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: Propane
This risk screen does not contain Clean Air Act (CAA) Confidential Business Information (CBI) and, therefore, may be disclosed to the public.
INTRODUCTION
Ozone-depleting substances (ODS) are being phased out of production in response to a series of diplomatic and legislative efforts that have taken place over the past two decades, including the Montreal Protocol and the Clean Air Act Amendments of 1990 (CAAA).  The U.S. Environmental Protection Agency (EPA), as authorized by Section 612 of the CAAA, runs the Significant New Alternatives Policy (SNAP) Program, which identifies acceptable and unacceptable substitutes for ODS in specific end-uses based on assessment of their health and environmental impacts.  

EPA's decision on the acceptability of a substitute is based largely on the findings of a screening assessment of potential human health and environmental risks posed by the substitute in specific applications.  EPA has already screened a large number of substitutes in many end-use applications within all of the major ODS-using sectors, including: refrigeration and air conditioning, solvent cleaning, foam blowing, aerosols, fire suppression, adhesives, coatings and inks, and sterilization. The results of these risk screens are presented in a series of Background Documents that are available in EPA's docket.

The purpose of this risk screen is to supplement EPA's Background Document on the refrigeration and air conditioning sector (EPA 1994) (hereinafter referred to as the Background Document) by adding to the list of potential substitutes for specific end-uses of CFC-12, HCFC-22 and R-502 in this sector.  The proposed end-use application considered in this analysis is retail food refrigeration, specifically ice cream cabinets.  The specific proposed CFC-12, HCFC-22 and R-502 substitute examined in this report is propane (R-290).  The substitute would only be used in new equipment, not in retrofitted equipment.  
The Background Document examines the potential risks associated with use of substitutes in retail food refrigeration, and includes a detailed discussion of the methodologies used to conduct this risk screen.  Occupational exposure modeling was performed to ensure that use of the proposed substitute in the application listed above did not pose unacceptable risk to workers during equipment manufacture.  Modeling was performed at the end-use to ensure that potential catastrophic releases of the substitute did not pose unacceptable risk to store employees and customers in locations where the ice cream cabinets are in use.  Lastly, general population exposure modeling was performed to ensure that the proposed substitute would not pose an unacceptable risk to the population at large.  

The proposed substitute, propane, may contain minute quantities of impurities.  Table 1 details the composition of the proposed substitute, including the maximum estimated concentration of impurities that may be present.

                Table 1. Composition of the Proposed Substitute
                                   Component
                                Concentration 
                                  Substitute
                               Percent by Weight
                                    Propane
                                     99.5%
                             Potential Impurities
                        Maximum Concentration (ppm v/v)
                                    Ethane
                                     1700
                                   Propylene
                                      300
                                   n-Butane
                                     1000
                                    Sulfur
                                       1
                                     Water
                                      12

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

         * Section 3: Atmospheric Assessment
         * Section 4: Discussion of End-Use Scenarios Modeled
         * Section 5: Flammability Assessment
         * Section 6: Asphyxiation Assessment
         * Section 7: Toxicity Assessment
         * Section 8: Volatile Organic Compound Assessment 
         * Section 9: References
SUMMARY OF RESULTS
Propane is recommended for SNAP approval for retail ice cream cabinets.  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.  No significant toxicity risks to workers, consumers, or the general population are expected according to occupational and end-use exposure modeling.  

Flammability models indicate that risks of explosions are not a concern for store employees or customers.  To further protect against the limited risk of explosion, it is recommended that the cabinets not be used in small, poorly ventilated spaces (see Section 4).  Caution must be used in manufacturing facilities and by refrigeration technicians to minimize explosion risk while in the presence of large quantities of the substitute.  This includes installation of proper safety equipment during manufacturing, transportation, and storage and providing proper training and certification to technicians.  EPA recommends that American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standards 15, 34 and 62 be followed.
ATMOSPHERIC ASSESSMENT
This section presents an assessment of the potential risks to atmospheric integrity posed by the use of propane in the retail food refrigeration sector.  The ozone depletion potential (ODP), global warming potential (GWP), and atmospheric lifetime (ALT) of the proposed substitute are presented in Table 2.	

