1 May 22, 2009

Significant New Alternatives Policy Program 

Refrigeration and Air Conditioning Sector

Risk Screen on Substitutes for CFC-12 in Household Refrigerators and Household 

Freezers.

Substitute: Isobutane

This risk screen contains no 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 (EPA 1994) (hereinafter referred to as the Background Document) by adding to the 

list of potential substitutes for specific end-uses of CFC-12 in this sector. Proposed end-use applications 

considered in this analysis include new replacements for household refrigerators and household freezers. 

The specific proposed CFC-12 substitute examined in this report is isobutane (R600a). Table 1 presents 

the composition of the substitute, including the maximum estimated concentrations of impurities which 

may be present in the substitute.

Table 1. Composition of the Substitute.

Component Concentration 

(Weight Percent)

Substitute

Isobutane 99.5%

Potential Impurities (maximum concentration)

Propane and N-butane 0.5%

1,3-Butadiene 0.0005%

N-Hexane 0.005%

Benzene 0.0001%
2 May 22, 2009

Table 1. Composition of the Substitute (continued)

Component Concentration 

(Weight Percent)

Sulfur 0.0001%

Liquid phase water 0.0005%

Air 0.05% (V/V)

The potential risks associated with use of substitutes in residential refrigeration 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. Presently, EPA’s SNAP Program has not approved 

any hydrocarbon blends as a substitute for CFC-12 in residential appliance end uses, such as those 

intended for this substitute (i.e., household refrigerators and household freezers). Of particular concern 

are the flammability risks associated with hydrocarbon blends during manufacturing, use, servicing, and 

disposal of household appliances. Occupational exposure modeling was performed to ensure that use of 

the proposed substitute in the application listed above did not pose unacceptable risk to workers. 

Consumer exposure modeling was performed to examine potential catastrophic releases of the substitute. 

Lastly, general population exposure modeling was performed to ensure that the proposed substitute would 

not pose unacceptable risk to the population at large. 

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: References

2. SUMMARY OF RESULTS

Isobutane is recommended for SNAP approval for household refrigerators and freezers. EPA's risk screen 

indicates that the use of the proposed substitute and its constituents will be less harmful to the atmosphere 

than the continued use of CFC-12. No significant toxicity risks to workers, consumers, or the general 

population are expected according to occupational and consumer exposure modeling. Flammability 

models indicate that risks of explosions are not a concern for consumers, provided the refrigerant is not 

used in small, poorly ventilated spaces (see Section 4). Caution must be used in manufacturing facilities 

and by refrigeration technicians to minimize 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) Standards 15 

and 34 be followed. 

3. ATMOSPHERIC MODELING

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

isobutane in the residential refrigeration sector. The ODP, GWP, and atmospheric lifetime (ALT) of the 

proposed substitute are presented in Table 2.

The environmental impacts resulting from use of isobutane are generally in the range of those predicted 

for other substitutes examined in the Background Document. The substitute is substantially less harmful
3 May 22, 2009

to the ozone layer, has less climate impact, and a shorter atmospheric lifetime compared to CFC-12.

Table 2. Atmospheric Impacts of Isobutane Compared to CFC-12.

Refrigerant Ozone Depleting Potential 

(ODP)

Global Warming Potential 

(GWP)

Atmospheric Lifetime

years (ALT)

Isobutane 0

a 8a <1b

CFC-12 1

c 8,100d 100c

a Isobutane SNAP Submission (GE 2008).

b Atmospheric lifetime (ALT) not provided in SNAP submission. Calm and Hourahan (2007) indicate the ALT is 0.019 years, or 

about one week.

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

d IPCC, Second Assessment Report (1996).

