3 x 10-7 Current analysis 

Probability of service technician exposure to CO2 above health based 

limits (per hour of vehicle service) 3 x 10-17 Current analysis 

1 There were 6,024,000 police reported vehicle collisions in the U.S. in 2007. This is divided by the number of registered vehicles (255,748,000, 

NHTSA 2009) and the average number of hours each vehicle is operated (approximately 500 hours based on SAE J2766, Table 6). 

2 Frequency divided by 8 hours assuming plane travels between 600 and 650 mph. 3 The New York State Department of Motor Vehicles reports that there were 1753 accidents in 2008 attributed to brake failure (NYSDMV, 2008) and 

that there were roughly 11 million registered vehicles in New York State in 2008. Combining these data with the "operating hours per year" 

suggested in J2766 (~500 as an average) yields the an accident frequency per vehicle operating hour. 4 Ahrens reports that 33 highway vehicle fires are reported per hour. This is divided by the number of registered vehicles in the U.S. (255,748,000, 

NHTSA 2009) . 

5 Based on NHTSA statistics for 1990 to 2001, the average number of confirmed air-bag related fatalities per year is 16. This is divided by the 

number of hours of vehicle operation per year ((approximately 500 hours based on SAE J2766, Table 6) and the number of vehicle drivers in the 

U.S. (200 million) to arrive at an average hourly risk rate. 

6 The rate (3.7 per 100 workers) for non-fatal workplace injuries reported for 2008 by the U.S. Bureau of Labor Statistics (BLS, 2008) divided by the 

number of working hours per year in the U.S. (i.e., 2080). 7 The U.S. Bureau of Labor Statistics (2007) reports a total of 3450 cases in 2005 of automotive repair technicians and mechanics being struck by an 

object at work resulting in an injury that required time off from work. This can be divided by the number of technicians and mechanics working in the 

U.S. (954,000, BLS, 2007) and 2,080 working hours per year to yield a risk per working hour. 

* Probabilities are given in these units to be consistent with international performance standards used by the automotive industry. Results of the 

Phase I and Phase II risk assessment were given in units of "per year". Conversion between these units is possible by accounting for the number of 

vehicle operating hours per year (roughly 500 hours) and the size of the relevant vehicle fleet (e.g., roughly 250 million vehicles in the U.S.). Note 

that the outcomes of individual fault trees should not all be combined because some have different denominators (events per vehicle operating hour, 

events per year). The values shown here represent the highest estimates when combining individual fault trees with common units of the top level 

events. 

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Table 7-3 

Overall Comparison R-744 to R-12 and R-134a 

Property R-12 R-134a R-744 Comparison among 

refrigerants

Toxicity 

Acute toxicity (LC50, ppm) 600,000 >500,000 ~400,000 equivalent 

Anesthetic potency (ppm) 51,000 81,000 40,000 less favorable to R-744 

Cardiac sensitization no 

effect level 

25,000 50,000 NA more favorable to R-744 

Genotoxicity/ 

Carcinogenicity 

negative negative negative equivalent 

ASHRAE ATEL (ppm, 

acute health effects value) 

18,000 50,000 40,000 slightly less favorable to R-744 

ASHRAE Safety 

Classification (ppm, 

chronic health effects 

value) 

A A A equivalent 

Ecological Toxicity low low low equivalent 

Flammability not 

flammable 

not flammable not flammable equivalent 

ODP 1.0 0 0 R-744 and R-134a equally 

superior to R-12 

100 year GWP (CO2=1) 10,600 1,430 1 much more favorable to R-744 

Estimated total lifetime 

annual refrigerant loss rate 

(grams/year) 

75 65 50 equivalent 

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