HFO-1234yf Non-CBI Risk Screen

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October 13, 2009

Significant New Alternatives Policy Program 

Refrigeration and Air Conditioning Sector

Risk Screen on Substitutes for CFC-12 in Motor Vehicle Air Conditioning.

Substitute: HFO-1234yf

This risk screen does not contain Clean Air Act (CAA) Confidential Business Information (CBI) 

and, therefore, may be disclosed to the public.

1. INTRODUCTION

Ozone-depleting substances (ODS) are being phased out of production in response to a series of 

diplomatic and legislative efforts that have taken place over the past decade, including the Montreal 

Protocol and the Clean Air Act Amendments of 1990 (CAAA). The U.S. Environmental Protection 

Agency (EPA), as authorized by Section 612 of the CAAA, is developing a program to evaluate the 

human health and environmental risks posed by alternatives to ODS. The main purpose of EPA's 

program, called the Significant New Alternatives Policy (SNAP) Program, is to identify acceptable and 

unacceptable substitutes for ODS in specific end uses. 

EPA’s decision on the acceptability of a substitute is based largely on the findings of a screening 

assessment of potential human health and environmental risks posed by the substitute in specific 

applications. EPA has already screened a large number of substitutes in many end uses within all of the

major ODS-using sectors, including refrigeration and air conditioning, solvent cleaning, foam blowing, 

aerosols, fire suppression, adhesives, coatings and inks, and sterilization. The results of these risk screens 

are presented in a series of Background Documents that are available in EPA's docket.

The purpose of this report is to supplement EPA’s Background Document on the refrigeration and air 

conditioning sector (U.S. EPA 1994) (hereinafter referred to as the Background Document) by adding to 

the list of potential substitutes for CFC-12 as a motor vehicle air conditioning (MVAC) refrigerant. The 

specific proposed CFC-12 substitute examined in this report is 2,3,3,3-Tetrafluoropropene, or “HFO1234yf.”

The potential risks associated with use of substitutes as MVAC refrigerants have been examined at length 

in the Background Document. The reader is referred to this reference for a detailed discussion of the 

methodologies used to conduct this risk screen. In this risk screen, reasonable worst-case occupational 

and consumer exposure analyses were performed to ensure that use of HFO-1234yf as an MVAC 

refrigerant will not pose unacceptable risks to workers or consumers. General population analyses were 

not conducted as chronic exposures are not expected for the general public.

Section 2 of this report summarizes the results of the risk screen for the proposed substitute. The 

remainder of the report is organized into the following sections:

 Section 3: Atmospheric Assessment

 Section 4: Flammability Assessment

 Section 5: Asphyxiation Assessment

 Section 6: Toxicity Assessment

 Section 7: Volatile Organic Compound Assessment 

 Section 8: Additional Environmental Impacts Analysis

 Section 9: References
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2. SUMMARY OF RESULTS

HFO-1234yf is recommended for SNAP approval as an MVAC refrigerant so long as all HFO-1234yf 

MVAC systems are designed to avoid occupant exposure to concentrations of HFO-1234yf above 6.5% in 

the passenger compartment for more than 15 seconds, even if in the event of the leak.

EPA's risk screen indicates that the use of the proposed substitute will be less harmful to the atmosphere 

than the continued use of CFC-12. No significant toxicity risks to workers or vehicle passengers are 

expected according to occupational and consumer modeling. However, because of the risk of exposure to 

large quantities of the substitute when it is used in servicing activities by an untrained worker, it is 

recommended that the refrigerant only be made available to qualified personnel. HFO-1234yf should not 

be made available for “do-it-yourself” workers. Flammability models indicate that risks of fire or 

explosion in the passenger compartment of a vehicle may be small for average sized cars, but could be 

more significant for cars with a high ratio of refrigerant charge to passenger compartment volume. To

protect against the risk of fire or explosion, systems using HFO-1234yf should be designed so that 

passenger exposures will not exceed the lower flammability limit of the refrigerant (6.5% by volume in 

air) for more than 15 seconds, even if a leak occurs. Additionally, to protect against flammability risks 

during servicing, systems should also be designed to prevent refrigerant leaks into the engine 

compartment and vehicle electric power source storage areas. Appropriate design features might include

using a secondary refrigerant containment shell around the refrigerant circuit, for example. Further, 

systems should also be engineered so that potential ignition sources are isolated from the refrigerant 

circuit and any areas into which the refrigerant might leak (should a leak occur). Caution must be used in 

manufacturing facilities and by refrigeration technicians to minimize fire and explosion risk while in the 

presence of large quantities of the substitute. This includes installation of proper safety equipment during 

manufacturing, transportation, and storage and providing proper training and certification to technicians. 

