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   SEQ Table \* ARABIC  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%



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

Table   SEQ Table \* ARABIC  2 .  Atmospheric Impacts of Isobutane
Compared to CFC-12.

Refrigerant	Ozone Depleting Potential (ODP)	Global Warming Potential
(GWP)	Atmospheric Lifetime years (ALT)

Isobutane	0a	8a	<1b

CFC-12	1c	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) is 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.   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.  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 full-sized 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.  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.    ICF
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   SEQ Table \* ARABIC  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.  

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, 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, 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 m3 (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   SEQ Table \* ARABIC  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

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   SEQ Table \* ARABIC  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

Table   SEQ Table \* ARABIC  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 - Short-term 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   HYPERLINK "http://www.epa.gov/riskassessment/glossary.htm#n" 
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.  
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.  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, 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   SEQ Table \* ARABIC  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 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   SEQ Table \* ARABIC  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/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 10-5 to 1.7 x 10-1,
depending on the type of release scenario.  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.

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 1x10-5 percent of all annual
anthropogenic VOC emissions.   Further, the calculated potential VOC
emissions due to use of isobutane in household refrigerators is less
than 1x10-4 percent of annual residential wood combustion emissions.  
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.

 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.

 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.

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

 http://www.freepatentsonline.com/6966365.html

 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.”

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

 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.    

 During disposal it is assumed that only 90 percent of the refrigerant
charge remains in the unit. 

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

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

 This figure determined using 2002 annual VOC emissions data from EPA
(2008b) and expected annual production levels from the GE SNAP
Submission (2008).

 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.

	          			                May 22, 2009

