November 17, 2011

1

Significant New Alternatives Policy Program 

Refrigeration and Air Conditioning Sector

Risk Screen on Substitutes for CFC-12 and HCFC-22 in Household Refrigerators 

and Household Freezers

Substitute: Isobutane

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 two decades, 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, runs the Significant New Alternatives Policy 

(SNAP) Program, which identifies acceptable and unacceptable substitutes for ODS in specific end-uses

based on assessment of their health and environmental impacts.

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-use applications 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 risk screen 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 and HCFC-22 in this sector. The proposed 

end-use application considered in this analysis is household refrigerators and household freezers. The 

specific proposed CFC-12 and HCFC-22 substitute examined in this risk screen is isobutane (R-600a).

The Background Document examines the potential risks associated with use of substitutes in residential 

refrigeration, and includes a detailed discussion of the methodologies used to conduct this risk screen. 

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. End-use 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 an unacceptable

risk to the population at large. 

The proposed substitute, isobutane, may contain minute quantities of impurities. Table 1 details the 

composition of the substitute, including the maximum estimated concentration of impurities that may be 

present in the substitute.
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Table 1. Composition of the Proposed Substitute

Component Concentration 

(Weight Percent)

Substitute

Isobutane 99.44%

Potential Impurities (maximum concentration)

Propane and N-butane 0.5%

1,3-Butadiene 0.0005%

N-Hexane 0.005%

Benzene 0.0001%

Sulfur 0.0001%

Liquid phase water 0.0005%

Air 0.05% (V/V)

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

screen is organized into the following sections:

 Section 3: Atmospheric Assessment

 Section 4: Discussion of End-Use Scenarios Modeled

 Section 5: Flammability Assessment

 Section 6: Asphyxiation Assessment

 Section 7: Toxicity Assessment

 Section 8: Volatile Organic Compound Assessment 

 Section 9: 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 will be less harmful to the atmosphere than the continued 

use of CFC-12 and R-502. 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 

refrigerator or freezer 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 151, 342, and 623be followed. 

 

1

Safety Standard for Refrigeration Systems ASHRAE Standard 15 establishes safeguards for life, limb, health, and 

property and prescribes safety requirements.

2 Designation and Safety Classification of Refrigerants ASHRAE Standard 34 is establishes a uniform system for 

assigning reference numbers, safety classifications, and refrigerant concentration limits to refrigerants. Safety 

classifications based on toxicity and flammability data are included.

3 Ventilation for Acceptable Indoor Air Quality ASHRAE Standard 62 establishes minimum ventilation rates and 

other measures intended to provide indoor air quality that is acceptable to human occupants and that minimizes 

adverse health effects.
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3. ATMOSPHERIC ASSESSMENT

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

isobutane in the residential refrigeration sector. The ozone-depletion potential (ODP), global warming 

potential (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 

to the ozone layer and has less climate impact, and a shorter atmospheric lifetime when compared to 

CFC-12 and HCFC-22.

Table 2. Atmospheric Impacts of Isobutane Compared to CFC-12 and HCFC-22

Refrigerant ODP GWP ALT (years)

Isobutane 0

a 8a <1 b

CFC-12 1

c 10,900 d 100 d

HCFC-22 0.055 c 1,810 d 12 d

a

Isobutane SNAP Submission (GE 2008).

b

IPCC/TEAP (2005). Atmospheric lifetime (ALT) not provided in SNAP submission.

cMontreal Protocol and available at: http://www.epa.gov/ozone/ods.html.

d

IPCC 4th Assessment Report (Forster et al. 2007).

4. DISCUSSION OF END-USE SCENARIOS MODELED

The proposed end-use for this substitute is household food refrigeration. The specific application 

includes household refrigerators and freezers. The submission states that the household refrigerators and 

freezers with this substitute will have a charge size of 50 grams. This charge size is consistent with the 

standard UL 250 Supplement SA, which calls for a maximum leak amount of 50 grams for refrigerants 

with flammability limits and a heat of combustion greater than 19,000 kJ/kg, such as isobutane.

For the analysis, a kitchen with an area of 7.5 m

2

and a height of 2.4 meters was assumed (EPA 1994). 