EPA recommends that American Society of Heating, Refrigerating and Air-Conditioning Engineers 

(ASHRAE) Standard 34 be followed. 

3. ATMOSPHERIC MODELING

This section presents an assessment of the potential risks to atmospheric integrity posed by the use of 

HFO-1234yf as an MVAC refrigerant. The ODP, GWP, and atmospheric lifetime (ALT) of the proposed 

substitute are presented in Table 1.

The environmental impacts resulting from use of HFO-1234yf are generally in the range of those 

predicted for other substitutes examined in the Background Document. The substitute is substantially less 

harmful to the ozone layer, has less climate impact, and a shorter atmospheric lifetime compared to CFC12.

Table 1. Atmospheric Impacts of HFO-1234yf Compared to CFC-12.

Refrigerant Ozone Depleting Potential 

(ODP)

Global Warming Potential 

(GWP)

Atmospheric Lifetime

years (ALT)

HFO-1234yf 0

a 4a <1b

CFC-12 1

c 8,100d 100c

a HFO-1234yf SNAP Submission (Honeywell 2007).

b Atmospheric lifetime (ALT) is 11 days (Honeywell 2007).

c Available at: http://www.epa.gov/ozone/ods.html.

d IPCC, Second Assessment Report (1996).

4. FLAMMABILITY ANALYSIS

HFO-1234yf is flammable when its concentration in air is in the range of 6.5%-12.3% by volume (this is 

equal to 65,000 ppm to 123,000 ppm). In the presence of an ignition source (e.g., static electricity, a 
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spark, or a cigarette), an explosion or a fire could occur when the concentration of HFO-1234yf is within 

these flammability limits. The remainder of this section addresses the potential for flammability risks at

the end-use, and during manufacture and servicing and provides a discussion of measures to be taken to 

ensure safe use. 

4.1. Flammability Risks at the End-Use

To determine whether flammability would be a concern for vehicle passengers, a reasonable worst-case

scenario analysis was performed to model catastrophic release of the refrigerant into a vehicle. Two key 

inputs to the model were passenger compartment volume (i.e., the space into which the refrigerant would 

leak) and the refrigerant charge size. Because passenger compartment volumes and refrigerant charge 

sizes can vary widely from model to model, the vehicle in each vehicle class1 with the highest ratio of 

charge size to compartment volume was identified (ICF 2008) and used in the model. Table 2 presents 

the compartment volume and charge size of the vehicles used in this analysis.

2

Table 2. Vehicles with the Highest Ratio of Refrigerant Charge to Passenger Compartment 

Volume in each Vehicle Class

Class Passenger 

Compartment 

Size (m3)

a

MVAC 

Charge Size 

(g) HFC134a b

Ratio of HFC134a Charge to 

Compartment 

Volume (g/m3)

Manufacturer Model c

TWO SEATERS 1.42 907.2 640.75 MERCEDES-BENZ SL65 AMG (MY 2007)

SMALL PICKUP 

TRUCKS 1.48 940 633.27 FORD RANGER

S.U.V. 3.01 1250 414.96 MERCURY MOUNTAINEER (w/ 

Aux Heat)

STANDARD PICKUP 

TRUCKS 1.59 632 397.84 TOYOTA

TOYOTA TACOMA 

PRERUNNER REG 

CAB

COMPACT CARS 2.63 935.55 355.25 MERCEDES-BENZ CLS Class (MY 2007)

SUBCOMPACT CARS 2.29 793.8 346.08 BMW 650i and M6 (MY 

2007)

MINICOMPACT CARS 2.10 725 345.99 JAGUAR JAGUAR XKR

MIDSIZE CARS 2.72 935.55 343.44 MERCEDES-BENZ E Class (MY 2007)

LARGE CARS 2.94 793.8 269.55 BMW 750i (MY 2007)

SMALL STATION 

WAGONS 2.66 670 251.71 PONTIAC VIBE

MINIVAN 4.42 1100 248.85 DODGE GRAND CARAVAN 

SXT WAGON

VANS 8.20 1600 195.08 CHEVROLET EXPRESS VAN 

(Passenger/Cargo)

MIDSIZE STATION 

WAGONS d

3.06 475 155.32 KIA RONDO

a Source: FuelEconomy.gov, and if not available there, BlackBookUSA.com. 2008 New Car Cost Guide. When volume 

unavailable, value was estimated using BlackBook data and methods at 40 CFR 600, Subpart D.

b Source: Motor Information Systems database.

 

1 Vehicle classes used are those defined by the U.S. DOE and EPA at the Fuel Economy.gov website (U.S. DOE and EPA 2009).