4. FLAMMABILITY ANALYSIS

Due to its flammable nature, isobutane could pose a significant safety concern for workers and consumers 

if it is not handled carefully. In the presence of an ignition source (e.g., static electricity, a spark resulting 

from a closing door, or a cigarette), an explosion or a fire could occur when the concentration of 

isobutane exceeds its lower flammability limit (LFL) of 18,000 ppm. Therefore, it is important to ensure 

that the levels of isobutane do not exceed 18,000 ppm. In production facilities or other facilities where

large quantities of the refrigerant will be stored, proper safety precautions should be in place to minimize 

the risk of explosion. White goods installed with isobutane should be clearly labeled as containing a 

flammable refrigerant charge and designed to prevent catastrophic leaks. Furthermore, only refrigerant 

technicians certified to work with flammable refrigerants should handle these units during manufacturing, 

installation, servicing, transportation and disposal. 

To determine whether flammability would be a concern for consumers, a reasonable worst-case scenario 

analysis was performed to model catastrophic release of the refrigerant. The analysis models the release 

of charge from a refrigerator into a kitchen. The kitchen volume is assumed to be 18 m3(635 ft3) or 

approximately 2.7 x 2.7 m (8.9 x 8.9 ft) for a square room with 2.4 m (8 ft) ceilings (EPA 1994). In the 

analysis, the maximum permissible leak per the standard UL 250 Supplement SA (50 g)1is assumed to be 

emitted within one minute and the kitchen is assumed to have an air flow rate of 2.5 air exchanges per 

hour. Given these conservative assumptions, the assumed reasonable “worst-case” scenario is highly 

unlikely. 

Horizontal stratification is also assumed since isobutane is denser than air and will settle in higher 

concentrations closer to the ground. 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 room, and the rest of the refrigerant 

mixes evenly in the remaining volume (Kataoka 1999).

Under catastrophic release scenarios, the maximum instantaneous concentration of isobutane in the lower 

stratum of the room would be approximately 37 percent of its LFL for the average room size assumed, as 

shown in Table 3. The maximum instantaneous concentration is lower in the upper stratum of the room, 

as only five percent of the leaked refrigerant is present in this stratum, and this stratum has a greater 

volume than the lower stratum. For flammability to be of concern under the conservative (protective) 

assumptions described above, the volume of the kitchen would have to be about 7 m3 (247 ft3). Assuming 

a square room with a ceiling height of 2.4 m (8.0 ft), this equates to a 1.7 x 1.7 m (5.6 x 5.6 ft) kitchen. 

 

1

Fifty grams is the maximum permissible leak for refrigerants with limits of flammability and heat of combustion greater than 

19,000 kj/kg, such as isobutane, per the standard UL 250 Supplement SA. Per this standard, a larger charge is not prohibited, if

the amount of refrigerant leaked during testing does not exceed 50 grams. Therefore, if a charge greater than 50 grams is to be 

used in these refrigerators, the system should be designed to ensure that a refrigerant leak would not exceed 50 grams.
4 May 22, 2009

Very limited data are available regarding the distribution of kitchen volumes in the United States. 

However, an analysis by Murray (1997) which aggregated data from over 60 different projects reported

the minimum kitchen volume encountered as 31 m3.

2

Further, only one percent of houses sampled had a 

kitchen smaller than 53 m3. 

To avoid the risk of fire and explosion, it is recommended that the refrigerators not be installed in small, 

poorly ventilated spaces such as very small ‘galley’ kitchens or storage closets (especially as other 

equipment or appliances in the space would reduce the effective volume of the room). However, based 

on the available data regarding kitchen volumes, it is not likely that a kitchen would exist which would be 

small enough for flammability to be a concern, even under a reasonable worst-case scenario. For fullsized kitchens the risk of explosion is minimal. The installation of leak prevention devices would further 

protect against the very limited risk of explosion. For example, in commercial refrigeration end-uses, 

refrigerant leak prevention systems can be used to capture leaked refrigerant in a receiving tank during 

over pressure events, rather than venting the refrigerant to the atmosphere.3 Additionally, in motor 

vehicle air-conditioning, an outflow prevention device can be used to prevent leakage events when the 

heat exchanger of a unit is damaged.4ICF recommends that similar safety devices (designed for 

residential refrigerators and freezers) be installed in units containing isobutane to prevent refrigerant leaks 

and thereby further reduce the risk of explosion.

