Document ID: EPA-HQ-OAR-2013-0748-0007
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2014-07-09T04:00Z

Significant New Alternatives Policy Program
Refrigeration and Air Conditioning Sector
Risk Screen on Substitutes for CFC-12 and R-502 in Household Refrigerators and Freezers
                          Substitute: Propane (R-290)
This risk screen is restricted to residential and light commercial window air conditioner applications and other self-contained room air conditioners meeting the requirements of UL 250: Household 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 decade, including the Montreal Protocol and the Clean Air Act Amendments of 1990 (CAAA).  The U.S. Environmental Protection Agency (EPA), as authorized by Section 612 of the CAAA, 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 propane as a substitute for CFC-12 and R-502 in the refrigeration and air-conditioning end use, specifically household refrigerators and freezers included under Underwriters Laboratory (UL) 250: Household Refrigerators and Freezers. The proposed substitute, propane, may contain minute quantities of impurities. Table 1 details the composition of the proposed substitute, including the maximum estimated concentrations of impurities which may be present in the proposed substitute.

                Table 1. Composition of the Proposed Substitute
                                   Component
                               Chemical Formula
                                  CAS Number
                                 Concentration
                               (Weight Percent)
                              Proposed Substitute
                                    Propane
                                     C3H8
                                    74-98-6
                                     99.5%
                 Potential Impurities (maximum concentration)
                                   Isobutane
                                     C4H10
                                    75-28-5
                                     0.14%
                                   Propylene
                                     C3H6
                                   115-07-1
                                    0.031%

Section 2 of this risk screen summarizes the results of the risk screen for the proposed substitute.  The remainder of the risk screen is organized into the following sections:

         * Section 3: Atmospheric Assessment
         * Section 4: Volatile Organic Compound Assessment 
         * Section 5: Discussion of End-Use Scenarios 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
Propane is recommended for SNAP approval for household refrigerators and freezers that comply with Underwriters Laboratory (UL) Standard 250: Household Refrigerators and Freezers.  EPA's risk screen indicates that the use of the proposed substitute will be less harmful to the atmosphere than the continued use of CFC-12, HCFC-22 and R-502 as it is less harmful to the ozone layer, has lower climate impact, and a shorter atmospheric lifetime.  No significant asphyxiation risks to consumers or toxicity risks to workers, consumers, or the general population are expected.  Propane is subject to volatile organic compound (VOC) regulations under the CAA (40 CFR 51.100(s)). Based on analysis of potential impacts of hydrocarbon refrigerant emissions on ground-level ozone concentrations, VOC emissions from the use of propane in household refrigerators and freezers are not anticipated to contribute significantly to ground level ozone concentrations in the United States. It is expected that procedures identified in the MSDS for propane 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 & Health Administration (OSHA) guidelines will be used during manufacture, installation, servicing, and disposal of household refrigerators and freezers using propane. Because units are to be installed in locations with adequate space and/or ventilation in accordance with the equipment maintenance manual for propane, as discussed in greater detail in Section 9, significant toxicity or flammability risks to consumers are also unlikely. Additional safeguards, including specified refrigerant concentration limits (RCL) for propane, 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 250: Household Refrigerators and Freezers.
 3. 		ATMOSPHERIC ASSESSMENT
This section presents an assessment of the potential risks to atmospheric integrity posed by the use of propane in household refrigerators and freezers. The ozone-depleting 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 has a shorter atmospheric lifetime than those of CFC-12 and R-502. In addition propane also has a lower climate impact and a shorter atmospheric lifetime than those predicted for other substitutes examined in the Background Document, as well as a commonly utilized substitute, HFC-134a. Thus, EPA believes that use of propane would result in substantially less harm to the ozone layer than the continued use of ODS and commonly used ODS substitutes. 