2 All analyses conducted in this risk screen assume a 1:1 HFC-134a to HFO-1234yf replacement ratio. The HFO-1234yf 

submission (Honeywell 2007) indicates the refrigerant would replace CFC-12 at a rate of approximately 87% and HFC-134a 

replaces CFC-12 at a rate of 80-90% (U.S. EPA 2008). As HFC-134a and HFO-1234yf have similar CFC-12 replacement ratios, 

it was assumed that HFO-1234yf would replace HFC-134a at a 1:1 rate. 
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c Model Year (MY) 2008, unless otherwise indicated.

d Data were available for only 1 vehicle in this class. By default it has the ‘highest’ ratio.

In the analysis, the full charge of the MVAC unit is assumed to be emitted within 140 seconds (Blackwell 

et al. 2006) and the vehicles are assumed to have an air flow rate of 0.3 air exchanges per hour (Blackwell 

et al. 2006). Horizontal stratification is also assumed since HFO-1234yf is denser than air and will settle 

in higher concentrations closer to the bottom of the passenger compartment.3 These assumptions are 

reasonable, but very conservative; therefore the assumed worst-case scenario has a low probability of 

occurring. 

In addition to this worst-case scenario analysis, which employed a simple box-model, modeling was also 

conducted by an industry consortium (Gradient 2008). The consortium’s model used computational fluid 

dynamics (CFD) to more realistically represent the behavior of the leaked refrigerant in a vehicle

(specifically, a Crown Victoria4). As reported in the industry consortium’s report, Honeywell also 

conducted CFD modeling; however Honeywell examined releases into a smaller vehicle (i.e., a vehicle 

with a smaller passenger compartment volume). Finally, the consortium also conducted field tests in 

which HFO-1234yf was released into an actual vehicle. Table 3 presents the results of all four analyses. 

Table 3. Results of Flammability Modeling

Flammability 

Model

Maximum 

Instantaneous 

Concentration 

of HFO-1234yf 

(ppm)

Situation under which 

Maximum Concentration was 

Measured

Minimum

Instantaneous 

Concentration 

of HFO-1234yf 

(ppm)

Situation under which 

Minimum Concentration was 

Measured

Worst-Case 

Scenario Boxmodel

291,193 Passenger compartment volume 

1.42 m3

Catastrophic release of total 

charge; all windows and doors 

closed; stationary vehicle.

70,752 Passenger compartment 

volume 3.06 m3

Catastrophic release of total 

charge; all windows and doors 

closed; stationary vehicle.

Industry 

Consortium 

CFD Model

(Gradient 2008)

65,000 a Passenger compartment volume 

3.1 m3

Vehicle idling, low fan, large leak 

(0.5 mm orifice), recirculation 

mode.

34,000a Passenger compartment 

volume 3.1 m3

Vehicle idling, low fan, large 

leak (0.5 mm orifice), outside 

air mode.

Honeywell

CFD Model

53,000 Passenger compartment volume 

2.5 m3

Large leak (0.5 mm orifice).

Not available. Not available.

Industry 

Consortium 

Field Test

29,774 Passenger compartment volume 

3.1 m3

Vehicle shielded from wind; six 

mannequins placed in car to 

simulate vehicle occupants; large 

leak (0.5 mm orifice); full charge 

released; low fan.

Not available. Not available.

a Only concentrations for the ‘worst-case scenarios’ (i.e., the large leaks occurring under the conditions noted in the table) were 

provided by Gradient (2008). Concentrations resulting from smaller leaks occurring under the conditions noted in the table are 

expected to be less than the concentrations reported in the table.

The worst-case scenario box-model produced concentrations above the LFL (65,000 ppm) in the 

 

3

In order to simulate the horizontal concentration gradient that will occur because of the weight differential between the 

refrigerant and air, it is assumed that 95 percent of the leaked refrigerant mixes evenly into the bottom 0.4 meter of the car, and 

the rest of the refrigerant mixes evenly in the remaining volume (Kataoka 1999).

4

The Crown Victoria was used by the industry consortium as it was the vehicle used in the risk assessment of HFC-152a and 

CO2(Blackwell et al. 2006) as well as in another SAE Cooperative Research Program Analysis. The Crown Victoria was 

chosen by Blackwell et al. (2006) as it represented a “possible worst-case scenario.”
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passenger compartment for all vehicles; however CFD modeling conducted by the industry consortium 

and the submitter produced concentrations at or below the LFL. Actual field testing of an HFO-1234yf 

release found concentrations equal to about 46% of the LFL in the passenger compartment.

