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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

April 4, 2012

MEMORANDUM

SUBJECT: Risk Assessment of Single Exposures to HFO-1234yf (P-07-0601)

FROM: Ernest V. Falke, Ph.D.

Senior Scientist

Risk Assessment Division (7403M)

Office of Pollution Prevention and Toxics

THRU: Oscar Hernandez, Acting Branch Chief

Science Support Branch

Risk Assessment Division (7403M)

Office of Pollution Prevention and Toxics

TO: Ken Moss

New Chemicals Notice Management Branch

Chemical Control Division (7405M)

Office of Pollution Prevention and Toxics

Contents

Background:.............................................................................................................................................. 1

Discussion and Conclusions: .................................................................................................................... 2

Area Under the Curve Assessment ....................................................................................................... 2

Concentration Dependent Risk Assessment ......................................................................................... 3

Conclusions........................................................................................................................................... 3

Hazard Assessment from Single Exposures: ............................................................................................ 3

Exposure Assessment from Single Exposures in Do-It-Yourself (DIY) Secenarios:............................... 4

Highest TWA ........................................................................................................................................ 5

Highest peak exposures......................................................................................................................... 6

References:................................................................................................................................................ 7

APPENDECES: ........................................................................................................................................ 8

APPENDIX 1. Review of rabbit one hour inhalation study. ................................................................ 8

Background:

One of the use scenarios for HFO-1234yf is from Do-It-Yourself (DIY) individuals charging 

their vehicle air conditioning system. In order to assess this risk, data on short duration exposures 

related to this scenario are required from both toxicology and exposure studies. The hazard assessment 

OFFICE OF

CHEMICAL SAFETY AND POLLUTION 

PREVENTION
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section below discusses the relevant toxicity studies and presents a brief description of the exposure 

study. The exposure results are presumed to be accurate for the purpose of this assessment.

Discussion and Conclusions:

It is important to note that this assessment is valid ONLY FOR SINGLE, SHORT DURATION 

EXPOSURE SCENARIOS THAT DO NOT REPEAT OVER A NUMBER OF DAYS.

In assessing risk from different exposures one must consider the mode of action. Typically the 

assumption is made that the toxicity is based upon the total dose delivered during a daily exposure. 

This presumes that the area under the curve (AUC) is the determining factor. Such toxicants follow 

Haber’s Rule where concentration x time = constant (c*t=k). In this case a high exposure for a short 

duration or a low exposure for a long duration can have the same toxic result as long as their products 

are the same. This mode of action is presumed in the time weighted approach (TWA) scenario 

discussed in the Exposure Assessment section.

A concentration dependent mode of action depends on exceeding a specific level in the blood to 

exhibit toxicity. One example would be anesthetic agents. As long as blood levels are below a 

threshold concentration, narcosis will not occur, regardless of duration of exposure. Another example 

would be cardiac sensitization where short duration exposures to high levels can cause cardiac 

sensitization within minutes whereas the same dose averaged over an hour would not result in a toxic 

effect.

Typically the AUC mode of action is assumed when performing risk assessments. A 

concentration dependent mode of action may be utilized where there are data to that effect. For HFO1234yf no toxicity was seen in the one hour exposure of rabbits to 0, 50,000, and 100,000 ppm 

discussed below. The Human Equivalent Concentration of the 100,000 ppm exposure in rabbits is 

190,000 and will be utilized for this assessment. No effect was seen at 100,000 ppm exposure in 

rabbits. If higher exposures were tested, it is unknown whether toxicity would be observed before 

hypoxia from low levels of oxygen occurred. Since no toxicity was observed, it is not possible to 

differentiate between a potential AUC or concentration dependent mode of action so a worst case 

analysis will be done for each mode of action.

Area	Under	the	Curve	Assessment

The No Observed Adverse Effect Level (NOAEL) HEC of the rabbit study is 190,000 ppm for 

60 minutes of exposure. This equates to an AUC of 11,400,000 ppm-min (190,000 ppm * 60 

min). The exposure in the exposure scenario (“full, upright can (340 g) released at LS Service 

Port with hood half open and condenser fan off” ) with the highest TWA for 30 minutes was

1,789 ppm. Since Figure 17 showed exposure levels were about the same from 3 to 60 minutes 

an exposure of 1,789 ppm will be assumed for 60 minutes for a worst case scenario. The 

cumulative exposure AUC is 107,340 ppm-min (1,789 ppm * 60 min). The MOE for the worst 

case scenario is 11,400,000 ppm-min/107,340 ppm-min = 106.