The environmental impacts resulting from use of propane are generally in the range of those predicted for other substitutes examined in the Background Document.  The substitute is substantially less harmful to the ozone layer, has less climate impact, and a shorter atmospheric lifetime when compared to CFC-12, HCFC-22 and R-502.

Table 2.  Atmospheric Impacts of Propane Compared to CFC-12, HCFC-22 and R-502
                                  Refrigerant
                                      ODP
                                      GWP
                                      ALT
                                    Propane
                                     0[a]
                                    3.3[a]
                                   0.041[b]
                                    CFC-12
                                     1[c]
                                   10,900[a]
                                    100[a]
                                    HCFC-22
                                   0.055[c]
                                   1,810[a]
                                     12[a]
                                   R-502 [d]
                                    0.33[c]
                                     4650
                                      NA
NA = Not Available
a IPCC 4[th] Assessment Report (Forster et al. 2007).
b IPCC/TEAP (2005).  Atmospheric lifetime not provided in SNAP submission.
[c] Montreal Protocol and available at: http://www.epa.gov/ozone/ods.html.
[d] R-502 is a blend consisting of CFC-115 (51%) and HCFC-22 (49%). The ODP and GWP presented for this blend were calculated using values from http://www.epa.gov/ozone/ods.html (Montreal Protocol) and IPCC 4th Assessment Report (Forster et al. 2007), respectively. Alternatively, WMO (2006) has estimated the ODP for CFC-115 to be 0.44 based on recent atmospheric modeling (rather than 0.6, as previously agreed by the Parties to the Montreal Protocol). Using the more current estimate of ODP would bring the overall ODP for R-502 to 0.24.
DISCUSSION OF END-USE SCENARIOS MODELED
The proposed use of this substitute is in ice cream cabinets which could reasonably be expected to be located either in the main area of a store, or in a storage room.  Because a storage room could be much smaller than the main area of a store, a reasonable worst-case scenario analysis was performed to model catastrophic release of the refrigerant in a storage room.  

For the analysis, a storage room with a floor area of 14 m[2] and a height of 3 meters was assumed.  This is equivalent to a room volume of 42 m[3].  In the analysis, the full charge of the unit is assumed to be emitted over the course of one minute and the model conservatively assumes 2.5 air exchanges occur per hour.  Horizontal stratification is also assumed since propane is denser than air and will settle in higher concentrations closer to the ground.  In order to simulate the horizontal concentration gradient that will occur because of the weight differential between the refrigerant and air, it is assumed that 95 percent of the leaked refrigerant mixes evenly into the bottom 0.4 meter of the room, and the rest of the refrigerant mixes evenly in the remaining volume (Kataoka 1999).  Table 3 details the assumptions used in the models.
                Table 3. Worst-Case Scenario Model Assumptions
                                   Parameter
                                  Scenario 1
                              Refrigeration Unit
                               Ice Cream Cabinet
                                     Room
                                 Storage Room
                                Charge size (g)
                                      150
                              % of charge emitted
                                      100
                          Length of release (minutes)
                                       1
                           Room size (volume - m[3])
                                      42
                   Room ventilation (air exchanges per hour)
                                      2.5
                           Horizontal stratification
                                      Yes
FLAMMABILITY ASSESSMENT
Propane is flammable when its concentration in air is in the range of 2.1%-9.5% by volume (this is equal to 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 when the concentration of propane is within these flammability limits.  The remainder of this section addresses the potential for flammability risks at the end-use, and during manufacture and servicing and provides a discussion of measures to be taken to ensure safe use. 