To prevent against the risk of fire or explosion, especially in cars which currently have high refrigerant 

charge to compartment volume ratios, it is recommended that all HFO-1234yf systems be designed to 

avoid occupant exposure to concentrations of HFO-1234yf above 6.5% in the passenger compartment for 

more than 15 seconds, even if in the event of the leak.

4.2. Flammability Risks During Manufacture and Servicing

Catastrophic releases of large quantities of refrigerant during manufacturing and servicing, especially in 

areas where large amounts of refrigerant are stored, could cause an explosion. For this reason, it is 

important that only properly trained and certified refrigerant technicians handle HFO-1234yf; the 

refrigerant should not be made available to unqualified personnel or “do-it-yourself” users. Also, to 

protect against the risk of fire during servicing, HFO-1234yf MVAC systems should be designed to 

prevent refrigerant leaks into the engine compartment and vehicle electric power source storage areas. 

Appropriate design features might include using a secondary refrigerant containment shell around the 

refrigerant circuit, for example. Systems should also be engineered so that potential ignition sources are 

isolated from the refrigerant circuit and any areas into which the refrigerant might leak (should a leak 

occur). As a further precaution, HFO-1234yf storage and transport equipment should be installed with 

safety devices that minimize the likelihood of catastrophic releases. For example, NFPA 58 Liquefied 

Petroleum Gas Code (NFPA 2008) requires the use of overfill protection devices (OPD) on cylinders to 

minimize the likelihood of leaks. The NPFA 58 Code also contains storage and transportation 

requirements/guidelines. Similar equipment safety and procedural requirements should be implemented

for this substitute. 

It is important that the strictest standards be followed during manufacturing. It is recommended that 

refrigerants be properly stored and caution used within manufacturing facilities to minimize explosion 

risk and that workers adhere to the requirements set by OSHA under 29 CFR 1910. OSHA requirements 

include proper ventilation and storage practices within manufacturing facilities to prevent fire and 

explosion. Proper ventilation should be maintained at all times during the manufacture of equipment 

containing HFO-1234yf through adherence to good manufacturing practices. If refrigerant levels in the air 

surrounding the equipment rise above one-fourth of the lower flammability limit,

5

the space should be 

evacuated and re-entry should only occur after the space has been properly ventilated. Ventilation is also 

of the utmost importance to mitigate the risk of fire or explosion when servicing vehicles using HFO1234yf. 

5. ASPHYXIATION ANALYSIS

The risk of asphyxiation in a vehicle following release of HFO-1234yf for a reasonable worst-case 

scenario was investigated. This analysis does not consider conditions that are likely to occur that would 

reduce the levels to which individuals would be exposed, such as open doors or windows, fans operating, 

or conditioned airflow (either heated or cooled).

To determine if release of HFO-1234yf would present a risk of asphyxiation, the charge of HFO-1234yf 

required to reduce oxygen levels to the No Observed Adverse Effect Level (NOAEL) for hypoxia6in each 

of the cars presented in Table 2 was calculated. These charge sizes were then compared to the actual 

charge size for each car. If the actual charge for a car exceeded the charge required to reduce oxygen 

 

5 OSHA regulation 29 CFR 1910.110 considers ventilation adequate “when the concentration of the gas in a gas-air mixture does 

not exceed 25 percent of the lower flammable limit.”

6

Twelve percent oxygen in air (i.e., 120,000 ppm) is the NOAEL for hypoxia (ICF 1997).
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levels to the NOAEL, there could be risk of asphyxiation in the car under a worst-case refrigerant release

scenario. Table 4 presents the results of the asphyxiation analysis.

Table 4. Reasonable Worst-Case Asphyxiation Assessmenta

Car Class Charge Required to 

Cause Impairment (g) b

Charge Size (g)b May Charge Cause 

Impairment?

TWO SEATERS 6,584 907 No

SMALL PICKUP TRUCKS 6,862 940 No

S.U.V. 13,956 1,250 No

STANDARD PICKUP TRUCKS 7,372 632 No

COMPACT CARS 12,194 936 No

SUBCOMPACT CARS 10,618 794 No

MINICOMPACT CARS 9,737 725 No

MIDSIZE CARS 12,612 936 No

LARGE CARS 13,632 794 No

SMALL STATION WAGONS 12,334 670 No

MINIVAN 20,494 1,100 No

VANS 38,021 1,600 No

MIDSIZE STATION WAGONS 14,188 475 No

a This analysis only includes those vehicles identified as having the highest “refrigerant charge to compartment volume ratio” in 

each vehicle class.

b Values provided in these columns refer to the charge required to cause impairment based on concentrations of the substitute 

in the occupants’ breathing area (i.e., the ‘upper stratum’ of the car). These data assume 5% of the refrigerant is in the upper 

stratum. Concentrations in the lower stratum of the car are not considered in this analysis as vehicle occupants are not 

expected to be in the foot well of the car. 