Table 2. Atmospheric Impacts of Propane Compared to CFC-12, R-502 and HFC-134a
                                  Refrigerant
                        Ozone Depleting Potential (ODP)
                 Global Warming Potential -  100 year (GWP)[a]
                       Atmospheric Lifetime years (ALT)
                                    Propane
                                     0[a]
                                      3.3
                                    0.03[a]
                                    CFC-12
                                     1[c]
                                    10,900
                                    100[a]
                                   R-502[d] 
                                    0.31[c]
                                     4,650
                                     NA[e]
                                   HFC-134a
                                     0[a]
                                     1,430
                                     14[a]
NA = Not Available
[a] [I]PCC 4th Assessment Report (Forster et al. 2007).
[b] IPCC/TEAP (2005).  
[c] ODP for R-502 was calculated using constituent ODPs from WMO 2010 Scientific Assessment Report (2011).
[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]).
 4. 		VOLATILE ORGANIC COMPOUND (VOC) ASSESSMENT
Propane is regulated as a VOC under CAA regulations (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 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 propane in vending machines are not anticipated to contribute significantly to ground level ozone concentrations in the United States.
 5. 	DISCUSSION OF END-USE SCENARIOS MODELED
Propane is proposed for use in household refrigerators and freezers. A charge size was not available for propane in the household refrigeration end-use.  Therefore, this analysis assumes that refrigeration units will conform to the standard UL 250 Supplement SA.  Per this standard, 50 grams is the maximum amount of refrigerant that may leak for refrigerants with flammability limits and a heat of combustion that is >19,000 kJ/kg, which is the case for propane. 
To represent a reasonable worst-case scenario, it was assumed that a catastrophic leak of refrigerant occurs while the refrigerator or freezer unit is located in a household kitchen. Because household refrigerators and freezers can be installed in locations with varying room volumes, the analysis in this risk screen conservatively assumes that the refrigeration unit is located in a kitchen with a volume of 18m[3]. This modeling scenario is also intended to be consistent with EPA's proposed use condition for hydrocarbon refrigerants that limits the refrigerant charge size of any refrigerator, freezer, or combination refrigerator and freezers to 57 grams, assuming that roughly 12 percent of the refrigerant charge is retained in the compressor oil and 88 percent of the charge leaks. 
Under the worst-case scenario, the full charge of the unit is assumed to be emitted over the course of one minute into a kitchen with a ventilation rate of 0.25 ACH. It is further assumed that an individual is present at the start of the leak and remains in the room with the door closed until the charge is completely released. A vertical concentration gradient is also assumed since propane is denser than air (specific gravity of propane relative to air is 1.52 [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 lower 0.4 meters of the room and the rest of the refrigerant mixes evenly in the remaining volume (Kataoka 2000). Modeling of the lower 0.4 meters of the room is consistent with the possibility that in a bedroom scenario individuals may be lying close to the floor. Table 3 details the assumptions used in the models throughout this risk screen (i.e., in Sections 7, 8, and 9).
                  Table 3. End-Use Scenario Model Assumptions
Parameter
                                  Assumption
Room Type
                               Household Kitchen
 Size (m[3])
                               18[a] (640 ft[3])
 Ventilation Rate (air changes per hour)
                                    0.25[a]
Refrigeration Unit
                       Household Refrigerator or Freezer
 Charge Size (g)
                                     50[b]
 Duration of Release (minutes)
                                       1
 Vertical Concentration Gradient
                                      Yes
                           [a] Background Document (EPA 1994)
                    [b] Maximum charge size for a household refrigerator or freezer under UL 250.
 6.		POTENTIAL HEALTH EFFECTS
To assess potential health risks from exposure to the proposed substitute in household refrigeration units, EPA identified the relevant toxicity threshold values for comparison with 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 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 with 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 with emergency guidance levels. In the absence of an established short-term exposure limit (STEL), acute exposure guideline (AEGL), or emergency response planning guideline (ERPG), potential short-term, end-use exposures were compared with an excursion limit, which was calculated according to the methodology described in ACGIH (2004).  Table 4 lists the relevant toxicity limits and RCLs of propane and its 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. Toxicity Levels of Propane
                                   Component
                           WGL (Long-term Exposure)
                                      ppm
                           EGL (Short-term Exposure)
                                      ppm
                   Refrigerant Concentration Limit (RCL) ppm
                                    Propane
                                    1,000a
                                  (OSHA PEL)
                           10,000c,d (10 min AEGL-1)
                            6,900d (30 min AEGL-1)
                                   5,300[g]
Potential Impurities 
                                   Isobutane
                            1,000[b]  (ACGIH TLV )
                                   6,900[e] 
                                   4,000[g]
                                   Propylene
                              500[b] (ACGIH TLV)
                     1,500f (30 min ACGIH Excursion Limit)
                                   1,000[g]
[a] OSHA PEL available at: http://www.cdc.gov/Niosh/npg/npgd0524.html
[b] ACGIH TLV for aliphatic hydrocarbon gases: alkane (C1 - C4) is 1,000 ppm TWA (ACGIH 2012)
[c] An IDLH of 2,100 ppm has been established for propane.  However, NIOSH (1996) states that "[b]ased on acute inhalation toxicity data in humans (ACGIH 1991; Braker 1980), a value much greater than 10,000 ppm would have been appropriate. However, the revised IDLH for propane is 2,100 ppm based strictly on safety considerations (i.e., being 10% of the lower explosive limit of 2.1%)."  Therefore, as this IDLH value is based on flammability concerns and not toxicity concerns, it is not used in the evaluation of toxicity risks in this risk screen.
[d] AEGL-1 available at: http://www.epa.gov/opptintr/aegl/pubs/results96.htm
[e] Because n-butane and isobutane have the same molecular formula, EGL for isobutane is conservatively assumed to be that for n-Butane. 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."
[f] Calculated as three times the TLV-TWA. (ACGIH 2004)
[g] ASHRAE (2010b)
         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
AIHA
American Industrial Hygiene Association
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 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.
ERPG
Emergency Response Planning Guideline 1  
The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hour without experiencing other than mild transient health effects or perceiving a clearly defined, objectionable odor according to AIHA. 