Table 3. Flammability Assessment

Room Type/Appliance 

Type

Reasonable Worst-Case Scenario Flammability Threshold Scenario

Room Size 

(m3)

Maximum 

Instantaneous 

Concentration 

(ppm) a,b

Room Size (m3) Maximum 

Instantaneous 

Concentration 

(ppm) a,b

Kitchen/Refrigerator 18 (635 ft3) 6,647 7 (247 ft3) 17,006

a Lower Flammability Limit of isobutane is equal to 18,000 ppm.

b Values provided in these columns refer to the concentration in the lower stratum of the room. 

 

Catastrophic releases of large quantities of refrigerant during servicing and manufacturing, 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 isobutane. The submitter 

has provided information regarding their training program for service technicians. The program includes 

detailed information regarding proper recovery of the refrigerant prior to service as well as information on 

servicing the refrigerator and charging the system with isobutane (GE SNAP Submission 2008). This 

training should be provided for all technicians who will be servicing refrigerators using isobutane. As a 

further precaution, isobutane 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. 

 

2 Murray (1997) uses data from the Brookhaven National Laboratory PFT database. Data regarding kitchen volumes were only 

available for the LA area and the kitchen volumes are for a “kitchen zone” which may include associated areas, such as utility 

rooms, dining rooms, living rooms, and family room. The inclusion of these other spaces in the “kitchen zone” volume is 

realistic, as the presence of adjoining rooms would increase the volume into which the refrigerant could leak.

3

http://www.freepatentsonline.com/5259204.html

4

http://www.freepatentsonline.com/6966365.html
5 May 22, 2009

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 isobutane 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 equipment using 

isobutane. During servicing operations, technicians should ensure that proper ventilation is in place 

through the use of fans (or other mechanical ventilation devices) and portable refrigerant detectors should 

be used to alert technicians to the presence of flammable gases in the area. 

5. ASPHYXIATION

The risk of asphyxiation for a reasonable “worst-case” scenario was investigated for isobutane. 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, conditioned airflow (either 

heated or cooled), or even seepage between the door and door frame. 

The maximum leak of isobutane necessary to reduce the oxygen levels to 12 percent in air,

6

in a kitchen 

of volume 18 m3(635 ft3) (EPA 1994), was calculated, assuming horizontal stratification of the 

refrigerant and the air. Horizontal stratification is assumed since isobutane is denser than air and will 

settle in higher concentrations closer to the ground. Assuming that nitrogen and oxygen retain the same 

relative volumes in the rooms with the balance composed entirely of isobutane, and that the pressure of 

the room does not increase significantly with the addition of the refrigerant, a leak of approximately 625 g 

would be necessary to reach 12 percent oxygen in the lower stratum. This amount is more than twelve

times the maximum permissible leak for a single isobutane refrigerator or freezer (50 g). The size of a 

leak needed to reach the same effect in the upper strata would be even greater because of the stratum’s 

larger volume. For asphyxiation to be of concern with the maximum permissible leak, under the 

conservative (protective) assumptions described above, the volume of the kitchen would have to be about 

1.5 m

3

(53 ft3). Assuming a square room with a ceiling height of 2.4 m (8.0 ft), this equates to a 0.8 x 0.8

m (2.6 x 2.6 ft) kitchen. 

The results of the asphyxiation assessment are summarized in Table 4 below. It is recommended that 

isobutane refrigerators not be installed in small, poorly ventilated spaces, such as very small ‘galley’ 

kitchens, to avoid the risk of asphyxiation. However, based on the available data regarding kitchen 

volumes (see Section 4), it is not likely that a kitchen would exist which would be small enough for 

asphyxiation to be a concern, even under a reasonable worst-case scenario. Therefore, EPA does not 

believe that the use of isobutane in this end-use poses a significant risk of asphyxiation or impaired 

coordination to consumers. 