Based on the results of the worst-case scenario analysis, use of HFO-1234yf in this end-use should not 

pose a significant risk of asphyxiation or impaired coordination to vehicle occupants. Further, as the 

NOAEL for hypoxia (120,000 ppm) is greater than HFO-1234yf’s lower flammability limit (65,000 

ppm), the recommendation at the end of Section 4.1 (which ensures concentrations of refrigerant will not 

exceed the LFL in the passenger compartment) will further protect against the limited risk of 

asphyxiation.

6. TOXICITY ANALYSIS

To assess potential health risks from exposure to this substitute when used as an MVAC refrigerant, EPA 

identified the relevant toxicity threshold values for comparison to modeled exposure concentrations for

different scenarios. For the occupational exposure analysis, potential risks from chronic and acute worker 

exposure were evaluated by comparing exposure concentrations to available occupational exposure limits. 

Occupational exposure limits are typically established for either an eight-hour or ten-hour time period for 

long-term exposure or for a 10 to 30-minute period for short-term exposure, as shown in Table 5. To 

assess short-term consumer (i.e., vehicle passenger) exposures, ICF conducted an analysis based on 

modeling of stratified exposure concentrations within the compartment of several different vehicles

(ICF’s analysis is included as Appendix A). Chronic general public exposures are not expected, so a 

reference concentration (RfC) was not determined. A list of the relevant toxicity limits is shown in Table 

5. Table 6 provides definitions of the acronyms used in Table 5. EPA’s approach for identifying or 

developing these values is discussed in Chapter 3 of the Background Document. 
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Table 5. Toxicity Levels of HFO-1234yf

Occupational

Long-term Exposure

ppm

Occupational

Short-term Exposure

ppm

Consumer 

Short-term Exposure

ppm

HFO-1234yf 250a NA See Appendix A

NA = Not Available

a US EPA (2009).

Table 6. Explanation of Toxicity-Related Acronyms

Exposure Limit Definition Explanation

LC50 Lethal Concentration “The concentration of the chemical in air that kills 50% of the test 

animals in a given time”

a

NOAEL No Observed Adverse Effect Limit “The highest exposure level at which there are no biologically 

significant increases in the frequency or severity of adverse effect 

between the exposed population and its appropriate control; some 

effects may be produced at this level, but they are not considered 

adverse or precursors of adverse effects.”b

RfC Reference Concentration A concentration “designed to protect the general population against 

adverse systemic (i.e., noncancer) effects.” c

a From http://www.ccohs.ca/oshanswers/chemicals/ld50.html

b From http://www.epa.gov/riskassessment/glossary.htm#n

c U.S. EPA (1994)

6.1. Occupational Exposure

Occupational exposure modeling was performed for the proposed substitute to ensure that use of the 

substitute does not pose an unacceptable risk to workers. The methodology used for this screening 

assessment is based on the one used in the occupational exposure and hazard analysis described in 

Chapter 5 of the Background Document. A box-model approach was used to evaluate potential worker 

exposure to alternative refrigerants. This approach has been widely used for many years to estimate 

probable worker exposures to hazardous airborne materials and has been described in detail by the 

National Institute for Occupational Safety and Health (NIOSH). This model takes into consideration the 

duration and magnitude of the resulting exposure which is influenced by 1) duration and intensity of the 

release, 2) rate at which contaminated air is diluted with uncontaminated air, 3) proximity of the worker 

to the source of the release, and 4) the length of time the worker remains in the affected space. 

Estimates of refrigerant release per event for various release scenarios and data on number of events were 

obtained from the Vintaging Model.7 The release per event was conservatively assumed to be 5 percent 

of the equipment charge during manufacturing and 3 percent of the equipment charge during disposal.8 

The release rate per event was multiplied by the number of events estimated to occur over a workday. For 

equipment manufacturing, the number of events per workday was assumed to equal the number of units 

containing the substitute produced per plant per year divided by 365 workdays per year. For the purposes 

of this model, it is assumed that HFO-1234yf will have a market penetration rate of 0.2% and that one

production facility will be in operation. These assumptions result in approximately 9

 

7

ICF International maintains the Vintaging Model for EPA in order to simulate the aggregate impacts of the ODS phaseout on 

the use and emissions of various fluorocarbons and their substitutes over a period of several years across more than 40 different 

applications. The model tracks the use and emissions of various compounds for the annual vintages of new equipment that enter 

service in each end-use. The vintage of each type of equipment determines such factors as leak rate, charge size, number of units 

in operation, and the initial ODS substance that the equipment contained. 