Emergency Response Planning Guideline 2  
The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action according to AIHA.

Emergency Response Planning Guideline 3  
The maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects according to AIHA.
Excursion Limit [d]
Excursion Limit 
Because methane (and other short-chain alkane gases through butane) does not have a TLV-short-term exposure limit, an excursion limit may be applied. Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded according to ACGIH. 
IDLH
Immediately Dangerous to Life and Health
If exposed to this concentration, room occupants are expected to be able to escape the room within 30 minutes without experiencing escape-impairing or irreversible health effects.
TEEL[e]
Temporary Emergency Exposure Limit 1
TEEL-1 is the maximum concentration in air below which it is believed nearly all individuals could be exposed without experiencing other than mild transient adverse health effects or perceiving a clearly defined, objectionable odor. 

Temporary Emergency Exposure Limit 2
TEEL-2 is the maximum concentration in air below which it is believed nearly all individuals could be exposed without experiencing or developing irreversible or other serious health effects or symptoms that could impair their abilities to take protective action.

Temporary Emergency Exposure Limit 3
TEEL-3 is the maximum concentration in air below which it is believed nearly all individuals could be exposed without experiencing or developing life-threatening health effects. 
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.
[d] ACGIH (2004)
[e] Chemical exposure guidelines to use for emergency planning if no AEGL or ERPG is available. TEELs are developed based on concentration limits or toxicology parameters (DOE 2008).

