June 6, 2014 -- Final

1

Significant New Alternatives Policy Program 

Refrigeration and Air Conditioning Sector

Risk Screen on Substitutes for CFC-12, HCFC-22 and R-502 in Retail Food 

Refrigeration

Substitute: Isobutane (R-600a)

This risk screen is restricted to retail food refrigeration applications covered under UL 471: 

Commercial Refrigerators and Freezers.

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, administers 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 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 ODSusing 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). This risk screen 

evaluates the potential use of isobutane (R-600a) as a substitute for CFC-12, HCFC-22, and R-502 in 

retail food refrigeration, specifically in stand-alone beverage coolers (e.g., for cans or bottles). Table 1 

presents the composition of the proposed substitute and its potential impurities.

Table 1. Composition of Isobutane

Constituent Chemical Formula CAS Number Concentration

(by weight)

Isobutane CH(CH3)3 75-28-5 99.5%

Impurities 0.5%

1,3-Butadiene C4H6 106-99-0 5 ppm 

n-Hexane C6H14 110-99-0 50 ppm

Benzene C6H6 71-43-2 1 ppm 

Sulfur S 7704-34-9 2 ppm

Section 2 summarizes the results of the risk screen for the proposed substitute blend listed in Table 1. 

The remainder of the risk screen is organized into the following sections:

 Section 3: Atmospheric Assessment
June 6, 2014 -- Final

2

 Section 4: Volatile Organic Compound Assessment

 Section 5: Discussion of End-Use Scenario Modeled

 Section 6: Potential Health Effects

 Section 7: Flammability Assessment

 Section 8: Asphyxiation Assessment

 Section 9: End-Use Exposure Assessment 

 Section 10: Occupational Exposure Assessment

 Section 11: General Population Exposure Assessment

 Section 12: References

2. SUMMARY OF RESULTS

Isobutane is recommended for SNAP approval for retail food refrigeration for stand-alone refrigeration 

equipment that comply with Underwriters Laboratory (UL) Standard 471: Commercial 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, HCFC-22, and R-502 as it is less harmful to the ozone 

layer, has lower climate impact, and a shorter atmospheric lifetime. Isobutane is subject to volatile 

organic compound (VOC) regulations under the CAA (40 CFR 51.100(s)), and as such, emissions of

isobutane should be controlled. Based on analysis of potential impacts of hydrocarbon refrigerant 

emissions on ground-level ozone concentrations, VOC emissions from the use of isobutane in vending 

machines are not anticipated to contribute significantly to ground level ozone concentrations in the United 

States. It is expected that procedures in the material safety data sheet (MSDS) for isobutane and good 

manufacturing practices will be adhered to. Additionally, it is expected that the appropriate safety and 

personal protective equipment (PPE) (e.g., protective gloves, tightly sealed goggles, protective work 

clothing, and suitable respiratory protection in case of leakage or insufficient ventilation) consistent with 

Occupational Safety and Health Administration (OSHA) guidelines will be used during manufacture, 

installation and servicing, and disposal of retail food refrigeration systems using isobutane. Because retail 

food refrigeration systems are to be installed in locations with adequate space and/or ventilation in 

accordance with EPA recommendations and the equipment maintenance manual for isobutane, as 

discussed in greater detail in Section 9, significant toxicity or flammability risk to consumers is also 

unlikely. Additional safeguards are also provided by adherence to industry standards including American 

Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standards 15,

1

34,

2

and 623

and Underwriters Laboratory (UL) Standard 471.4

3. ATMOSPHERIC ASSESSMENT

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

isobutane in retail food refrigeration. The ozone depletion potential (ODP), global warming potential 

(GWP), and atmospheric lifetime (ALT) of the proposed substitute are presented in Table 2.

The proposed substitute is substantially less harmful to the ozone layer, has lower climate impact, and a 

shorter atmospheric lifetime when compared to CFC-12, HCFC-22, and R-502. In addition, isobutane

also has lower climate impact and a shorter atmospheric lifetime than those predicted for other substitutes 

 

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.

4 UL 471: Commercial Refrigerators and Freezers establishes safety requirements for the construction, electrical system, 

electrical components, refrigeration system, and performance of commercial refrigerators and freezers. 
June 6, 2014 -- Final

3

examined in the Background Document as well as commonly utilized substitutes R-410A and R-404A. 

Thus, EPA believes that the use of isobutane would result in substantially less harm to the climate and 

ozone layer than the continued use of ODS and commonly used ODS substitutes.

Table 2. Atmospheric Impacts of Isobutane Compared to Other Retail Food Refrigerants

Refrigerant Ozone Depleting Potential

(ODP)

a

Global Warming Potential

(GWP)

b

Atmospheric Lifetime in 

Years (ALT)

Isobutane 0 ~4 0.016b

1,3-Butadiene 0 NA NA

n-Hexane 0 NA 0.01c

Benzene 0 NA 0.046c

Sulfur 0 NA NA

CFC-12 0.82 10,900 100b 

R-502d 0.2 4,650 NAe

HCFC-22 0.04 1,810 12b

HFC-134a 0 1,430 14b

R-404Af 0 3,922 NAg

aWMO 2010 Scientific Assessment Report (2011)

b

IPCC 4th Assessment Report (Forster et al. 2007), unless otherwise noted.

c Hewitt (1998)

d R-502 is a blend consisting of HCFC-22 (49%) and CFC-115 (51%). 

e Atmospheric lifetimes are not given for blends, because the components separate in the atmosphere. The ALT for HCFC-22 is 12 years and 

the ALT for CFC-115 is 1,700 years (IPCC 4th Assessment Report [Forster et al. 2007]). 

f R-404A is a blend consisting of HFC-143a (52%), HFC-125 (44%) and HFC-134a (4%). 

g Atmospheric lifetimes are not given for blends, because the components separate in the atmosphere. The ALT for HFC-143a is 52 years, the 

ALT for HFC-125 is 29 years, and the ALT for HFC-134a is 14 years.