Table 4. Asphyxiation Assessment

Room Type/Appliance 

Type

Maximum 

Permissible 

Leak from 

Appliance (g)

a

Reasonable Worst-Case Scenario Asphyxiation Threshold Scenario

Room Size (m3) Leak Size

Causing 

Impairment (g)b

Room Size (m3) Leak Size

Causing 

Impairment (g)b

Kitchen/Refrigerator 50 18 (635 ft3) 625 1.5 (53 ft3) 50

 

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 is the No Observed Adverse Effect Level (NOAEL) for hypoxia (ICF 1997). 
6 May 22, 2009

a

Fifty grams is the maximum permissible refrigerant leak per the standard UL 250 Supplement SA. 

b Values provided in these columns refer to the leak size required to cause impairment in the lower stratum of the room. 

6. TOXICITY REFERENCE VALUES FOR SUBSTITUTES

To assess potential health risks from exposure to this substitute in the residential refrigeration sector, 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, such as the Workplace Guidance Levels (WGLs), or for a 10 to 30-minute period for 

short-term exposure, such as the Emergency Guidance Levels (EGLs), as shown in Table 5. Because they 

are designed to assess risks from acute exposure, emergency guidance levels are used to assess risks from 

short-term consumer exposures. Reference concentrations (RfCs) are used to assess risks to the general 

population from exposure to ambient air releases and to assess potential risks associated with chronic 

consumer exposures. 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. 

Table 5. Toxicity Levels of Isobutane and Potential Impurities

Long-term Exposure

ppm

Short-term Exposure

ppm

Reference Concentration (RfC)

mg/m3

Substitute

Isobutane 800 a(NIOSH REL) 18000 b(NOAEL) 0.95 c

Potential Impurities

Propane 1000 d

(OSHA PEL/NIOSH REL)

2100 d(IDLH) 0.9 c

n-Butane 800 e(NIOSH REL) NA 0.95 c

1,3-Butadiene 1 f(OSHA PEL) 5 f(OSHA PEL - STEL)

670 g(10 minute interim AEGL-1)

2 x 10-3 h

N-Hexane 50 I (NIOSH REL) 1100 i (IDLH) 0.7 j

Benzene 0.1 k(NIOSH REL) 1 k(NIOSH STEL)

500 k(IDLH)

3 x 10 -2 l

Sulfur NA NA NA

NA = Not Available

a http://www.cdc.gov/niosh/npg/npgd0350.html

b OSHA (2004) notes the following regarding isobutane: “OSHA does not have a PEL for isobutane, which is affirmed as 

"generally recognized as safe" as a direct human food ingredient (21 CFR 184.1165). No toxic effects reported below 

18,000 ppm.”

c SNAP Refrigerant Background Document (EPA 1994).

d http://www.cdc.gov/Niosh/npg/npgd0524.html

e http://www.cdc.gov/niosh/npg/npgd0068.html

f http://www.cdc.gov/niosh/npg/npgd0067.html

g http://www.epa.gov/opptintr/aegl/pubs/rest148.htm

h http://www.epa.gov/ncea/iris/subst/0139.htm#refinhal

i http://www.cdc.gov/niosh/npg/npgd0322.html

j http://www.epa.gov/ncea/iris/subst/0486.htm#refinhal

k http://www.cdc.gov/niosh/npg/npgd0049.html

l http://www.epa.gov/iris/subst/0276.htm#refinhal
7 May 22, 2009

Table 6. Explanation of Toxicity-Related Acronymsa

Organization Definition

OSHA Occupational Safety and Health Administration

NIOSH National Institute for Occupational Safety and Health

Exposure Limit Definition Explanation

AEGL-1 Acute Exposure Guideline Level The AEGL-1 “is the airborne concentration... above which it is 

predicted that the general population, including susceptible 

individuals, could experience notable discomfort, irritation, or certain 

asymptomatic nonsensory effects. However, the effects are not 

disabling and are transient and reversible upon cessation of 

exposure” (EPA 2008a).

IDLH Immediately Dangerous to Life and 

Health

If exposed to this concentration, room occupants are expected to be 

able to escape the room within 30 minutes without experiencing 

escape-impairing or irreversible health effects.

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

PEL Permissible Exposure Limit This is an 8-hour time-weighted average exposure limit set by 

OSHA. 

PEL - STEL Permissible Exposure Limit - Shortterm exposure limit.This is a 15-minute time-weighted average exposure limit set by 

OSHA.