8 During disposal it is assumed that only 90 percent of the refrigerant charge remains in the unit. 
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events/manufacturing facility/day. For disposal, it was conservatively assumed that 100 units are 

disposed during an 8-hour work day. 

The maximum time weighted average (TWA) exposure was estimated for each exposure scenario and 

these values were compared to the occupational long-term exposure limit for HFO-1234yf. The results 

are presented in Table 7. 

Table 7. Occupational Risk Assessment

Maximum 15-

minute TWA 

Exposure Levels 

Box-Model

(ppm)

Maximum 8-hr 

TWA Exposure 

Levels 

Box-Model

(ppm)

Occupational 

Long-Term 

Exposure Limit 

(ppm)

Workplace 

Guidance Levels 

Time Period

HFO-1234yf 27.76 8.5 250 8-hr TWA

The modeling results indicate that both the short-term (15-minute) and long-term (8-hour) worker 

exposure concentrations at no point exceed 12 percent of the occupational long-term exposure limit. 

Even the maximum estimated short-term occupational level (i.e., maximum 15-minute TWA exposure) is

significantly lower than the 8-hour long-term exposure limit and therefore occupational exposure to HFO1234yf is not considered a toxicity threat for trained professionals.

Gradient (2008) found that those consumers undertaking “do-it-yourself” car repair could be exposed to 

concentrations of HFO-1234yf ranging from 32,800 ppm (30-minute TWA) to 38,900 ppm (maximum 

instantaneous concentrations). These concentrations are significantly higher than the occupational 

exposure limit; therefore it is recommended that HFO-1234yf not be made available to untrained workers, 

including for “do-it-yourself” car repair.

6.2. Consumer Exposure

This section presents estimates of potential consumer exposures to HFO-1234yf in motor vehicles. A 

consumer exposure analysis was performed to examine potential catastrophic release of the substitute

under a reasonable worst-case scenario.

9

 The analysis was undertaken to determine the 15- and 30-

minute TWA for the substitute, which were then compared to the short-term consumer toxicity limit 

presented in Table 5 to assess the risk to consumers. However, the TWA values are fairly conservative as 

the analysis does not consider opened windows, conditioned airflow (either heated or cooled) and other 

variables that would reduce the levels to which individuals would be exposed.

The model involves a refrigerant leak from an MVAC into an enclosed vehicle. The same set of vehicles 

used in the flammability and asphyxiation analyses (i.e., those vehicles identified by ICF (2008) as having

the highest refrigerant charge to compartment volume ratios in each vehicle class) are used in this model.

The model assumes that the individual is present at the start of the leak and the individual remains in the 

car while the refrigerant is released. It is also assumed that horizontal stratification causes most of the 

refrigerant to settle in higher concentrations closer to the bottom of the passenger compartment.

10 The 

full charge of the MVAC unit is assumed to be emitted within 140 seconds (Blackwell et al. 2006) and 

the vehicles are assumed to have an air flow rate of 0.3 air exchanges per hour (Blackwell et al. 2006). 

 

9 Acute exposure due to a catastrophic leak was selected as the ‘worst-case scenario,’ rather than chronic exposure to a long, slow 

leak because chronic exposures are not expected for vehicle passengers. In the chronic exposure toxicity study conducted for this 

substitute, rabbits were exposed to HFO-1234yf for 6 hours a day, for 28 gestation days. These sorts of exposures are not 

expected for vehicle occupants as the average trip length in the U.S. is 10 miles (U.S. DOT 2007).

10 In order to simulate the horizontal concentration gradient that will occur because of the weight differential between the 

refrigerant and air, it is assumed that 95 percent of the leaked refrigerant mixes evenly into the bottom 0.4 meter of the car, and 

the rest of the refrigerant mixes evenly in the remaining volume (Kataoka 1999). 
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Exposure concentrations were calculated using the box-model described in the Background Document,

which was adapted to estimate concentrations on a minute-by-minute basis. Concentrations in the 

occupants’ breathing area (i.e., the upper stratum of the car) were used for comparison to the toxicity 

limit. Vehicle occupants are not expected to be in the foot well (i.e., the lower stratum) of the car. 

The calculated levels of consumer exposure for the “worst-case” vehicle in each vehicle class are 

presented in Table 8 below.