4. VOLATILE ORGANIC COMPOUND (VOC) ANALYSIS

Isobutane is regulated as a VOC under the CAA (40 CFR 51.100 [s]). Through regulations and standard 

industry practices, VOC emissions should be controlled. A separate analysis was prepared by EPA 

(2014) to evaluate the potential impact of the use of hydrocarbon refrigerants on ground level ozone 

concentrations in the United States. The analysis estimated refrigerant emissions from refrigeration and 

air conditioning equipment which were assumed to contain propylene, isobutane, and/or propane 4under 

different scenarios. Under the most conservative scenario it was assumed that propylene was used in all 

refrigeration and air conditioning equipment. In the most realistic scenario, all three hydrocarbons were 

assumed to be used in certain types of refrigeration and air conditioning equipment, depending on the 

proposed use of each alternative under submissions received by the SNAP Program at the time of the 

analysis. End-uses included in the evaluation included a number of types of smaller, self-contained 

refrigeration and room air-conditioning units, for which SNAP hydrocarbon applications have been 

received and/or UL Standards covering flammable refrigerants exist. The hydrocarbon emissions from 

these scenarios were estimated based on U.S. EPA’s Vintaging Model, and their potential contributions to 

ozone concentrations were assessed using U.S. EPA’s Community Multiscale Air Quality (CMAQ) 

model. 

CMAQ modeling was performed for April through the end of September, as these months presented the 

largest releases of hydrocarbon refrigerant as well as weather conditions favorable for ozone 

formation. The ozone concentrations were estimated for the Atlanta, Houston and Los Angeles regions, 

due to their distinctive geographic setting and chronic high levels of ground level ozone, and then scaled 

for national emission estimates. The results of the CMAQ modeling indicated that under the most realistic 

 

4 EPA uses the maximum incremental reactivity (MIR) of ethane (0.28 g-O3

/g-substance) as the threshold for determining 

whether a compound has negligible reactivity. Propylene (MIR = 11.57), isobutane (MIR = 1.34), and propane (MIR 0.56) are all 

considered to be reactive compounds and are therefore subject to VOC regulations.
June 6, 2014 -- Final

4

scenario, hydrocarbon refrigerants could potentially increase the maximum 8-hour average ground level 

ozone by no more than 0.15 ppb in Los Angeles, the city with the greatest ozone problem. This is roughly

0.2 percent of the current National Ambient Air Quality Standard (NAAQS) for ozone of 75 ppb. In the 

most conservative case which assumed that the most reactive hydrocarbon, propylene, was used in all 

refrigeration and air conditioning equipment, there could be an incremental maximum increase of the 8-

hour average as high as 6.61 ppb ozone, or an increase of up to 9 percent of the NAAQS. However, this 

upper bound level of increase is not likely, as most ozone nonattainment areas are not VOC-limited (i.e., 

the formation of ozone in these areas are not by limited by VOC emissions, but by other compounds such 

as nitrogen oxides [NOx]). In addition, the analysis assumed no use of VOC-exempt refrigerants which 

may be used in the refrigeration and air conditioning end-uses. Based on the results of this analysis, VOC 

emissions from the use of isobutane in self-contained retail food refrigeration equipment are not 

anticipated to contribute significantly to ground level ozone concentrations in the United States.

5. DISCUSSION OF END-USE SCENARIOS MODELED

Isobutane has been proposed for the retail food refrigeration end-use, specifically for stand-alone 

beverage coolers. The submission states that isobutane stand-alone beverage coolers will contain a 

maximum charge size of 59 grams.

5 UL Standard 471, however, states that commercial refrigeration units

containing flammable refrigerants can have a charge size of up to 150 grams. Both maximum charge 

sizes, 59 grams and 150 grams, are modeled in this risk screen.

To represent a reasonable worst-case scenario, it is assumed that a catastrophic leak of refrigerant occurs 

while a stand-alone beverage cooler is installed at the end-use. Because retail food refrigeration units can 

be installed in a wide range of locations with varying room volumes, the analysis in this risk screen 

conservatively assumes that the beverage cooler contains the maximum charge size indicated by UL 471 

(150 grams) and is located in a convenience store. The convenience store is assumed to have an effective

volume of 244 m

3

(8,620 ft3), with an effective floor area of 102 m

2

(i.e., excluding the space filled by the 

beverage cooler, furniture, boxes, etc.) and a height of 2.4 meters. Under the worst-case scenario, the full 

charge of the unit is assumed to be emitted over the course of one minute, into the convenience store with

1 air change per hour (ACH).

6

 A vertical concentration gradient is also assumed since isobutane is denser 

than air (specific gravity of isobutane relative to air is 2 [air = 1]) and will settle in higher concentrations 

closer to the ground. In order to simulate the vertical concentration gradient, 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 2000). Additionally, to account for retail food 

refrigeration models which could utilize the maximum amount of refrigerant allowed under UL 471, the 

same scenario is modeled using a charge size of 150 grams. Table 3 summarizes the end-use modeling 

assumptions used throughout the risk screen (i.e., in Sections 7, 8, and 9).