This is equivalent to a volume of 18 m

3

. In the analysis, the full charge of the unit as specified in the 

submission is assumed to be emitted over the course of one minute and the model conservatively assumes

2.5 air changes occur per hour.

4

 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).Table 3 details the 

assumptions throughout the risk screen for modeling at the end-use (i.e., in Sections 5, 6, and 7.3).

Table 3. Worst-Case Scenario Model Assumptions for Submission

Parameter Assumption

Refrigeration Unit Household Refrigerator

Room Household Kitchen

Charge size (g) 50

Length of release (minutes) 1

Room size (volume - m3) 18

Room ventilation (air changes

per hour) 2.5

Horizontal stratification Yes

 

4 The range of potential flow rates for a home is believed to be 2.5 to 4.5 air changes per hour (ach) (Sheldon 1989). The lowest and thus 

most conservative, possible rate – 2.5 air changes per hour – was used in the models.
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5. FLAMMABILITY ASSESSMENT

Isobutane is flammable when its concentration in air is in the range of 1.8%-9.6% by volume (18,000 ppm 

to 96,000 ppm). 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 from a switch malfunction, 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 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. Household refrigerators and freezers 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. 

Under the catastrophic release scenario (see Section 4), 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 4. 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. 

Table 4. 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. If 18,000 ppm were used in the analysis, the room size required to reach the 

threshold of flammability would be 6.6 m3 or 234 ft3.

b

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

 

For flammability to be of concern under the conservative (protective) assumptions described in Section 4, 

the volume of the kitchen would have to be about 7 m

3 (247 ft3

) (see Table 4). 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. Although very 

limited data are available regarding the distribution of kitchen volumes in the United States, an analysis 

by Murray (1997) which aggregated data from over 60 different projects reported the minimum kitchen 

volume encountered as 31 m3.

5

Further, only one percent of houses sampled had a kitchen smaller than 53 

m

3

. 

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 

 

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

were only available for the Los Angeles 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.
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protect against the very limited risk of explosion.

To mitigate the risks of flammability at the end-use, it is recommended that isobutane refrigerators' and 

freezers’ charge sizes are determined according to the UL 250 Supplement SA standard. Per the UL 

standard, the maximum permissible leak for refrigerants with limits of flammability and heat of 

combustion greater than 19,000 kJ/kg (such as isobutane) is 50 g. Additional charge is not prohibited, 

however, if the amount of refrigerant leaked during testing does not exceed 50 grams. This is also 

consistent with EPA’s proposed use condition for hydrocarbon refrigerants that limits the refrigerant 

charge size of any refrigerator, freezer, or combination refrigerator and freezers to 576grams, assuming 

that roughly 12% of the refrigerant charge is retained in the compressor oil and 88% of the charge leaks. 

By ensuring isobutane refrigerators and freezers conform to the aforementioned standard, flammability 

risks associated with the use of isobutane in these appliances should be mitigated.

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 additional information regarding its 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 2008). This 

training should be provided for all technicians who will be servicing refrigerators that use isobutane. As 

a further precaution, isobutane storage and transport equipment, including the recovery cylinders used to 

recover refrigerant on-site during the initial product run (GE 2008), 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 during servicing and manufacture. 

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 Part 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,

7

it is 

recommended that 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. 

6. ASPHYXIATION ASSESSMENT

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 

 

6 EPA 40 CFR Part 82 Proposed Rulemaking EPA-HQ-OAR-2009-0286; FRL-9147-9. Available at: 

http://www.gpo.gov/fdsys/pkg/FR-2010-05-10/html/2010-10959.htm 

7 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.”
November 17, 2011

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

screening analysis with these restrictive assumptions in place, it can be reasonably assumed that no risks 

of asphyxiation will be present under real-world conditions.

The results of the asphyxiation assessment are summarized in Table 5 below. The maximum charge of 

isobutane necessary to reduce the oxygen levels to 12 percent in air,

8

in a kitchen of volume 18 m3(635 

ft3) (see Section 4), was calculated, assuming horizontal stratification of the refrigerant and the air. 

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 charge of approximately 625 g would be necessary to reach 12 percent 

oxygen in the lower stratum. This amount represents more than twelve times the intended charge of 

approximately 50 g for a single isobutane refrigerator or freezer. Charge requirements to reach the same 

effect in the upper stratum would be even higher because of the stratum’s larger volume. For 

asphyxiation to be of concern with the proposed charge size, under the conservative (protective) 

assumptions described above, the volume of the kitchen would have to be about 1.5 m

3

(53 ft3) (see Table 

5). 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. 