Table 8. Reasonable Worst-Case Consumer Exposure Assessment

Car Type 15-minute TWA Consumer Exposure 

(ppm)

30-minute TWA Consumer Exposure 

(ppm)

TWO SEATERS 11,664 11,282 

SMALL PICKUP TRUCKS 11,595 11,216 

S.U.V. 7,582 7,334 

STANDARD PICKUP TRUCKS 7,257 7,020 

COMPACT CARS 6,494 6,282 

SUBCOMPACT CARS 6,328 6,122 

MINICOMPACT CARS 6,303 6,097 

MIDSIZE CARS 6,279 6,074 

LARGE CARS 4,929 4,768 

SMALL STATION WAGONS 4,598 4,448 

MINIVAN 4,543 4,395 

VANS 3,562 3,446 

MIDSIZE STATION WAGONS 2,834 2,741 

ICF analysed the maximum concentration in Table 8 to determine if consumer exposures to HFO-1234yf 

under a reasonable worst-case scenario may pose a toxicity threat. ICF concluded that human exposure to 

HFO-1234yf following the accidental release of an MVAC refrigerant charge should not pose a 

toxicological concern (see Appendix A for the full analysis).

6.3. General Population Exposure

Chronic exposures to the substitute are not expected for the general population.

7. VOLATILE ORGANIC COMPOUND (VOC) ANALYSIS

To determine if emissions of HFO-1234yf may adversely affect local air quality by increasing levels of 

ground-level ozone, ICF (2009) modeled potential emissions of HFO-1234yf and examined the affects of 

these potential emissions on air quality in two regions where high levels of ground-level ozone are a 

chronic problem.11 ICF (2009) concluded that while there is a level of uncertainty surrounding the 

impacts of HFO-1234yf emissions on air quality, “non-attainment resulting from HFO-1234yf emissions 

is not likely to be a major concern for local air quality in most locations.”

8. ADDITIONAL ENVIRONMENTAL IMPACTS ANALYSIS

The production of trifluoroacetic acid (TFA) occurs during the atmospheric breakdown of many 

fluorocarbons, including HFO-1234yf. TFA can be wet- or dry-deposited to the earth’s surface and is 

highly soluble and very persistent in aquatic environments (ICF 2009). As TFA is also mildly phytotoxic, 

ICF completed an analysis to determine if emissions of HFO-1234yf could pose a significant risk of 

 

11 These regions are the Houston-Galveston region and the South Coast Air Basin in California.
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October 13, 2009

ecotoxicity. ICF (2009) concluded that “TFA production resulting from HFO-1234yf emissions is not 

expected to pose significant harm to aquatic communities.” 

9. REFERENCES

Blackwell, N., L. Bendixen and E. Birgfeld. 2006. Risk Analysis for Alternative Refrigerant in Motor 

Vehicle Air Conditioning. September.

Honeywell. 2007. Significant New Alternatives Policy Program Submission to the United States 

Environmental Protection Agency for HFO-1234yf. October. 

Gradient. 2008. Risk Assessment for Alternative Refrigerant HFO-1234yf. Confidential report prepared 

for SAE International Cooperative Research Program 1234. February.

ICF. 2008. Air Conditioning Refrigerant Charge Size to Passenger Compartment Volume Ratio Analysis. 

Confidential Memorandum prepared for the U.S. Environmental Protection Agency. August.

ICF. 2009. Revised Final Draft Assessment of the Potential Impacts of HFO-1234yf and the Associated 

Production of TFA on Aquatic Communities and Local Air Quality. Prepared for the U.S. 

Environmental Protection Agency. August.

ICF. 1997. Physiological Effects of Alternative Fire Protection Agents - Hypoxic Atmospheres

Conference. Stephanie Skaggs prepared the proceedings of the conference held May 22, 1997 in 

New London, CT.

Kataoka. 1999. “Allowable Charge Limit of Flammable Refrigerants and Ventilation Requirements.” 

Draft Proposal. O. Kataoka/Daikin/Japan. June.

NFPA. 2008. NFPA 58: Liquefied Petroleum Gas Code. National Fire Protection Agency.

U.S. Department of Energy (DOE) and U.S. Environmental Protection Agency (EPA). 2009. 

FuelEconomy.gov Frequently Asked Questions. Available online at: 

<http://fueleconomy.gov/feg/info.shtml#sizeclasses>.

U.S. Department of Transportation (DOT). 2007. Bureau of Transportation Statistics Special Report: 

Trends in Personal Income and Passenger Vehicle Miles. October 2007. Available online at: <

http://www.bts.gov/publications/bts_special_report/2007_10_03/>.

U.S. EPA. 1994. Significant New Alternatives Policy Technical Background Document: Risk Screen on 

the Use of Substitutes for Class I Ozone-depleting Substances: Refrigeration and Air 

Conditioning. Stratospheric Protection Division. March.