Table 3. End-Use Scenario Model Assumptions for Submission

Parameter Assumption

Refrigeration Unit Stand-Alone Beverage Cooler

Room Convenience Store

Charge Size (g) 59a

; 150b

Length of Release (minutes) 1

Effective Room Size (volume - m3) 244c

Room Ventilation (ACH) 1

Vertical Concentration Gradient Yes

a VestFrost 2010

b Maximum charge size for a vending machine under UL 471.

 

5 The submitter states that the maximum charge size of 59 grams is also anticipated to be the most typical. 

6 The air exchange rate is derived from the requirements in ANSI/ASHRAE Standard 62.1-2007, Table 6.1 (ANSI/ASHRAE 

2007). Ventilation requirements (presented as cubic feet per minute per square foot of ventilated space in the standard) were 

converted to air exchanges per hour using the assumed room size for the scenario.
June 6, 2014 -- Final

5

c Specific information was not available regarding which types of stores would utilize the beverage cases, so a small store (i.e., a 

convenience store) was used in the model. The minimum convenience store size found during a California survey (Armines 2009) –

100 m2 – was used for this scenario and an 8-foot ceiling was assumed.

EPA recognizes that beverage coolers and other small retail food refrigeration units may be placed in a 

variety of stores, establishments, and storage rooms with different sizes and ventilation rates. When units 

are installed in smaller, enclosed spaces, there is a higher risk for flammability, asphyxiation, or exposure 

concerns. To address these concerns, this risk screen incorporates threshold analyses in addition to the 

worst-case scenario modeling. The results from the threshold analyses are used to establish guidelines for 

choosing an effective room size, charge size, and ventilation rate such that use of isobutane in small retail 

food refrigeration does not present risk to consumers or servicing technicians.

6. POTENTIAL HEALTH EFFECTS

To assess potential health risks from exposure to the proposed substitute in retail food refrigeration, EPA 

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

different scenarios. To protect consumers from the potential dangers of a catastrophic leak from the 

refrigeration unit, ASHRAE Standard 34 determined refrigerant concentration limits (RCLs) to reduce the 

risks of acute toxicity, asphyxiation, and flammability hazards in occupied spaces (ASHRAE 2010b). 

ASHRAE Standard 15 implements the aforementioned standard, requiring that “the concentration of 

refrigerant in an enclosed space following a complete discharge of a high-probability system shall not 

exceed the RCL” (ASHRAE 2010a). As such, this risk screen references the RCL in addition to the lower 

flammability limit, hypoxia No Observed Adverse Effect Level (NOAEL), and exposure limits, as an 

additional, conservative limit to ensure that significant flammability, asphyxiation, and end-use exposure 

risks, respectively, do not occur.

For the occupational exposure analysis, potential risks from chronic and acute worker exposure were 

evaluated by comparing exposure concentrations to available occupational exposure limits. Potential risks 

of chronic worker exposure were evaluated using workplace guidance levels (WGL). Risks from potential 

short-term consumer exposures were evaluated by comparing exposure concentrations to emergency 

guidance levels (EGL). Table 4 lists the relevant toxicity limits and RCLs of isobutane and its potential 

impurities, and is followed by Table 5, which provides an explanation of each toxicity limit. EPA’s 

approach for identifying or developing these values is discussed in Chapter 3 of the Background 

Document. 

Table 4. Exposure Limits of Isobutane and Potential Impurities

NA = Not Available

a ACGIH TLV for aliphatic hydrocarbon gases: alkane (C1 - C4) is 1,000 ppm TWA (ACGIH 2012)

b Because n-Butane and isobutane have the same molecular formula, EGL for isobutane is conservatively assumed to be that for n-Butane 

(i.e., 30-min AEGL-1 of 6,900 ppm). However,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 ASHRAE (2010b)

d OSHA PEL and STEL for 1,3-butadiene available at: http://www.osha.gov/SLTC/butadiene/

e ACGIH TLV and STEL for n-hexane available at: http://www.osha.gov/dts/chemicalsampling/data/CH_245401.html 

Component

8-hr WGL

(Long-term Exposure)

ppm

EGL 

(Short-term Exposure)

ppm

Refrigerant Concentration 

Limit (RCL) ppm

Isobutane 1,000a (ACGIH TLV) 6,900b 4,000c

1,3-Butadiene 1

d

(OSHA PEL) 5

d

(15-min OSHA STEL) NA

n-Hexane 500e(ACGIH TLV) 1,000e(15-min ACGIH STEL) NA

Benzene 1

f

(OSHA PEL) 5

f

(15-min OSHA STEL) NA

Sulfur NA NA NA
June 6, 2014 -- Final

6

fOSHA PEL and STEL for benzene available at: http://www.osha.gov/dts/chemicalsampling/data/CH_220100.html

Table 5. Explanation of Exposure Limit-Related Terminologya

Organization Definition

OSHA Occupational Safety and Health Administration

NIOSH National Institute for Occupational Safety and Health

ACGIH American Conference of Governmental Industrial Hygienists

Exposure Limit Definition Explanation

Short-Term Exposure

RCL Refrigerant Concentration 

Limit

The RCL for a refrigerant is intended to reduce the risks of acute toxicity, 

asphyxiation, and flammability hazards in normally occupied, enclosed 

spaces. The RCL for each refrigerant is the lowest of the Acute-Toxicity 

Exposure Limit (ATEL), Oxygen Deprivation Limit (ODL), and Flammable 

Concentration Limit (FCL). Determination assumes full vaporization with 

no removal by ventilation, dissolution, reaction, or decomposition and 

complete mixing of refrigerant in the space to which it is released.

STEL Short-Term Exposure Limit A 15-minute time-weighted average (TWA) exposure that should not be 

exceeded at any time during a workday, even if the 8-hour TWA is within 

the TLV–TWA, set by ACGIH. 