Under the assumptions outlined under the catastrophic release scenario  (see Section 4), the maximum instantaneous concentration of propane in the lower stratum of the room would be approximately 66 percent of its lower flammability limit (LFL), as shown in Table 4.  The maximum instantaneous concentration is even lower in the upper stratum of the room, as only five percent of the leaked refrigerant is present in this stratum, and this stratum has a greater volume than the lower stratum.  

                       Table 4. Flammability Assessment
                                       
                                       
                                  Room Type/
                                Appliance Type
                        Reasonable Worst-Case Scenario
                        Flammability Threshold Scenario
                                       
                               Room Size (m[3])
                      Maximum Instantaneous Concentration
                                  (ppm) a[,b]
                  Room Size Causing Flammability Risk (m[3])
                      Maximum Instantaneous Concentration
                                  (ppm) a[,b]
                                 Storage Room/
                               Ice Cream Cabinet
                               42 (1,483 ft[3])
                                    13,780
                             28.5 (1,006.5 ft[3])
                                    20,308
[a] Lower Flammability Limit of propane is equal to 21,000 ppm
[b] Values provided in these columns refer to the concentration in the lower stratum of the room.  

For flammability to be of concern under the conservative (protective) assumptions described above, the volume of the storage room would have to be 28.5 m3 (1,006.5 ft[3])  (See Table 4); assuming a square room with a ceiling height of 3 m (9.8 ft), the storage room floor area would be less than  3.1 x 3.1 m (10.1 x 10.1 ft) .  Although these dimensions equate to a very small space, since storage rooms can be of varying size and because boxes and other equipment in the room may reduce the effective volume of the room, it is recommended that propane cabinets not be installed in small, poorly ventilated spaces, to avoid the risk of fire and explosion.  However, for average-sized rooms the risk of explosion is minimal.  Creating a hole in the bottom of storage room doors (to act as a vent and increase air circulation) and installing leak prevention devices will further protect against the limited risk of explosion.  For example, in commercial refrigeration end-uses, refrigerant leak prevention systems can be used to capture leaked refrigerant in a receiving tank during over pressure events, rather than venting the refrigerant to the atmosphere (McKeown 1993).  

Catastrophic releases of large quantities of refrigerant during servicing and manufacturing, especially in areas where large amounts of refrigerant are stored, could cause an explosion.  For this reason, it is important that only properly trained and certified refrigerant technicians handle propane.  The submitter has provided information regarding their training program for service technicians.  The program includes detailed information covering important topics such as potential ignition sources, proper tools to use with flammable refrigerants, safe workplace behavior and service procedures.  At the end of the training, technicians must pass a multiple choice assessment (Ben and Jerry's/Unilever 2008).  All technicians who will be servicing cabinets using propane should receive this training and be required to pass the assessment.  As a further precaution, 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 2008) requires the use of overfill protection devices (OPD) on cylinders to minimize the likelihood of leaks.  The NPFA 58 Code also contains storage and transportation requirements/guidelines.  Similar equipment safety and procedural requirements should be implemented for this substitute.  

It is important that the strictest standards be followed during the manufacture of these ice cream cabinets.  It is recommended that refrigerants be properly stored and caution used within manufacturing facilities to minimize explosion risk and that workers adhere to the requirements set by OSHA under 29 CFR 1910.  OSHA requirements include proper ventilation and storage practices within manufacturing facilities to prevent fire and explosion.  Proper ventilation should be maintained at all times during the manufacture of equipment containing propane through adherence to good manufacturing practices. If refrigerant levels in the air surrounding the equipment rise above one-fourth of the lower flammability limit, the space should be evacuated and re-entry should only occur after the space has been properly ventilated.  Ventilation is also of the utmost importance to mitigate the risk of fire or explosion when servicing equipment using propane.  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.  
ASPHYXIATION ASSESSMENT
The risk of asphyxiation for a reasonable "worst-case" scenario was investigated for propane.  This analysis considers the modeling assumptions outlined in Section 4. This analysis does not consider additional ventilation conditions (such as open doors or windows, fans operating, conditioned airflow, or seepage between the door and door frame) that are likely to occur and would further reduce the exposure concentration of propane.  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 5 below.  The maximum charge of propane necessary to reduce the oxygen levels to 12 percent in air in a storage room of volume 42 m[3] (1,483 ft[3]) was calculated, assuming horizontal stratification of the refrigerant and the air.  Horizontal stratification is assumed since propane is denser than air and will settle in higher concentrations closer to the ground.  Assuming that nitrogen and oxygen retain the same relative volumes in the rooms with the balance composed entirely of propane, and that the pressure of the room does not increase significantly with the addition of the refrigerant, a charge of approximately 904 g would be necessary to reach 12 percent oxygen in the lower stratum.  This amount represents more than six times the maximum intended charge size, 150 g, of a single propane ice cream cabinet.  Charge requirements to reach the same effect in the upper stratum would be even higher because of the stratum's larger volume. 