According to the MSDS, exposure to propane may be hazardous if inhalation, skin contact, or eye contact with propane occurs. The most likely pathway of exposure is through inhalation. At concentrations high enough to significantly lower oxygen concentrations below 19.5 percent by volume, propane can cause symptoms of asphyxiation such as headaches, ringing in the ears, dizziness, drowsiness, nausea, vomiting, depression of all senses, and unconsciousness. Exposure when propane causes oxygen levels to fall below 6 percent by volume may cause death. In addition, at high concentrations, propane can act as a narcotic and cause central nervous system depression, including dizziness, drowsiness, and headaches. 
If propane 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.  Rescuers should not attempt to assist victims of exposure to propane without adequate PPE. At a minimum, a self-contained breathing apparatus (SCBA) should be worn. Exposures of propane to the skin may cause frostbite. In the case of dermal exposure, the MSDS for propane recommends that person(s) immediately wash the affected area with water and remove all contaminated clothing; if frostbite occurs, bathe (do not rub) the affected area with lukewarm, not hot, water. 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. Exposures of propane to the eyes could cause eye irritation. In case of ocular exposure, the MSDS for propane recommends that person(s) immediately flush the eyes, including under the eyelids, with copious amounts of water for 15 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 propane can be minimized by following the exposure guidelines and ventilation and PPE recommendations outlined in the MSDS for propane and in this risk screen.
 7. 		FLAMMABILITY ASSESSMENT
ASHRAE Standard 34 classifies propane as a Class A3 refrigerant. Propane is flammable when its concentration in air is in the range of 2.1 percent to 9.5 percent by volume (21,000 ppm to 95,000 ppm).  In the presence of an ignition source (e.g., static electricity, a spark resulting from a closing door, or a cigarette), an explosion or a fire could occur if the concentration of propane were to exceed the lower flammability limit (LFL) of 21,000 ppm.  The remainder of this section addresses 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
Release of or exposure to propane may be possible at the charging and sealing steps on the production line at the manufacturing site. It is therefore important that all relevant standards be followed during the manufacture of refrigeration units, specifically UL 250. If the manufacture of household refrigerator and freezer units charged with propane occurs in the United States, EPA recommends that engineering controls should include normal and local ventilation (e.g., chemical hoods) for standard manufacturing procedures so workers can avoid physical contact with the refrigerant and to limit emissions. In general, use of appropriate PPE consistent with OSHA guidelines is recommended, such as splash goggles, mechanically-resistant gloves when handling cylinders, chemically-resistant gloves (e.g., butyl rubber, chlorinated polyethylene, or neoprene) when handling the gas mixture, and protective clothing. 

All propane 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 2007) for liquefied propane requires the use of overfill protection devices (OPD) on cylinders to minimize the likelihood of leaks.  The NPFA 58 Code also contains propane storage and transportation requirements/guidelines. It is expected that refrigerants be properly stored and caution used within manufacturing facilities to minimize explosion risk and that workers adhere to the requirements set by OSHA under 29 CFR 1910. OSHA requirements include proper ventilation and storage practices within manufacturing facilities to prevent fire and explosion. Good manufacturing practices should be adhered to during the manufacture of equipment containing propane, e.g., using proper ventilation. 

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 propane. Both servicing and end-use of propane household refrigeration units are expected to take place in the same space (i.e., the place of installation and use). In order to determine the potential flammability risks during servicing or end-use in case of a catastrophic release of refrigerant, modeled concentrations of propane immediately following the release of refrigerant were compared with the LFL for propane. As demonstrated in Table 6, the maximum instantaneous concentration in the lower 0.4 meters of the room at a charge size of 50 grams would be approximately 8,717 ppm. The maximum instantaneous concentration in the upper portion in the room is much lower, as only 5 percent of the leaked refrigerant is present in this area, which has greater volume than the lower 0.4 meters. The results of the flammability assessment are presented in Table 6. 