AEGLb,c

Acute Exposure Guideline 

Level 1

AEGL-1 is the airborne concentration of a substance 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.

Acute Exposure Guideline 

Level 2

AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a 

substance above which it is predicted that the general population, 

including susceptible individuals, could experience irreversible or other 

serious, long-lasting adverse health effects or an impaired ability to 

escape.

Acute Exposure Guideline 

Level 3

AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a 

substance above which it is predicted that the general population, 

including susceptible individuals, could experience life-threatening health 

effects or death.

Long-Term Exposure

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

TLV-TWA Threshold Limit Value –TimeWeighted AverageThe TWA concentration for a conventional 8-hour workday and a 40-hour 

workweek, to which it is believed that nearly all workers may be 

repeatedly exposed, day after day, for a working lifetime without adverse 

effect according to ACGIH. 

a All information in this table taken from EPA (1994) except where otherwise noted.

b EPA (2012)

c Applicable to emergency exposure periods ranging from 10 minutes to 8 hours.

According to the MSDS, exposure to isobutane may be irritating if inhalation, skin contact, or eye contact 

with isobutane occurs. The most likely pathway of exposure is through inhalation. If isobutane is inhaled, 

person(s) should be immediately removed and exposed to fresh air. In accordance with the MSDS, EPA 

further recommends that if breathing is difficult, person(s) be given oxygen, provided a qualified operator 

is present, and medical attention be sought. Isobutane is not irritating to skin, but rapid evaporation of 

isobutane on the skin may cause frostbite. In the case of dermal exposure, the MSDS for isobutane 

recommends that person(s) immediately wash the affected area with water; if frostbite occurs, bathe (not 

rub) the affected area with lukewarm, not hot, water and consult a doctor. If water is not available, cover 

the affected area with a clean, soft cloth. Alternatively, if the fingers or hands are frostbitten, warm the 

affected area by placing it in the armpit; gently exercise the affected part while being warmed, and seek 

medical attention immediately. Isobutane exposure to the eye can cause irritation. In case of ocular 
June 6, 2014 -- Final

7

exposure, the MSDS for isobutane recommends that person(s) immediately flush the eyes, including 

under the eyelids, with copious amounts of water for several minutes. EPA’s review of the human health 

impacts of this proposed substitute is contained in the public docket for this decision. The potential health 

effects of isobutane can be minimized by following the exposure guidelines and ventilation and PPE 

recommendations outlined in the MSDS for isobutane and this risk screen.

7. FLAMMABILITY ASSESSMENT

ASHRAE Standard 34 classifies isobutane as a Class A3 refrigerant. Isobutane is flammable when its 

concentration in air is in the range of 1.8 percent to 8.5 percent by volume (18,000 ppm to 85,000 ppm). 

In the presence of an ignition source (e.g., static electricity, a spark resulting 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, posing a significant safety concern for workers and consumers if 

it is not handled carefully. The remainder of this section assesses flammability risks and summarizes the 

recommended measures to ensure safe handling and use of the refrigerant during manufacture, servicing,

and end-use. 

7.1 Flammability Risks at Manufacture

As indicated by the submitter, the manufacture of isobutane refrigerant (i.e., formulation mixing) and the 

charging of isobutane beverage coolers (i.e., receiving, blending. and filling operations) occurs in a closed 

area; all units are completely sealed before delivery and installation. The charging area is equipped with 

detectors and a low alarm that is activated when 10 percent of the LFL for isobutane is reached, and a 

high alarm that is activated when 25 percent of the LFL for isobutane is reached. In addition, 

manufacturing facilities are automatically ventilated, anticipated to maintain proper ventilation at all 

times during the manufacture of equipment containing isobutane, and will adhere to good manufacturing 

practices at all times. As a result, significant releases of isobutane during these manufacturing and 

installation operations in the presence of an ignition source are not anticipated. 

All 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, 2014) 

for liquefied isobutane requires the use of overfill protection devices (OPD) on cylinders to minimize the 

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

requirements/guidelines. EPA believes that because workers follow relevant safety standards in 

manufacturing facilities, the MSDS for isobutane, and OSHA requirements under 29 CFR 1910 (e.g., 

proper ventilation and storage practices within manufacturing facilities to prevent fire and explosion), 

flammability during manufacture is not expected to be of concern. 

7.2 Flammability Risk at Servicing and End-Use

The risk of flammability during servicing and end-use for the reasonable worst-case scenario (see Section 

5) was investigated for isobutane. Both servicing and end-use of isobutane beverage coolers are expected 

to take place in the same room. In order to determine the potential flammability risks during servicing or 

end-use in case of a catastrophic release of refrigerant in a convenience store (see Section 5), 

concentrations of isobutane immediately following a complete release of refrigerant, either accidental or 

deliberate (e.g., through vandalism or theft), were compared to the LFL for isobutane. The maximum 

instantaneous concentration of isobutane in the lower 0.4 meters of the room at a charge size of 59 grams 

would be approximately 566 ppm, which is 3 percent of the proposed substitute’s LFL. At a charge size 

of 150 grams, the maximum instantaneous concentration of isobutane in the lower 0.4 meters of the room 

would be approximately 1,440 ppm, which is 8 percent of the LFL for isobutane (see Table 6). The 

maximum instantaneous concentration in the upper portion of the room for both modeled charge sizes is 

much lower, as only 5 percent of the leaked refrigerant is present in this area, which also has a greater 

volume than the lower 0.4 meters of the room. Table 6 presents the results of the analysis.
June 6, 2014 -- Final

8

Table 6. Flammability Assessmenta

Scenario Charge Size (g) Effective Room 

Size (m3)

Maximum Instantaneous 

Concentration

(ppm) b,c

Reasonable Worst-Case 59 244 (8,620ft3) 566 

Reasonable Worst-Case 150 244 (8,620ft3) 1,440 

Threshold Analysis 1: Charge Size 1,560 244 (8,620ft3) 15,000

Threshold Analysis 2a: Room Size 59 9.2 (325 ft3) 15,000

Threshold Analysis 2b: Room Size 150 23.4 (826 ft3) 15,000

 Bold font indicates modeling results.

 a Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.

b Lower Flammability Limit of isobutane is equal to 15,000 ppm

c Values provided in these columns refer to the concentration in the lower 0.4 meters of the room.