                       Table 5.  Asphyxiation Assessment
                                  Room Type/
                                Appliance Type
                        Reasonable Worst-Case Scenario
                        Asphyxiation Threshold Scenario
                                       
                               Room Size (m[3])
                       Charge Causing Impairment (g)[a]
                      Room Size Causing Impairment(m[3])
                           Assumed Leak Size (g)[a]
                                Storage Room/ 
                               Ice Cream Cabinet
                               42 (1,483 ft[3])
                                      904
                              6.96 (245.8 ft[3])
                                      150
 a Values provided in these columns refer to the charge required to cause impairment in the lower stratum of the room. In this scenario, 100% of the charge leaks. Charge requirements to reach the same effect in the upper strata would be even higher because of the stratum's larger volume. 

 For asphyxiation to be of concern with the proposed charge size under the conservative (protective) assumptions described above, the volume of the storage room would have to be approximately 6.96 m[3] (245.8 ft[3]), which is significantly smaller than the 42 m[3] sized room assumed in the worst case scenario (see Section 4). Further, as the "threshold" room sizes for asphyxiation risks are smaller than those for flammability risks, flammability risks would be realized before asphyxiation risks.  Therefore, EPA does not believe that the use of propane in this end-use poses a significant risk of asphyxiation or impaired coordination to room occupants under the proposed end-use.

Based on the results discussed above, EPA does not believe that the use of propane in this end-use poses a significant risk of asphyxiation or impaired coordination to store employees or customers. 
TOXICITY ASSESSMENT
Section 7 summarizes the results of the toxicity assessment for propane in the retail refrigeration sector. This section is organized as follows: 
   * Section 7.1: Toxicity Reference Values
   * Section 7.2: Occupational Exposure
   * Section 7.3: End-Use Exposure
   * Section 7.4: General Population Exposure
7.1 	TOXICITY REFERENCE VALUES
To assess potential health risks from exposure to this substitute in the retail food refrigeration sector, EPA identified the relevant toxicity threshold values for comparison to modeled exposure concentrations for different scenarios.  For the occupational exposure analysis, potential risks from chronic and acute worker exposure were evaluated by comparing exposure concentrations to available occupational exposure limits.  Occupational exposure limits are typically established for either an eight-hour or ten-hour time period for long-term exposure, such as the Workplace Guidance Levels (WGLs), or for a 10 to 30-minute period for short-term exposure, such as the Emergency Guidance Levels (EGLs), as shown in Table 6.  Because they are designed to assess risks from acute exposure, emergency guidance levels are used to assess risks from short-term consumer exposures.  Reference concentrations (RfCs) are used to assess risks to the general population from exposure to ambient air releases and to assess potential risks associated with chronic consumer exposures.  Table 6 lists the relevant toxicity limits.  Table 7 provides definitions for acronyms used in Table 6.  EPA's approach for identifying or developing these values is discussed in Chapter 3 of the Background Document. 