REL Recommended Exposure Limit This is a 10-hour time-weighted average exposure limit set by 

NIOSH.

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

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

aAll information in this table taken from EPA (1994), except where noted otherwise.

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

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 exposures of workers 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 1 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 

 

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. 
8 May 22, 2009

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.9 For the 

purposes of this model, the isobutane market penetration rate was estimated using data from the SNAP 

submission and the Vintaging Model (version 4.3_2.16.09) and one production facility was assumed to be 

in operation. These assumptions result in approximately 44 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 

this value compared to the Workplace Guidance Levels (WGLs) for isobutane and the potential impurities 

in the substitute. The modeling results indicate that both the short-term (15-minute and 30-minute) and 

long-term (8-hour) worker exposure concentrations at no point exceed 2 percent of the WGLs. Table 7 

displays the maximum estimated 15-minute TWA occupational exposure levels of isobutane and the 

potential impurities in the substitute. Even these maximum estimated short-term occupational levels are 

significantly lower than the 8-hour or 10-hour long-term WGLs, and therefore occupational exposure to 

isobutane and the potential impurities is not considered a toxicity threat. 

Table 7. Occupational Risk Assessment

Maximum 15-minute 

TWA Occupational 

Exposure Levels 

(ppm)

Workplace 

Guidance Levels 

(ppm) a

Workplace 

Guidance Levels 

Time Period

Substitute

Isobutane 10.16 800 10-hour TWA

Potential Impurities

Propane and Butane b 5.8 x 10-2

1000 (propane)

800 (butane) 10-hour TWAc

1,3-butadiene 5.5 x 10-5 1 8-hour TWA

n-Hexane 3.4 x 10-4 50 10-hour TWA

Benzene 7.6 x 10-6 0.1 10-hour TWA

Sulfur 1.9 x 10-5 NA NA

NA = Not Available

a. See Table 5 for more information.

b. Propane and Butane weight percentage in substitute presented as one single value in 

submission. Exposure level is well below the WGL for either constituent.

c. The WGL for Propane is set as a 1000 ppm 8-hr TWA by OSHA and a 10-hr TWA by 

NIOSH. The Butane value is a 10-hr TWA.

6.2 CONSUMER EXPOSURE

This section presents estimates of potential consumer exposures to isobutane in home appliances. A 

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

under a reasonable “worst-case” scenario. Estimates for acute/short-term consumer exposures resulting 

from catastrophic leakage of refrigerant from residential refrigerators were examined. The analysis was 

undertaken to determine the 15- and 30-minute TWA for the substitute, which were then compared to the 

standard toxicity limits 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, fans operating, 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 a refrigerator into an enclosed kitchen of volume 18 m3(635 

ft3). The model assumes that the individual is present at the start of the leak and the individual remains in 

 

9

Submission indicates this is the maximum number of days per year that manufacturing will occur.
9 May 22, 2009

the room 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 ground. Considering the horizontal 

stratification is important because children, a particularly vulnerable segment of the population, breathe 

air that is closer to the ground. 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 room, and the rest of the refrigerant 

mixes evenly in the remaining volume (Kataoka 1999). 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. It was assumed that the maximum permissible leak per the standard UL 250 

Supplement SA (50 g) would be released during a time span of 1 minute, at which time the concentration 

of isobutane will peak, and then steadily decline onwards. Refrigerant concentrations were modeled 

under two air change scenarios believed to represent the range of potential flow rates for a home, 

assuming flow rates of 2.5 and 4.5 air changes per hour (ACH) (Sheldon 1989). 

The highest concentrations of the refrigerant occur in the lower stratum of the room and when assuming 

2.5 ACH. The highest expected levels of consumer exposure based on this analysis are presented in

Table 8 below.

Table 8. Consumer Exposure Assessment

15-minute TWA 

Consumer Exposure 

(ppm)

30-minute TWA 

Consumer Exposure 

(ppm)

Substitute

Isobutane 5,025 3,844

Potential Impurities

Propane and Butane a 29 22

1,3-butadiene 0.03 0.02

n-Hexane 0.17 0.13

Benzene 0.004 0.003

Sulfur 0.009 0.007

TWA = Time Weighted Average

a. Propane and Butane weight percentage in substitute presented as one single value 

in submission. Exposure level is well below the exposure limits for either constituent.