The results of the threshold analyses indicate that the charge size would have to be significantly larger, or 

the room size smaller, than what is assumed in the worst-case scenario (see Section 5) for a flammability 

risk to be posed. As shown in Threshold Analysis 1, for flammability to be of concern within a room size 

of 244 m

3

, the charge size released would need to be at least 1,560 grams. As shown in Threshold 

Analysis 2a, for flammability to be of concern with a charge size of 59 grams, the effective volume of the

room would have to be 9.2 m3 (325 ft3). With a charge size of 150 grams, the effective volume of the

room would have to be greater than 23.4 m3 (826 ft3) to remove the flammability risk, as shown in 

Threshold Analysis 2b. 

It is unlikely that a store as small as those determined in the threshold analyses would exist and it is 

further unlikely that the entire refrigerant charge would accidentally release into the convenience store. 

The refrigerant system is located in a protected compartment on the back side of the retail food 

refrigeration unit and accidents resulting in a puncture to the compressor, whether through attempted theft 

of the machine contents or vandalism, would be unlikely. End-users, however, should still ensure that 

there is adequate space (i.e., the effective volume of the room should be larger than 9.2 or 23.4 m

3

, 

depending on the charge size of the beverage cooler [see Table 6]) and ventilation (in accordance with the 

MSDS for isobutane) in any room where the beverage coolers are installed to mitigate the risk of fire or 

explosion resulting from a catastrophic leak during end-use or servicing activities.

Catastrophic releases of large quantities of refrigerant, especially in areas where refrigerant is stored, 

could lead to an explosion in the presence of an ignition source. For this reason, it is important that only 

properly trained and certified technicians handle isobutane. The submitter has provided safety guidelines 

for handling isobutane, which should be followed. As a further precaution, certification requirements and 

training programs for technicians that handle isobutane should be developed using these guidelines. 

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. Furthermore, EPA recommends that 

consumers should not be present during servicing activities.

Beverage coolers should not be installed in small, poorly ventilated spaces such as small storage rooms 

(especially as other equipment or appliances in the space would reduce the effective volume of the room). 

For beverage coolers installed in larger areas, the risk of fire and explosion is minimal. In addition, 

beverage coolers installed with isobutane should be clearly labeled as containing a flammable refrigerant 

and designed to prevent catastrophic leaks. The installation of leak prevention devices would further 

protect against the very limited risk of explosion. Because relevant safety standards and the MSDS for 

isobutane are followed by service technicians and because isobutane refrigeration units will be installed in 

areas with adequate space and ventilation, EPA believes that flammability during servicing and end-use is 

not expected to be of concern. 
June 6, 2014 -- Final

9

8. ASPHYXIATION ASSESSMENT

The risk of asphyxiation for the reasonable worst-case scenario was investigated for isobutane. In this 

section, risk of asphyxiation is assessed in three ways: 1) modeling the oxygen concentration under the 

charge size and room size specified in the worst-case scenario, 2) performing a threshold analysis of 

minimum charge size needed to cause an asphyxiation risk in the room size specified in the worst-case 

scenario 3) performing a threshold analysis of the maximum room size needed to cause an asphyxiation 

risk with the charge size specified in the worst-case scenario. This analysis does not consider ventilation 

or conditions that are likely to occur that would increase oxygen levels to which individuals would be 

exposed, such as open doors or windows, fans operating, conditioned airflow (either heated or cooled), or 

even openings at the bottom of doors that allow air to flow in and out. As specified in Section 5, this 

analysis assumes a vertical concentration gradient. If the proposed 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 7 below. 

Table 7. Asphyxiation Assessmenta

Scenario Charge Size (g)a

Effective Room Size 

(m3)

Percent Oxygen 

Concentration in 

Lower 0.4 Metersb

Reasonable Worst-Case 59 244 (8,620 ft3) 21

Reasonable Worst-Case 150 244 (8,620 ft3) 21

Threshold Analysis 1: Charge Size 41,400 244 (8,620 ft3) 12

Threshold Analysis 2a: Room Size 59 0.35 (12 ft3) 12

Threshold Analysis 2b: Room Size 150 0.88 (31 ft3) 12

 Bold font indicates modeling results.

 a Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.

b The typical concentration of oxygen in air is considered to be 21 percent (Mackenzie & Mackenzie 1995).

In order for a risk of asphyxiation to occur, the normal concentration of oxygen in air (21 percent) in the 

lower 0.4 meters of the room must be reduced to 12 percent.7 Based on the worst-case scenario modeling 

assumptions, isobutane in a beverage cooler does not present a significant risk of asphyxiation. The 

concentration of isobutane in the air following the release of the 59-gram and 150-gram charge size does 

not exceed 0.1 percent or 0.2 percent, respectively, which has an insignificant impact on the normal 

concentration of oxygen in air. Because this modeling does not take into account any ventilation which is 

likely to occur, such as conditioned airflow, open doors, or even openings at the bottom of doors that 

allow air to flow in and out, as mentioned above, the actual asphyxiation risk is likely to be even smaller 

than modeled.