         Table 6.  Toxicity Levels of Propane and Potential Impurities
                                       
                              Long-term Exposure
                                      ppm
                              Short-term Exposure
                                      ppm
                     Reference Concentration (RfC) mg/m[3]
                                  Substitute
                                    Propane
                          1000a (OSHA PEL/NIOSH REL)
                         10,000b,c (10 minute AEGL-1)
                                    0.9[d]
                                       
                                       
                          6,900 b  (30 minute AEGL-1)
                                       
                             Potential Impurities
                                    Ethane
                                     NA[e]
                                     NA[e]
                                      NA
                                   Propylene
                                     NA[f]
                                     NA[f]
                                      NA
                                   n-Butane
                              800[g] (NIOSH REL)
                                      NA
                                    0.95[d]
                                    Sulfur
                                      NA
                                      NA
                                      NA
NA  = Not Available
[a] http://www.cdc.gov/Niosh/npg/npgd0524.html
b http://www.epa.gov/oppt/aegl/pubs/results96.htm 
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]SNAP Refrigerant Background Document (EPA 1994).
[e] Ethane is a "simple asphyxiant."  http://www.osha.gov/dts/chemicalsampling/data/CH_238900.html
[f] Propylene is a "simple asphyxiant." http://www.osha.gov/dts/chemicalsampling/data/CH_264442.html
g http://www.cdc.gov/niosh/npg/npgd0068.html

             Table 7.  Explanation of Toxicity-Related Acronyms[a]
Organization 
Definition
OSHA
Occupational Safety and Health Administration
NIOSH
National Institute for Occupational Safety and Health
Exposure Limit
Definition
Explanation
AEGL-1
Acute Exposure Guideline Level 1
The AEGL-1 "is the airborne concentration, expressed as parts per million or milligrams per cubic meter (ppm or mg/m3) 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."  "Airborne concentrations below the AEGL-1 represent exposure levels that can produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects."[b]
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.
PEL
Permissible Exposure Limit
This is an 8-hour time-weighted average exposure limit set by OSHA. 
REL
Recommended Exposure Limit
This is a 10-hour time-weighted average exposure limit set by NIOSH.
RfC
Reference Concentration
A concentration "designed to protect the general population against adverse systemic (i.e., noncancer) effects."
[a]All information in this table taken from EPA (1994).
b http://www.epa.gov/oppt/aegl/pubs/define.htm
7.2	OCCUPATIONAL EXPOSURE 
Occupational exposure modeling was performed for the proposed substitute to ensure that use of the substitute does not pose an unacceptable risk to workers.  The methodology used for this screening assessment is based on the one used in the occupational exposure and hazard analysis described in Chapter 5 of the Background Document. A box-model approach was used to evaluate potential worker exposure to alternative refrigerants.  This approach has been widely used for many years to estimate probable exposures of workers to hazardous airborne materials, and has been described in detail by the National Institute for Occupational Safety and Health (NIOSH).  This model takes into consideration the duration and magnitude of the resulting exposure which is influenced by 1) duration and intensity of the release, 2) rate at which contaminated air is diluted with uncontaminated air, 3) proximity of the worker to the source of the release, and 4) the length of time the worker remains in the affected space.  

Estimates of refrigerant release per event for various release scenarios and data on number of events were obtained from the Vintaging Model.  The release per event was conservatively assumed to be 1 percent of the equipment charge during manufacturing and 3 percent of the equipment charge during disposal.  The release rate per event was multiplied by the number of events estimated to occur over a workday.  For equipment manufacturing, the number of events per workday was assumed to equal the number of units containing the substitute produced per plant per year divided by 365 workdays per year.  For the purposes of this model, it is assumed that propane will have a market penetration rate of 23% and that one production facility will be in operation.  These assumptions provided by the submitter result in approximately 274 events/manufacturing facility/workday.  For disposal, it was conservatively assumed that 1 unit is disposed during an 8-hour work day nationwide.  