U.S. EPA. 2008. Guidance on Retrofitting to HFC-134a. R-134a Refrigerant – How Much to Charge 

into the System. Last updated December 17, 2008. Available online at: 

<http://www.epa.gov/ozone/title6/609/technicians/retrguid.html#134a>.

U.S. EPA. 2009. “Risk Assessment: PMN 07-0601. Reflecting Deliberations And Decisions From the 

03/04/09 RAD [Risk Assessment Division] Disposition Meeting
Evaluation of accidental refrigerant release of HFO-1234yf in a motor vehicle

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Appendix A

Evaluation of accidental refrigerant release of HFO-1234yf in a motor vehicle

Background

ICF evaluated the accidental release of a refrigerant charge of HFO-1234yf into the passenger 

compartment of a motor vehicle.12 The evaluation included modeling of stratified exposure 

concentrations within the compartment of several different vehicles. It is important to note that exposure 

to the whole refrigerant charge would result from a catastrophic event in the automobile (such as a car 

accident). In this event, exposure to car refrigerant is unlikely to be the primary risk to the car’s driver or 

passengers.

We have not analyzed other scenarios in detail in the risk screen for HFO-1234yf. This is because as 

shown in the analysis below, there is no risk to human health from accidental release of an entire 

refrigerant charge resulting from a car crash. Exposures to consumers undertaking “do-it-yourself” car 

repair will be prevented (as HFO-1234yf is not to be made available to them), so other scenarios were not 

considered. This analysis focuses on a worst case scenario.

Refrigerant Release Scenarios and Resulting Concentrations 

Based on the modeling, the most conservative scenario involves a small Mercedes sedan with a high 

charge size to compartment volume ratio (640.75 g/m3). The highest concentration of HFO-1234yf was 

estimated in the area of the floorboards at 282,000 ppm. At the breathing zone, however, the estimated 

concentration is approximately 11,700 ppm (Table 1). 

Table 9: Worst-case scenario and resulting concentrations

Car type

Passenger 

Compartment 

Size (m3)

a

MAC Charge 

Size (grams) 

HFC-134ab,c

Ratio of HFC134a Charge to 

Compartment 

Volume (g/m3)

Maximum 15-Minute TWA 

Exposure (ppm)

Manufacturer

Model Lower 

compartment

Upper 

compartment

TWO 

SEATERS 1.42 907.2 640.75 282,205 11,664

MERCEDESBENZ

SL65 AMG (MY 

2007)

a

Source: FuelEconomy.gov, and if not available there, BlackBookUSA.com. 2008 New Car Cost Guide. When volume was 

unavailable, the value was estimated using BlackBook data and methods at 40 CFR 600, Subpart D.

b

Source: Motor Information Systems database.

cAssuming a one-to-one replacement ratio for HFO-1234yf substituting for HFC-134a.

Toxicity Evaluation for Reasonable Worst Case Scenario

This study considers the 11,700 ppm concentration for the purposes of toxicity evaluation. An accidental 

release of car refrigerant due to a car crash is a very low probability event involving acute exposure only. 

Most toxicity studies involve exposures lasting several hours per day for several days (e.g., 6 hours/day, 5 

days/week for 3 months for sub-chronic studies). Acute toxicity studies are the most appropriate for 

comparison to modeled human exposure under these conditions. The acute (4-hour) Lowest Observed 

Adverse Effect Level (LOAEL) value for single inhalation exposure to HFO-1234yf in rats is 201,600 

ppm. 

When the worst-case breathing zone exposure concentration following a catastrophic release of HFO-

 

12 This modeling and related assumptions are discussed in detail in the risk screen for HFO-1234yf. The most conservative 

scenario is discussed here.
Evaluation of accidental refrigerant release of HFO-1234yf in a motor vehicle

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1234yf (11,700 ppm) is compared to the acute LOAEL value for the compound in rats (201,600 ppm), it 

is apparent that even the worst case exposure concentration is more than one order of magnitude lower 

(about 17 times lower). This indicates that acute toxicity is not a risk in this low-probability event.

Conclusion

ICF believes that the most appropriate toxicity study to use as a comparison for acute exposures in 

humans is the 4-hour acute inhalation study in rats, which had a LOAEL of 201,600 ppm. The most 

conservative modeled breathing-zone exposure concentration resulting from an accidental complete 

discharge of the car’s refrigerant is 11,700 ppm, much lower than the top exposure concentration to which 

rats were exposed for 4 hours. Therefore, acute exposure to elevated concentrations of HFO-1234yf is not 

expected to be a toxicological concern in humans who are exposed following a worst-case, accidental 

release of a car refrigerant charge.