 

7 Twelve percent oxygen in air is the No Observed Adverse Effect Level (NOAEL) for hypoxia (ICF 1997).
June 6, 2014 -- Final

10

For the asphyxiation threshold analyses shown in Table 7, the conditions at which an asphyxiation 

concern would exist were determined. As shown in Threshold Analysis 1, the minimum charge of 

isobutane necessary to reduce the oxygen levels to 12 percent in air in the lower 0.4 meters of a room of

effective volume 244 m

3

(8,620 ft3) was calculated to be 41,400 grams, which is 276 times the maximum 

UL 471 charge size of 150 grams. This analysis assumed that 1) nitrogen and oxygen retain the same 

relative volumes in the rooms with the balance composed entirely of isobutane, and 2) the pressure of the 

room does not increase significantly with the addition of the refrigerant. As shown in Threshold Analysis 

2a and 2b, for asphyxiation to be of concern with the charge size of 59 grams and maximum charge size 

of 150 grams, under the reasonable worst case scenario described above in Section 5, the effective volume 

of the room would have to be about 0.35 m3(12 ft3) and 0.88 m3(31 ft3), respectively (see Table 7). 

Based on the worst-case modeling results and threshold analysis, EPA does not believe that the use of 

isobutane in beverage coolers poses a significant risk of asphyxiation or impaired coordination to 

consumers.

9. END-USE EXPOSURE ASSESSMENT

This section presents estimates of potential end-user exposures to isobutane in beverage coolers. An enduse exposure analysis was performed to examine potential catastrophic releases for isobutane and its 

anticipated impurities under the reasonable worst-case scenario outlined in Section 5.

For the end-use exposure assessment scenario, 30-min TWA exposures for the proposed substitute were 

calculated using the box model described in the Background Document, which was adapted to estimate 

concentrations on a minute-by-minute basis. Estimates for acute/short-term end-user exposures resulting 

from catastrophic leakage of refrigerant from stand-alone beverage coolers were examined. The analysis 

was undertaken to determine the 30-minute TWA for isobutane and its potential impurities, and then to 

compare the derived TWAs to the standard toxicity limits presented in Table 4 to assess the risk to endusers. 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. Modeling results are presented in Table 8.

Table 8. End-Use Exposure Assessmenta

Component

30-minute TWA End-Use

Exposure (ppm)

Charge Size: 59 grams

30-minute TWA End-Use

Exposure (ppm)

Charge Size: 150 grams

Short-term (30-min) 

Exposure Limits 

(ppm)b

Isobutane 540 1,380 6,900

1,3-Butadiene 0.3 0.6 5

c

n-Hexane 1.6 4.0 1,000c

Benzene 0.003 0.01 5

c

Sulfur 0.02 0.04 NA

 a Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.

 b See Table 4 for more information

 c STEL values are 15-min TWAs

Under the reasonable worst-case scenario described in Section 5, catastrophic releases of isobutane and its 

impurities from a beverage cooler with a charge size of 59 grams and 150 grams were modeled. For a 

charge size of 59 grams and 150 grams, the estimated 30-min TWA exposures of isobutane do not exceed 

7.8 percent and 20 percent of the STEL for isobutane, respectively. Furthermore, the 30-min TWA 

exposures of the potential impurities of isobutane do not exceed 6 percent and 12 percent of their 

respective STELs for a 59-gram and 150-gram charge size, respectively. The modeling results indicate 

that the end-use exposures to isobutane and its impurities would be well below the STELs. As such, use 

of isobutane in beverage coolers is not anticipated to present a significant risk end-users. To prevent 

exposure and potential serious side effects during larger releases, EPA recommends that the charge size 

for an isobutane beverage cooler does not exceed the RCL (9.6 g/m3) and proposes to require compliance
June 6, 2014 -- Final

11

with UL 471 in any space containing a beverage cooler, unless proper leak protection devises are in place 

in order to prevent exposures of isobutane beyond the recommended limits. Proper leak protection 

devices and engineering control requirements and adherence to the MSDS will further ensure that 

exposure limits are maintained below those described in Table 8.

10. OCCUPATIONAL EXPOSURE ASSESSMENT

This section assesses potential exposures to workers during manufacture, installation, servicing, and 

disposal of isobutane. As indicated by the submitter, the manufacture of isobutane refrigerant (i.e., 

formulation mixing) and the charging of isobutane stand-alone beverage coolers (i.e., receiving, blending,

and filling operations) occurs in a closed system; all units are completely sealed before delivery and 

installation. As a result, exposure during these manufacturing and installation operations is not 

anticipated. 

To ensure that use of the proposed substitute in stand-alone beverage coolers does not pose an 

unacceptable risk to workers during servicing and disposal, occupational exposure modeling was 

performed using a box-model approach. For a detailed description of the methodology used for this 

screening assessment, the reader is referred the occupational exposure and hazard analysis described in 

Chapter 5 of the Background Document. Estimates of refrigerant release per event for various release 

scenarios and data on number of events in 2010 were obtained from the Vintaging Model.8 For the standalone beverage cooler end-use, the release per event was conservatively assumed to be 0.25 percent of the 

equipment charge during servicing and 35 percent of the equipment charge during disposal.9 Additionally, 

EPA is proposing to allow intentional venting or release of isobutane as a refrigerant during installation, 

servicing, maintenance and disposal from beverage coolers and other stand-alone retail food refrigeration 

equipment. If venting does occur, EPA recommends that it is done in a well-ventilated area (e.g.,

outdoors). Therefore, scenarios in which 100 percent of the equipment charge was released during 

servicing and disposal were also considered. The release rate per event was multiplied by the number of 

events estimated to occur over a workday. For equipment servicing, the number of events per 8-hour 

work day was assumed to equal the maximum number of units anticipated to be serviced (eight units). For

disposal, it was conservatively assumed that 10 units are disposed during an 8-hour work day. The 

modeled exposure concentrations were compared to short-term occupational exposure limits at 

installation and servicing and long-term exposure limits at disposal.

