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 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). 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
         * 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, 34, and 62 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 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
                                   0.016[b] 
                                 1,3-Butadiene
                                       0
                                      NA
                                      NA
                                   n-Hexane
                                       0
                                      NA
                                    0.01[c]
                                    Benzene
                                       0
                                      NA
                                   0.046[c]
                                    Sulfur
                                       0
                                      NA
                                      NA
                                    CFC-12
                                     0.82
                                    10,900
                                     100b 
                                   R-502[d]
                                      0.2
                                     4,650
                                     NA[e]
                                    HCFC-22
                                     0.04
                                     1,810
                                    12[b] 
                                   HFC-134a
                                       0
                                     1,430
                                    14[b] 
                                   R-404A[f]
                                       0
                                     3,922
                                     NA[g]
[a] WMO 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  under 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 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. 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 ft[3]), 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).  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) 
                                59[a]; 150[b] 
Length of Release (minutes)
                                       1
Effective Room Size (volume - m[3])
                                    244[c] 
Room Ventilation (ACH)
                                       1
Vertical Concentration Gradient
                                      Yes
      [a] VestFrost 2010
      [b] Maximum charge size for a vending machine under UL 471.
      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 m[2]  -  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. 
                                   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 
                                   4,000[c]
                                 1,3-Butadiene
                                1[d] (OSHA PEL)
                            5[d] (15-min OSHA STEL)
                                      NA
                                   n-Hexane
                              500[e] (ACGIH TLV)
                         1,000[e] (15-min ACGIH STEL)
                                       NA
                                    Benzene
                                1[f] (OSHA PEL)
                            5[f] (15-min OSHA STEL)
                                       NA
                                    Sulfur
                                      NA
                                      NA
                                      NA

        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  
f OSHA PEL and STEL for benzene available at: http://www.osha.gov/dts/chemicalsampling/data/CH_220100.html
         Table 5. Explanation of Exposure Limit-Related Terminology[a]
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. 
AEGL[b,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  - Time-Weighted Average
The 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 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.
                      Table 6. Flammability Assessment[a]
                                   Scenario
                                Charge Size (g)
                          Effective Room Size (m[3])
                      Maximum Instantaneous Concentration
                                  (ppm) b[,c]
                             Reasonable Worst-Case
                                      59
                               244  (8,620ft[3])
                                     566 
                             Reasonable Worst-Case
                                      150
                               244  (8,620ft[3])
                                    1,440 
                       Threshold Analysis 1: Charge Size
                                     1,560
                               244  (8,620ft[3])
                                    15,000
                       Threshold Analysis 2a: Room Size
                                      59
                                9.2 (325 ft[3])
                                    15,000
                       Threshold Analysis 2b: Room Size
                                      150
                               23.4 (826 ft[3])
                                    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 ft[3]). With a charge size of 150 grams, the effective volume of the room would have to be greater than 23.4 m3 (826 ft[3]) 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. 
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 Assessment[a]
                                   Scenario
                              Charge Size (g)[a]
                          Effective Room Size (m[3])
             Percent Oxygen Concentration in Lower 0.4 Meters[b] 
                             Reasonable Worst-Case
                                      59
                              244  (8,620  ft[3])
                                      21
                             Reasonable Worst-Case
                                      150
                               244 (8,620 ft[3])
                                      21
                       Threshold Analysis 1: Charge Size
                                    41,400 
                              244  (8,620  ft[3])
                                      12
                       Threshold Analysis 2a: Room Size
                                      59
                                0.35 (12 ft[3])
                                      12
                       Threshold Analysis 2b: Room Size
                                      150
                                0.88 (31 ft[3])
                                      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. 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.
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 ft[3]) 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 m[3] (12 ft[3]) and 0.88 m[3] (31 ft[3]), 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 end-use 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 end-users.  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 Assessment[a]
                                   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
                                   1,000[c]
                                    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/m[3]) and proposes to require compliance 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.  For the stand-alone 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. 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 m[3] (200 ft[3]) 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 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 Servicing[a]
                                   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
                                   1,000[c]
                                    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 long-term 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 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. 
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