                     Table 6.  Flammability Assessment[a]
                                   Scenario
                                Charge Size (g)
                          Effective Room Size (m[3])
                      Maximum Instantaneous Concentration
                                  (ppm) b[,c]
                            Reasonable Worst-Case 1
                                      50
                                18 (640 ft[3])
                                     8,717
                       Threshold Analysis 1: Charge Size
                                      310
                                18 (640 ft[3])
                                    21,000
                        Threshold Analysis 2: Room Size
                                      50
                                 3 (110 ft[3])
                                    21,000
    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 propane is 21,000 ppm.
    [c] Values provided in this column refer to the concentration in the lower 0.4 meters of the room, which presents the most conservative risk as 95 percent of the leaked refrigerant is present in this space. 

For the flammability threshold analysis shown in Table 6, the conditions at which a flammability concern would exist (i.e., when the maximum instantaneous concentration in the lower 0.4 meters of the room equals the LFL for propane) were determined. Within the reasonable worst-case room size of 18 m[3] (640 ft[3]), the charge size of the propane refrigeration unit would have to be at least 310 grams for a flammability concern to exist, as shown in Threshold Analysis 1. This is more than the charge size of 57 grams which EPA's proposed use condition requires as a limit for propane in any refrigerator, freezer, or combination refrigerator and freezer. Using a 50-gram refrigeration unit, as shown under Threshold Analysis 2, the volume of the room would have to be 3 m[3] (110 ft[3]) or smaller for a flammability risk to occur. 
According to the results of this flammability analysis, the risk of fire is minimal if a household refrigeration unit is installed in a room with volumes in accordance with standards and regulations. As kitchens can be smaller than those modeled in this assessment and because persons or furniture in the room may reduce the effective volume of the space, refrigeration units containing propane should not be installed in enclosed areas with effective volumes less than indicated by UL 250 unless additional steps are taken to protect against a flammability risk (e.g., higher ventilation rates).  According to ASHRAE Standard 34, the RCL for propane (9.5 g/m[3] of propane or 5,300 ppm by volume) should not be exceeded in any room that contains a propane refrigeration unit. Although the RCL for propane is less conservative than the anticipated release amount of propane under the modeled worst-case scenario (i.e., 50 grams in a 18 m[3] space, or 2.8 g/m[3]), the RCL is calculated assuming that refrigerant is evenly mixed in the space, whereas this analysis assumes a vertical concentration gradient occurs due to the higher density of propane than that of air (propane = 1.52, air = 1). End-users, therefore, should ensure that propane refrigeration units are installed in locations with areas that are consistent with manufacturer recommendations and the guidelines of relevant safety standards (e.g., ASHRAE 15 and 34 and UL 250).
EPA recommends that warning labels are present on the equipment indicating that flammable refrigerants are inside the unit as well as notifying the user to read the equipment use instructions that provide safety guidelines for handling propane. These guidelines should be followed to mitigate flammability risk. The installation of leak prevention devices would further protect against the very limited risk of explosion. During servicing operations, EPA recommends that the ventilation requirements outlined in the MSDS for propane are followed through the use of fans or other mechanical ventilation devices and that portable refrigerant detectors are used to alert technicians to the presence of flammable gases in order to mitigate the risk of fire or explosion during catastrophic leak.  It is also important that only properly trained and certified technicians handle propane. As a further precaution, certification requirements and training programs for technicians that handle propane should be developed using these guidelines.  
 8.		ASPHYXIATION ASSESSMENT
The risk of asphyxiation for the reasonable worst-case scenario (see Section 5) was investigated for propane. 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 conditions that are likely to occur that would increase the 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)
                          Effective Room Size (m[3])
           Percent Oxygen Concentration in the Lower 0.4 Meters[b] 
                            Reasonable Worst-Case 1
                                      50
                                18 (640 ft[3])
                                      21
                       Threshold Analysis 1: Charge Size
                                     3,250
                                18 (640 ft[3])
                                      12
                        Threshold Analysis 2: Room Size
                                      50
                                0.28 (9.8ft[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) applicationsne.  mmended that pens r than the room assumed in the worst case scenario (see Section 4). 