OSHA (2004) states no toxic effects are reported with exposures to isobutane below 18,000 ppm. Even 

under the very conservative assumptions used in the consumer exposure modeling, both the estimated 15-

minute and 30-minute consumer exposures to isobutane are lower than this level and thus should not pose 

a toxicity threat. The exposure levels of the impurities are also lower than their respective short-term 

toxicity levels (see Table 5) and thus should not pose a toxicity threat.

6.3 GENERAL POPULATION EXPOSURE

In the SNAP Background document for refrigerants (EPA 1994), the RfC value for isobutane is 0.95 

m/m

3

. We compared this RfC to estimated factory releases and on-site releases. This gives a ratio of 

exposure concentration to RfC that varies between 1.3 x 10-5to 1.7 x 10-1, depending on the type of 

release scenario.10 Ratios of exposure concentration to RfC for the substitute’s impurities were even 

lower. Since the exposure concentrations for the substances are lower than the RfC values, the substitute 

is not expected to pose a toxicity threat to the general population.

 

10 Release scenarios include factory, on-site and recycling/salvage yard releases, with and without recycling of the refrigerant.
10 May 22, 2009

7. VOLATILE ORGANIC COMPOUND (VOC) ANALYSIS

Isobutane has not been exempted as a VOC under the CAA (40 CFR 51.000). However, through 

regulations and standard industry practices, VOC emissions should be controlled. Chapter 8 of the 

Background Document shows that potential emissions of VOCs from all substitutes for all end uses in the 

refrigeration and air conditioning sector are likely to be insignificant relative to VOCs from all other 

sources (i.e., other industries, mobile sources, and biogenic sources). Additional analysis shows that even 

if all refrigerators produced by GE in one year were to leak the maximum permissible leak per the 

standard UL 250 Supplement SA over the course of the year (extremely unlikely), the resulting annual

VOC emissions would be less than 1x10-5percent of all annual anthropogenic VOC emissions.

11

 

Further, the calculated potential VOC emissions due to use of isobutane in household refrigerators is less 

than 1x10-4percent of annual residential wood combustion emissions.12 As these emissions of isobutane 

are several orders of magnitude less than other anthropogenic emissions, including other residential 

emissions (i.e., residential wood combustion), the environmental impacts of these VOCs are not 

considered a threat.

8. REFERENCES

Calm, J.M. and G.C. Hourahan. 2007. “Refrigerant Data Update.” Heating and Refrigerant Data 

Update. 79(1):50-64. January 2007. 

EPA 2008a. Acute Exposure Guideline Levels: Definitions. Accessed 27 February 2009. Available at: 

<http://www.epa.gov/opptintr/aegl/pubs/define.htm>.

EPA 2008b. Volatile Organic Compounds – National Summary of VOC Emissions. Last updated 21 

October 2008. Accessed 4 March 2009. Available at 

<http://www.epa.gov/air/emissions/voc.htm>.

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, 1994.

GE SNAP Submission. 2008. Significant New Alternatives Policy Program Submission to the United 

States Environmental Protection Agency, October 2008. 

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, 1999.

Murray, D.M. 1997. “Residential House and Zone Volumes in the United States: Empirical and 

Estimated Parametric Distributions.” Risk Analysis. 17(4): 439-446. 

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

OSHA. 2004. “Safety and Health Topics: Isobutane.” February 2004. Available online at: 

<http://www.osha.gov/dts/chemicalsampling/data/CH_247840.html>.

Sheldon, L.S., et al. 1989. "An Investigation of Infiltration and Indoor Air Quality." New York State 

Energy Research & Development Authority, Report 90-11.

 

11 This figure determined using 2002 annual VOC emissions data from EPA (2008b) and expected annual production levels from 

the GE SNAP Submission (2008).

12 This figure determined using 2002 annual VOC emissions data from EPA (2008b). Residential wood combustion is the only 

residential source included in the EPA (2008b) analysis.