For this mode of action and chemical the recommended MOE is 30. Typically an MOE of 100 

is recommended; a factor of ten for potential differences in species susceptibility and a factor of 

10 for differences between individual human susceptibility. The factors of ten comprise 

uncertainties due to differences in pharmacokinetics and pharmacodynamics. Since 

pharmacokinetic modeling was used to estimate the HEC, the pharmacokinetic component of 
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the interspecies uncertainty is known, and the interspecies factor of 10 is reduced to 3. This 

results in a total MOE of 30.

The MOE of 106 is acceptable because it exceeds the recommended MOE of 30. The 

assessment is inherently conservative because no toxicity was identified at the highest dose 

tested; it is unlikely that the user would remain for an hour in the “Passenger side under the 

engine” location after releasing a whole can of refrigerant; and it is unknown whether higher 

exposures would have resulted in toxicity being observed before hypoxia set in.

Concentration	Dependent	Risk	Assessment

For HFO-1234yf there are no identified concentration dependent modes of action. HFO-1234yf 

is in a class of chemicals that sometimes cause cardiac sensitization. It is replacing HVC-134a 

which is a cardiac sensitizer. However, HFO-1234yf has been tested at 120,000 ppm and did 

not cause cardiac sensitization. It is considered negative in this test. For comparison, the HEC 

of this concentration is 228,000 ppm.

Conclusions

Toxicity studies of HFO-1234yf in rabbits and dogs did not show signs of toxicity or cardiac 

sensitization at HEC exposure concentrations that are borderline hypoxic in humans. Using an 

AUC mode of action the calculated MOE for worst case exposure scenarios is 106, higher than 

the 30 recommended. There are no risk concerns identified for the exposure scenarios 

presented in Honeywell, 2012. The only potential for toxicity due to short duration (on the 

order of one hour), single exposures is if exposure levels reach high enough levels to deplete 

oxygen. These levels were not reached in the Honeywell, 2012 exposure scenarios. IT IS 

IMPORTANT TO NOTE THAT THIS ASSESSMENT ONLY ADDRESSES SHORT 

DURATION EXPOSURES THAT ARE NOT REPEATED OVER A PERIOD OF DAYS.

Hazard Assessment from Single Exposures:

This discussion will focus on the one hour acute toxicity study in rabbits (Honeywell, 2011) 

because the rabbit is the most sensitive species. The full report on the study is included in Appendix 1. 

The reason to focus on the rabbit is the observation that repeated exposures can cause cardiac 

histopathology and death in pregnant rabbits. In order to address the toxicity of HFO-1234yf from 

single short duration exposures, Honeywell, 2011 exposed pregnant and non-pregnant female rabbits as 

well as males to 0, 50,000, and 100,000 ppm for one hour. Animals were necropsied 14 days postexposure and histopathology performed on heart, kidneys, liver and lungs. No treatment related 

differences were noted between control and exposed animals. The No Observed Adverse Effect Level 

(NOAEL) for this study is 100,000 ppm exposure for 1 hour. The Human Equivalent Concentration 

(HEC) of the NOAEL is the exposure level required to raise blood levels in humans to those seen in the 

rabbit at 100,000 ppm. Based on earlier work the HEC is 190,000 ppm.

For this class of compounds, cardiac sensitization may be of concern. HFO-1234yf is not a 

cardiac sensitizer at the highest dose tested of 120,000 ppm (DuPont, 2010). HFO-1234yf is replacing 

HFC-134a. It is interesting to note that HFC-134a is a cardiac sensitizer in dogs at 75,000 ppm but not 

50,000 ppm (NRC, 2002). Thus, for single high exposure scenarios, HFC-134a is a greater risk than 

HFO-1234yf.
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Exposure Assessment from Single Exposures in Do-It-Yourself (DIY) Secenarios:

Honeywell, 2012 conducted experiments with HFC-134a to determine exposure levels in the 

event a DIY user did not properly connect the refrigerant can and the entire contents were released in a 

short period of time. This section will discuss the data used for the risk assessment. The validity of the 

study is determined by the exposure experts.

For the purpose of this exposure assessment, two of the six test cases will be considered. They

give either the highest time weighted average (TWA) for 30 minutes or the highest peak exposures. 