Table 5. Asphyxiation Assessment

Room Type/Appliance 

Type

Appliance 

Charge (g)

Reasonable Worst-Case Scenario Asphyxiation Threshold Scenario

Room Size (m3) Charge Causing 

Impairment (g)a

Room Size 

Causing 

Impairment (m3)

Assumed Leak 

Size (g)a

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

a

Values provided in these columns refer to the charge required to cause impairment in the lower stratum of the room. In this scenario, 100% 

of the charge size leaks. Charge requirements to reach the same effect in the upper stratum would be even higher because of the stratum’s 

larger volume.

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. Further, as the 

“threshold” room sizes for asphyxiation risks are smaller than those for flammability risks, flammability 

risks would be realized before asphyxiation risks. 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.

7. TOXICITY ASSESSMENT

Section 7 summarizes the results of the toxicity assessment for isobutane in the household refrigeration 

sector. This section is organized as follows: 

 Section 7.1: Toxicity Reference Values

 Section 7.2: Occupational Exposure

 Section 7.3: End-Use Exposure

 Section 7.4: General Population Exposure

 

8 Twelve percent oxygen in air is the No Observed Adverse Effect Level (NOAEL) for hypoxia (ICF 1997). 
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7.1 TOXICITY REFERENCE VALUES

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 6. 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. Table 6 lists the relevant toxicity limits. Table 7 provides definitions of the 

acronyms used in Table 6. EPA’s approach for identifying or developing these values is discussed in 

Chapter 3 of the Background Document. 

Table 6. 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)

10,000 ppme,f(10 min AEGL-1)

6,900 ppmf(30 min AEGL-1)

0.9 c

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

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

670 i(10 minute interim AEGL-1)

2 x 10-3 j

N-Hexane 50 k (NIOSH REL) 1100 k(IDLH) 0.7 l

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

500 k(IDLH)

3 x 10 -2 n

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 An IDLH of 2,100 ppm has been established for propane. However, NIOSH (1996) states that "[b]ased on acute inhalation toxicity data 

in humans (ACGIH 1991; Braker 1980), a value much greater than 10,000 ppm would have been appropriate. However, the revised IDLH 

for propane is 2,100 ppm based strictly on safety considerations (i.e., being 10% of the lower explosive limit of 2.1%)." Therefore, as this 

IDLH value is based on flammability concerns and not toxicity concerns, it is not used in the evaluation of toxicity risks in this risk screen.

f http://www.epa.gov/oppt/aegl/pubs/results96.htm

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

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

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

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

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

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

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

n http://www.epa.gov/iris/subst/0276.htm#refinhal
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Table 7. 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 effectsab

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

7.2 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.9 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.10 

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

 

9

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. 

10 During disposal it is assumed that only 90 percent of the refrigerant charge remains in the unit. 
November 17, 2011

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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.11 For the 

purposes of this model, it is assumed that isobutane will have a market penetration rate of 0.1%

12 and that 

one production facility will be in operation (GE 2008). 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 occupational 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 8 displays the maximum estimated 15-minute TWA occupational exposure levels of 

isobutane and the potential impurities in the substitute. The maximum estimated 15-minute TWA 

occupational exposure levels are the most conservative of the modeled results and indicate the highest 

predicted exposure concentrations of the short-term (15-minute and 30-minute) and long-term (8-hour) 

exposure scenarios. Even the maximum estimated short-term occupational levels for isobutane and the 

potential impurities are significantly lower than their 8-hour or 10-hour long-term WGLs. Therefore,

occupational exposure to the substitute is not considered a toxicity threat. 

Table 8. 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.

7.3 END-USE 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 6 to assess the risk to consumers. However, the TWA values 

are fairly conservative as the analysis does not consider opened windows, fans operating, conditioned 

 

11 Submission indicates this is the maximum number of days per year that manufacturing will occur.

12 Calculated by dividing the number of units the submitter anticipates manufacturing per year (starting in 2010) by the number 

of predicted new units produced in 2010, from the Vintaging Model.
November 17, 2011

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airflow (either heated or cooled) and other variables that would reduce the levels to which individuals 

would be exposed.