10.1 Occupational Exposure at Manufacture

Isobutane is produced around the world and is used for a wide variety of uses. Where manufacture occurs 

in the United States, the MSDS for isobutane should be referenced and proper engineering controls and 

PPE used. As indicated by the submitter, charging of the beverage coolers occurs at the manufacturing 

site. Charging occurs within an enclosed area with automatic ventilation and that is equipped with 

refrigerant detectors and alarm systems. The submitter has indicated that charging equipment is operated 

only by trained technicians. 

The submitter’s controlled charging area has a volume of 5.7 m3(200 ft3) and the production line charges 

30 units per hour, with an estimated release of 1.3 g of isobutane per unit charged or 5.9 ppm. Assuming 

an 8-hour work day, workers could be exposed to up to approximately 47.2 ppm of isobutane per day, 

which is approximately 6 percent of the 8-hour recommended exposure limit for isobutane. Because of 

 

8

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 60 different end-uses. 

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. 

9 During servicing and disposal it is assumed that 100 percent of the refrigerant charge remains in the unit.
June 6, 2014 -- Final

12

established equipment design and installation practices, significant exposures to isobutane will be 

unlikely. Thus, EPA does not believe that manufacturing, including charging, of retail food refrigeration 

units with isobutane presents a significant concern to workers.

10.2 Occupational Exposure at Servicing

As indicated by the submitter, when isobutane beverage coolers are serviced, the used refrigerant is 

pumped into a foil-lined bag designed for the purpose of storing recovered refrigerant, where it is stored 

until it is recycled (i.e., filtered and reused) (A/S Vestfrost 2013). During the transfer to the initial bag, 

small incidental refrigerant releases may occur during connection and disconnection of the recharging 

hose fittings. 

The potential occupational exposure during servicing was analyzed. The maximum 30-TWA exposure for 

isobutane and its potential impurities were estimated for the servicing exposure scenario and compared to 

their respective STELs (see Table 9). As summarized in Table 9, modeling results indicate that the 

maximum anticipated occupational exposure concentrations for isobutane and its impurities are 

significantly lower than the short term exposure limits for retail food refrigeration units with either a 59-

gram or 150-gram charge size during the 0.25 percent or 100 percent release scenarios. Because the 

exposure concentrations modeled under the conservative assumptions is significantly lower than the 

exposure limits for each constituent, occupational exposure to the proposed substitute during servicing is 

not considered a significant risk to workers. Further, the estimated exposures were derived using 

conservative assumptions, and represent a worst-case scenario with a low probability of occurrence. 

These types of systems are typically serviced by trained personnel using proper industrial hygiene 

techniques. 

Table 9. Occupational Risk Assessment at Servicinga

Component

30-minute TWA Occupational Exposure 

(ppm)

Charge Size: 59 grams

30-minute TWA Occupational 

Exposure (ppm)

Charge Size: 150 grams

Short Term (30-

min) Exposure 

Limits (ppm)b

0.25% Release 100% Release 0.25% Release 100% Release

Isobutane 0.22 89 0.56 226 6,900

1,3-Butadiene 1.0 x 10-4 0.04 2.0 x 10-4 0.10 5

c

n-Hexane 0.001 0.29 0.002 0.73 1,000c

Benzene 1.0 x 10-5 0.004 3.0 x 10-5 0.01 5

c

Sulfur NA NA NA NA NA

a Cells highlighted in green are the scenarios that are deemed to be acceptable given various modeling assumption options.

b See Table 4 for more information

c STEL values are 15-min TWAs

10.3 Occupational Exposure at Disposal

Disposal of isobutane beverage coolers is expected to occur with limited frequency at disposal facilities 

and with limited duration of exposure to the refrigerant charge. Potential exposures to the refrigerant 

during disposal are expected to occur during draining (i.e., recovery) of the refrigerant from the retail 

food refrigeration unit into cylinders (e.g., connecting of pipes). Such activities and related exposure is 

anticipated to occur within 15-30 minutes (per event/day).

Table 10 displays the maximum estimated 8-hour TWA occupational exposure levels of isobutane and its 

potential impurities during disposal. Based on the assumptions described in the beginning of Section 10, 

the modeling indicates that 8-hour worker exposure concentrations to isobutane and trace impurities

during disposal of isobutane beverage coolers will at no point exceed 40 percent of their respective longterm exposure limits during the 35 percent and 100 percent release scenarios. Because each value is 

significantly lower than the exposure guidelines for the chemical in question, occupational exposure to 
June 6, 2014 -- Final

13

the proposed substitute during disposal is not considered a toxicity threat. Further, the estimated 

exposures were derived using conservative assumptions, and represent a worst-case scenario with a low 

probability of occurrence. These types of systems are typically disposed of by trained personnel using 

proper industrial hygiene techniques.