In order for a risk of asphyxiation to occur, the normal concentration of oxygen in air (21 percent) must be reduced to 12 percent. Based on the worst-case scenario modeling assumptions, propane in household refrigeration units does not present a significant risk of asphyxiation. The concentration of propane in the air following the release of the 50-gram charge size does not exceed 0.11 percent, which has an insignificant impact on the concentration of oxygen in air. 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; therefore, the actual asphyxiation risk to person(s) 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. Within the reasonable worst-case room size of 18 m[3] (640 ft[3]), as shown under Threshold Analysis 1, the charge size of the propane refrigerator or freezer unit would have to be at least 19,500 grams for asphyxiation to be a concern, which is more than 340 times the charge size anticipated in EPA's proposed use condition for hydrocarbon refrigerants in any refrigerator, freezer, or combination refrigerator and freezers. In Threshold Analysis 2, the volume of the room would have to be 0.05 m3 (1.8 ft[3]) or smaller for asphyxiation to be of concern.
To further protect against an asphyxiation risk, end-users should ensure household refrigerators or freezers with propane are installed in rooms with areas consistent with manufacturer recommendations and the guidelines of relevant safety standards (e.g., ASHRAE 15 and 34 and UL 250). Minimum room sizes according to the RCL stated in ASHRAE 34 can be found in Section 7.2. Leak prevention, detection, and/or mitigation devices should be installed and the ventilation requirements outlined in the MSDS for propane should be closely followed. If these guidelines are followed, a catastrophic leak of propane is not anticipated to result in a significant asphyxiation risk. 
 9. 		END-USE EXPOSURE ASSESSMENT
This section presents estimates of potential end-use exposures to propane and each of its impurities in household refrigerators and freezers.  An exposure analysis was performed to examine potential catastrophic release of the proposed substitute in a kitchen under the reasonable worst-case scenario outlined in Section 5.  

For the end-use exposure assessment scenario, 30-minute TWA exposures for the proposed substitute and its impurities were calculated using the box model described in the Background Document, which was adapted to estimate concentrations on a minute-by-minute basis.  These exposures were then compared with 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 was performed for propane and each of its impurity constituents; however, only the results for propane are presented in Table 8. As propane is approximately 99.5 percent pure, it presents the greatest risk of exceeding the STEL and thus the most conservative assessment. 

                   Table 8. End-Use Exposure Assessment[a] 
                                   Scenario
                                Charge Size (g)
                          Effective Room Size (m[3])
                  30-minute TWA End-Use Exposure (ppm)[b][,c]
                           Reasonable Worst-Case 1 
                                      50
                                18 (640 ft[3])
                                     3,300
                       Threshold Analysis 1: Charge Size
                                      104
                                18 (640 ft[3])
                                     6,859
                        Threshold Analysis 2: Room Size
                                      50
                                8.7 (310 ft[3])
                                     6,822
      Bold font indicates modeling results. 
		[a] Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.
      [b] 30-min AEGL-1 STEL for propane is 6,900 ppm; See Table 4 for more information.
      [c] Values provided in this column refer to the concentration in the lower 0.4 meters of the room, which presents the most conservative risk as 95 percent of the leaked refrigerant in present in this space. 