Exposure concentrations were measured at eight locations. Results below are given for the highest 

concentration measured.
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Highest	TWA

Test case #6-“full, upright can (340 g) released at LS Service Port with hood half

open and condenser fan off.” These test conditions gave the highest 30 minute TWA of 1,789 

ppm at the Passenger Side Under Engine location (see Figure 17 below from the report). The 

highest peak concentration was approximately 15,000 ppm, less than Test Case #4 discussed 

below. The peak was reached within approximately one minute, decreased to near background 

levels within about 2 minutes and then increased to approximately 2,000 ppm for about 60 

minutes before falling to background levels.
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Highest	peak	exposures

Test case #4- “full, inverted cans (2 cans, 680 g) released at LS Service Port with hood 

open half and condenser fan off.” In this case two cans were released within a minute of 

each other giving two peaks with maximum concentrations being achieved within 

approximately 30 seconds of opening the can. The maximum peaks were approximately

30,000 ppm each. The exposure levels rapidly decreased to near background levels after 

a total duration of 180 seconds at the Driver Side Under Engine location (see Figure 15 

below from the report). The highest time weighted average was 1,030 ppm, less than 

seen with test case #6.
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References:

DuPont. 2010. Material Safety Data Sheet. ,23,3,3-Tetrafluoro-1-propene (Research Sample). 

http://msds.dupont.com/msds/pdfs/EN/PEN_09004a2f806513e4.pdf

Honeywell. 2011. HFO-1234yf: A 2-phase inhalation screening study and single exposure study in rabbits via 

whole-body inhalation exposure (GLP).

Honeywell. 2012. Refrigerant exposure to service personnel or DIYers due to leakage of 12 oz 

charging cans or “small cans”. Experiments Conducted at Honeywell’s Research Laboratory in 

Buffalo, NY USA. 14 pp.

NRC. 2002. Acute Exposure Guideline Levels for Selected Airborne Chemicals. 1,1,1,2-

Tetrafluoroethane. (HFC-134a) Acute Exposure Guideline Levels.
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APPENDECES:

APPENDIX	1.	Review	of	rabbit	one	hour	inhalation	study.

Reviewer:

Ernest V. Falke

April 18, 2011

Reference/Report:

HFO-1234yf: A 2-PHASE INHALATION SCREENING STUDY AND SINGLE EXPOSURE 

STUDY IN RABBITS VIA WHOLE-BODY INHALATION EXPOSURE (GLP)

Study Type:

One hour inhalation study in pregnant rabbits.

Study Date:

February 28, 2011

Test Material:

Name:

HFO-1234yf (2,3,3,3 – Tetrafluoropropene)

CAS No:

754-12-1

Structure:

Test Material Composition:

HFO-1234yf was administered as a gas and was 99.99% pure.

Sponsor:

Honeywell

101 Columbia Road

PO Box 2245R

Morristown, New Jersey 07962-1057

Test Lab:

Huntingdon Life Sciences

100 Mettlers Road

East Millstone, New Jersey 08875-2360

Materials Methods/Results

In earlier developmental toxicity studies in rabbits, lethality of the dams was observed after 

multiple exposures. While deemed unlikely, the outcome of lethality from a single exposure was not 

known. This experiment was designed to address that question.

New Zealand White rabbits were exposed to target concentrations of 0; 50,000; and 100,000 

ppm for 1 hour and observed for 14 days post exposure. Analytical concentrations for male/female 

were 0/0; 45,000/47,000; and 100,000/102,000 respectively. Five males and 6 pregnant females were 

exposed at each dose. Five non-pregnant females were exposed at 0 and 100,000 ppm.
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No mortality was observed for any condition of the experiment. No differences in body weight 

over the experiment, gross pathology, and histopathology (control and high dose) of heart, kidney, liver 

and lung were observed.

Metabolites were analyzed from urine samples taken at 12 hour intervals after exposure up to 48 

hours post-exposure. Metabolite patterns were similar in all exposed groups except an unidentified 

metabolite approximately twice as much in females (both groups) as males.

In conclusion, a single exposure of pregnant rabbits to high levels of HFO-1234yf (100,000 

ppm) will not cause lethality, changes in body weight, or pathology and histopathology of major 

organs. The cause of lethality in dams exposed multiple time in developmental toxicity studies remains 

unknown.