The scenario modeled (see Section 4) 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 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 100 percent of the unit’s charge would 

be released during a time span of 1 minute, at which time the concentration of isobutane will peak, and 

then steadily decline onwards. The highest expected levels of consumer exposure based on this analysis 

are presented in Table 9 below.

Table 9. End-Use Exposure Assessment

15-minute TWA EndUse Exposure (ppm)30-minute TWA EndUse Exposure (ppm)

Substitute

Isobutane 5,025 3,844

Potential Impurities

Propane and n-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 n-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 6) and thus should not pose a toxicity threat.

7.4 GENERAL POPULATION EXPOSURE

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

mg/m3. 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.13 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.

 

13 Release scenarios include factory, on-site and recycling/salvage yard releases, with and without recycling of the 

refrigerant.
November 17, 2011

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8. VOLATILE ORGANIC COMPOUND (VOC) ASSESSMENT

Isobutane has not been exempted as a VOC under the CAA (40 CFR 51.000). VOC emissions from the 

production of refrigerators and freezers using isobutane as a refrigerant are controlled through standard 

industry practices, and as such, emissions from manufacture of units are likely to be minimal. For units in 

operation for consumer applications, using leak estimates from the Vintaging Model, the annual release 

rate (including operating, servicing, and disposal leak rates) for household refrigerators was calculated to 

be approximately 4% of refrigerant charge size, annually.

14

 Assuming this release rate, two assessments 

were performed to compare the annual VOC emissions from use of isobutane in household refrigerators 

to other anthropogenic sources of VOC emissions. The first assessment approximates the annual VOC 

emissions that would be emitted from isobutane refrigerators produced in one year, assuming the 

submitter’s current production plans. The second analysis approximates the annual VOC emissions if all 

refrigerators in U.S. households were charged with isobutane.15

Table 10. VOC Annual Emissions Analysis

Set of Sources Assessed

Annual Isobutane VOC Emissions 

Compared to VOC Emissions from All 

Anthropogenic Sourcesa(%)

Isobutane Refrigerators Produced 

by GE in One Yearb

3.0x10-7%

All U.S. Household Refrigeratorsc 3.4x10-3%

a Calculations based on 2008 EPA annual VOC emissions data (EPA 2009b).

b 2010 manufacture projections based on 2006 GE sales data (GE 2008). 

c Calculations based on GE SNAP Submission and 2010 Residential Equipment Stock Projections (GE 2008 and U.S. EIA 2011).

As illustrated in Table 10, the VOC emissions from the use of isobutane under both scenarios are minimal 

compared to other sources of VOC emissions. Assuming the submitter’s current production plans, the 

resulting VOC emissions from isobutane refrigerators manufactured in one year would be equal to 

approximately 2.7x10-3percent of the VOC emissions caused by the generation of electricity used to 

power household refrigerators and freezers,

16 4.8x10-4

percent of all the VOC emissions caused by the 

generation of electricity for all refrigeration and air conditioning applications, or 3.0x10-7percent of all 

annual anthropogenic VOC emissions in the U.S. 

Moreover, if all the refrigerators in U.S. households were charged with isobutane (the second, more 

conservative, scenario), the resulting VOC emissions would be equal to approximately 3.4x10-3 percent of 

all annual anthropogenic VOC emissions in the U.S. Because these emissions of isobutane are several 

orders of magnitude lower than the VOC emissions generated by other anthropogenic emissions, and the 

VOC emissions from the anticipated production of refrigerators by the submitter is only a small fraction 

of VOC emissions from the use of electricity to power household refrigerators and freezers, the 

environmental impacts of these VOCs are not considered a threat.

 

14 The average annual leak rate (0.2%), annual service rate (0.7%) and EOL disposal rate (50%) for household 

refrigerators and freezers was approximated using data from the Vintaging Model (v4.4_3.23.11). Using an average 

lifetime of 14 years for household refrigerators and freezers, a total annual release rate of 4% was determined. 

15This scenario, conservatively assumes that there is immediate market saturation in 2010, and does not consider 

commercialization and initial market uptake delays.

16 Calculations based on ICF assumptions and 2010 Electricity Projections (U.S. EIA 2011).
November 17, 2011

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