In the reasonable worst-case scenario, the catastrophic release of propane from household refrigerators and freezers with a charge size of 50 grams was modeled. The modeling results indicate the maximum anticipated end-use exposure does not exceed 48 percent of the 30-min AEGL-1 for propane. To mitigate risk during end-use, EPA recommends that for a minimum room volume of 18 m[3], the charge size does not exceed 104 grams, and that for a charge size of 50 grams, the household refrigerator or freezer not be installed in a room of volume less than 8.7 m3 (310 ft[3]). Because the modeling was done using a conservative scenario, personnel exposure to the proposed substitute at end-use is not considered a significant threat. To minimize the risk of exposure, EPA recommends that the charge size for a household refrigeration unit does not exceed the RCL (9.5 g/m[3]) and proposes to require compliance with UL 250 in any space containing a household refrigeration unit, unless proper leak protection devises are in place in order to prevent exposures of propane 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 propane in household refrigerators and freezers. To ensure that use of the proposed substitute in household refrigeration units 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 were obtained from input parameters to the Vintaging Model.  To determine the estimated level of occupational exposure for a household refrigerator charged with propane, it was conservatively assumed that 1 percent of the equipment charge would be released per manufacturing event, 0.4 percent during servicing, and 3 percent during disposal.  Additionally, EPA is proposing to allow intentional venting or release of propane as a refrigerant during installation, servicing, maintenance and disposal from household refrigeration equipment. If venting does occur, it should be done in a well-ventilated area (i.e. outdoors). Therefore, scenarios in which 100 percent of the equipment charge is 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 household refrigeration unit servicing, it was conservatively assumed that eight events of servicing a unit by one servicing technician occur in an 8-hour work day. For disposal, it was conservatively assumed that 10 units are disposed of in one facility during an 8-hour work day.  The modeled exposure concentrations were compared with short-term occupational exposure limits at installation and servicing and long-term exposure limits at disposal. 

10.1	Occupational Exposure during Manufacture
Propane is produced around the world, including in the United States, and is used for a wide variety of refrigerant and non-refrigerant uses. Potential points of release relevant to this analysis are during the charging of propane into the refrigeration system within the refrigerator or freezer (prior to being hermetically sealed) or through punctures (after sealing) during the subsequent manufacturing, finishing, assembling, packaging or handling processes. Releases prior to sealing or leaks in the hermetic system could release up to 50 grams of propane into the indoor air. The maximum 8-hour time-weighted average (TWA) exposures for propane and the trace impurities were estimated for a manufacturing scenario and  compared with their respective short-term exposure limits (see Table 8). For manufacturing of household refrigeration units, it was assumed that the release per event was 1 percent of the unit's charge. 

           Table 8.  Occupational Risk Assessment at Manufacture[a]
                                   Component
                    8-hour TWA Occupational Exposure (ppm)
                   Long Term (8-Hour) Exposure Limits (ppm)
                                    Propane
                                      273
                                   1,000[b]
Potential Impurities 
                                   Isobutane
                                     0.29
                                   1,000[c]
                                   Propylene
                                     0.09
                                    500[c]
      [a] Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.
      [b] OSHA PEL
      [c] ACGIH TLV 
	       
The modeling results indicate that the maximum anticipated occupational exposure concentrations for propane and its impurities during manufacture are significantly lower than the short-term exposure limits for household refrigerators or freezers with a 50-gram charge size. EPA believes that when proper handling and storage guidelines are followed in accordance with both good industrial hygiene and manufacturing practices and the MSDS for propane significant exposures to propane during manufacture will be unlikely. Thus, EPA does not believe that manufacturing, including charging, of household refrigeration units with propane presents a significant concern to workers.

10.2	Occupational Exposure at Installation and Servicing 
The potential occupational exposure during installation and servicing for propane household refrigeration units was analyzed. The maximum 30-minute time-weighted average (TWA) exposure for propane and its potential impurities were estimated for servicing household refrigeration units charged with 50 grams of refrigerant. As summarized in Table 9, occupational exposure to propane is anticipated to be significantly below the STEL for propane and its impurities with a 50-gram charge size for both 0.5 percent release and 100 percent release of refrigerant during servicing. Therefore, worker exposure to propane during installation and servicing is not a significant concern. Technicians should follow the procedures outlined in the MSDS and maintenance manual, undergo proper training, and wear appropriate PPE (e.g., gloves and safety glasses). Adequate ventilation should always be present during any use, handling, or storage of propane. Systems should be installed by following procedures of ASHRAE Standard 14 including conducting leak checks once a system is installed (ASHRAE 2002). In addition, prior to adding refrigerant during servicing events, all pipes should be inspected for leakages, and repairs should be performed only after all refrigerant has been recovered from the system. By adhering to the safety guidelines mentioned above and in Section 6, worker exposure to propane during installation and maintenance is unlikely.