Table 10. Occupational Risk Assessment at Disposal

Component

8-Hour TWA Occupational Exposure 

(ppm)

Charge Size: 59 grams

8-Hour TWA Occupational Exposure 

(ppm)

Charge Size: 150 grams

Long Term (8-

Hour) Exposure 

Limits (ppm)a

35% Release 100% Release 35% Release 100% Release

Isobutane 55 160 140 400 1,000

1,3-Butadiene 0.02 0.07 0.06 0.17 1

n-Hexane 0.18 0.51 0.45 1.3 500

Benzene 0.003 0.01 0.01 0.17 1

Sulfur NA NA NA NA NA

a See Table 4 for more information

TWA = Time-weighted average

Although anticipated occupational exposures are well below the exposure limits for isobutane and each of 

the trace impurities, the recommendations for proper engineering controls and PPE in the MSDS for 

isobutane should be followed. Adequate ventilation should always be established during any use, 

handling, or storage of isobutane. Engineering controls should include vapor-in air detection systems and 

local exhaust ventilation during use of isobutane to prevent dispersion of isobutane throughout the work 

place. In addition, an eye wash and safety shower should be near the manufacturing facility and locations 

where isobutane is stored and ready for use. In general, use of appropriate PPE is recommended, such as 

splash goggles, mechanically-resistant gloves when handling cylinders and chemically-resistant gloves 

when handling the gas mixture (e.g., butyl rubber, chlorinated polyethylene, or neoprene), and protective 

clothing. Self-contained breathing apparatuses and fire retardant protective clothing should be worn in 

case of an accidental release (A/S Vestfrost 2012). EPA believes that if proper handling and disposal 

guidelines are followed in accordance with good industrial hygiene and manufacturing practices and the 

MSDS for isobutane, there is no significant risk to workers during the manufacturing, installation, 

servicing, and disposal of isobutane in beverage coolers.

11. GENERAL POPULATION EXPOSURE ASSESSMENT

Isobutane is not expected to cause a threat to human health in the general population when manufactured 

for use and used as a refrigerant in retail food refrigeration. The proposed substitute will be manufactured 

in a closed process and is proposed for use in hermetically sealed systems, and thus, significant releases 

are not anticipated. At room temperature, isobutane is a gas and, therefore, releases to ground or surface 

water are not anticipated, as isobutane is anticipated to dissipate into the atmosphere upon release to 

outside air (i.e., because natural ventilation rates would be higher and there is no enclosed space to keep 

isobutane concentrated). Should air releases during manufacturing operations occur, engineering controls 

should be used (e.g. carbon absorption scrubbers) to collect isobutane and prevent the release of isobutane 

to the atmosphere. EPA believes that by using proper engineering controls and by following disposal and 

containment recommendations outlined in the proposed substitute’s MSDS, exposure to isobutane is not 

expected to pose a significant toxicity risk to the general population. 
June 6, 2014 -- Final

14

12. REFERENCES

Armines. 2009. Inventory of Direct and Indirect GHG Emissions from Stationary Air Conditioning and 

Refrigeration Sources, with Special Emphasis on Retail Food Refrigeration and Unitary Air Conditioning. 

CARB Agreement No. 06-325.

ACGIH. 2012. Guide to Occupational Exposure Values. American Conference of Governmental 

Industrial Hygienists.

ANSI/ASHRAE. 2010a. Standard 15: Safety Standard for Refrigeration Systems. American Society of 

Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ANSI/ASHRAE. 2010b. Standard 34: Designation and Safety Classification of Refrigerants. American 

Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 

ANSI/ASHRAE. 2013a. Standard 62: Ventilation for Acceptable Indoor Air Quality. American Society 

of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

A/S Vestfrost. 2012. Isobutane in Retail Food Refrigeration. Significant New Alternatives Policy 

Program Submission to the United States Environmental Protection Agency. June 2012. 

A/S Vestfrost. 2013. Follow-up information provided to EPA. Isobutane in Retail Food Refrigeration. 

Significant New Alternatives Policy Program Submission to the United States Environmental Protection 

Agency. January 2013. 

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. 

Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland. 2007. Changes in 

Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007:The Physical Science 

Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel 

on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and 

H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Hewitt, Nicholas C., ed. 1998. Reactive Hydrocarbons in the Atmosphere. Academic Press. London, UK. 

Available at: http://books.google.com/books?id=Fn5368-Gv7AC&dq=nhexane+atmospheric+lifetime&source=gbs_navlinks_s. 

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, O., M. Yoshizawa, & T. Hirakawa 2000. “Allowable Charge Limit of Flammable Refrigerants 

and Ventilation Requirements.” Daikin Industries. International Refrigeration and Air Conditioning 

Conference. Paper 506. Available online at: http://docs.lib.purdue.edu/iracc/506.

Mackenzie, F.T. and J.A. Mackenzie. 1995. Our changing planet. Prentice-Hall.

NFPA. 2014. NFPA Liquified Petroleum Gas Code. 2014 Edition.

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

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

UL. 2010. UL 471: Commercial Refrigerators and Freezers. Underwriters Laboratory.
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15

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

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

Stratospheric Protection Division. March, 1994.

U.S. EPA. 2012. Acute Exposure Guidelines (AEGLs) Definitions. Available online at:

http://www.epa.gov/oppt/aegl/pubs/define.htm.

U.S. EPA. 2014. Draft Assessment of the Potential Impact of Hydrocarbon Refrigerants on Ground Level 

Ozone Concentrations. Prepared for the U.S. EPA Stratospheric Protection Division by ICF International. 

February 24, 2014.

WMO (World Meteorological Organization), 2011. Scientific Assessment of Ozone Depletion: 2010, 

Global Ozone Research and Monitoring Project—Report No. 52, 516 pp., Geneva, Switzerland, 2011.