             Table 9. Occupational Risk Assessment at Servicing[a]
                                      Component
                   30-minute TWA Occupational Exposure (ppm)
                            Charge size:  50 grams
                    30-min Short Term Exposure Limits (ppm)
                                       
                                 0.4% Release
                                 100% Release
                                       
                                    Propane
                                     0.40
                                      100
                                   6,900[b]
Potential Impurities
                                   Isobutane
                                  4 x 10[-4]
                                     0.11
                                   6,900[c]
                                   Propylene
                                  2 x 10[-4]
                                     0.03
                                   1,500[d]
      [a] Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.
   	[b] AEGL-1
   	[c] AEGL-1; See Table 4 for more information 
   	[d] ACGIH (2004)
      
10.3	Occupational Exposure at Disposal
Disposal of propane household refrigeration units is expected to occur at disposal facilities with limited duration of exposure to the refrigerant. Potential exposures to the refrigerant during recovery and disposal are expected to occur during activities related to draining the refrigerant from the refrigeration units into cylinders (e.g., connecting of pipes). Such activities and related exposure is anticipated to occur within 15-30 minutes (per event/day). 
Based on the assumptions described in the beginning of Section 10, the modeling indicates that 8-hour worker exposure concentrations for propane and its impurities during the 3 percent release and 100 percent release scenarios will not exceed the long-term exposure limits of any of the individual components during disposal of propane for household refrigeration units with a charge size of 50 grams. Table 10 displays the maximum estimated 8-hour TWA occupational exposure levels of the proposed substitute and its impurities during disposal. Because modeled exposure levels in both scenarios are significantly lower than the exposure guidelines for propane and its impurities, occupational exposure to the proposed substitute during disposal is not considered a toxicity threat. Further, estimated exposures were derived using conservative assumptions (e.g., no ventilation or use of PPE assumed) 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[a]
                                   Component
                    8-Hour TWA Occupational Exposure (ppm)
                             Charge size: 50 grams
                    8-Hour Long Term Exposure Limits (ppm)
                                       
                                  3% Release
                                 100% Release
                                       
                                    Propane
                                     5.28
                                      175
                                    1,000b
Potential Impurities
                                   Isobutane
                                     0.01
                                     0.19
                                   1,000[c]
                                   Propylene
                                     0.002
                                     0.06
                                    500[b]
                [a] Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.
               [b] OSHA PEL 
              [c] ACGIH TLV 
                      
Although anticipated occupational exposures are well below the exposure limits for propane and each of its impurities in both disposal release scenarios, the recommendations for proper engineering controls and PPE in the MSDS for propane should be followed. Adequate ventilation should always be established during any use, handling, or storage of propane. Engineering controls should include vapor-in-air detection systems and local exhaust ventilation during use of propane to prevent dispersion throughout the workplace. In addition, an eye wash and safety shower should be near the manufacturing facility and locations where propane is stored and ready for use. In general, use of 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). EPA believes that if proper handling and disposal guidelines are followed in accordance with good industrial hygiene and manufacturing practices and the MSDS for propane, there is no significant risk to workers during the manufacturing, installation, servicing, and disposal of propane in household refrigeration units. 
 11.		GENERAL POPULATION EXPOSURE ASSESSMENT
Propane is not expected to cause a concern for human health in the general population when used as a refrigerant in household refrigeration units. At room temperature, propane is a gas and, therefore, releases to ground or surface water are not anticipated, as propane is anticipated to dissipate into the atmosphere upon release to outside air because natural ventilation rates would be higher and there is no enclosed space to keep propane concentrated. Should air releases during manufacturing operations occur, including filling of refrigeration units, engineering controls should be used (e.g., carbon absorption units or scrubbers) to collect propane and prevent the release of propane 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 propane is not expected to pose a significant toxicity risk to the general population.

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