The maximum time weighted average (TWA) exposure was estimated for each exposure scenario, and this value compared to the Workplace Guidance Levels (WGLs) for propane and the potential impurities in the substitute.  The modeling results indicate that the short-term (15-minute and 30-minute) and long-term (8-hour) worker exposure concentrations are, at a maximum, about 50 percent of the WGLs.  Table 8 displays the maximum estimated 15-minute TWA occupational exposure levels of propane and the potential impurities in the substitute.  The maximum estimated 15-minute TWA occupational exposure levels are the most conservative of the modeled results and indicate the highest predicted exposure concentrations of the short-term (15-minute and 30-minute) and long-term (8-hour) exposure scenarios.  Even the maximum estimated short-term occupational levels for propane and the potential impurities are significantly lower than their 8-hour or 10-hour long-term WGLs. Therefore, occupational exposure to the substitute is not considered a toxicity threat. 

                    Table 8.  Occupational Risk Assessment
                                       
           Maximum 15-minute TWA Occupational Exposure Levels (ppm)
                       Workplace Guidance Levels (ppm) a
                     Workplace Guidance Levels Time Period
                                  Substitute
                                    Propane
                                      502
                                     1000
                                10-hour TWA[b]
                             Potential Impurities
                                    Ethane
                                      0.9
                                      NA
                                      NA
                                   Propylene
                                      0.2
                                      NA
                                      NA
                                   n-Butane
                                      0.5
                                      800
                                  10-hour TWA
                                    Sulfur
                                 5.4 x 10[-4]
                                      NA
                                      NA
 NA = Not Available
 a See Table 5 for more information. 
 [b] The WGL for Propane is set as a 1000 ppm 8-hr TWA by OSHA and a 10-hr TWA by NIOSH.  
7.3	END-USE EXPOSURE
This section presents estimates of potential store employee and customer exposures to propane in ice cream cabinets.  An exposure analysis was performed to examine potential catastrophic release of the substitute in a small storage room (as in Sections 5 and 6) under a reasonable "worst-case" scenario.  The analysis was undertaken to determine the 15- and 30-minute TWA exposures for the substitute, which were then compared to the standard toxicity limits presented in Table 6 to assess the risk to consumers.  However, the TWA values are fairly conservative as the analysis does not consider opened windows, fans operating, conditioned airflow (either heated or cooled) and other variables that would reduce the levels to which individuals would be exposed.

The model involves a refrigerant leak from an ice cream cabinet into an enclosed storage room of volume 42 m[3] (1,483 ft[3]) (see Section 4). The model assumes that the individual is present at the start of the leak and the individual remains in the room while the refrigerant is released. It is also assumed that horizontal stratification causes most of the refrigerant to settle in higher concentrations closer to the ground.  In order to simulate the horizontal concentration gradient that will occur because of the weight differential between the refrigerant and air, it is assumed that 95 percent of the leaked refrigerant mixes evenly into the bottom 0.4 meter of the room, and the rest of the refrigerant mixes evenly in the remaining volume (Kataoka 1999).  Exposure concentrations were calculated using the box model described in the Background Document, which was adapted to estimate concentrations on a minute-by-minute basis. In the analysis, the full charge of the unit is assumed to be emitted over the course of one minute and the model conservatively assumes 2.5 air exchanges occur per hour.  

The highest expected levels of exposure at the end-use based on this analysis occur in the lower stratum of the room.  The results of the assessment are presented in Table 9.
                     Table 9.  End-Use Exposure Assessment
                                       
                         15-minute TWA Exposure (ppm)
                         30-minute TWA Exposure (ppm)
                                  Substitute
                                    Propane
                                    10,414
                                     7,963
                             Potential Impurities
                                    Ethane
                                      19
                                      15
                                   Propylene
                                      3.4
                                      2.6
                                   n-Butane
                                      11
                                      8.6
                                    Sulfur
                                     0.01
                                     0.009
               TWA = Time Weighted Average
               
The 15-minute TWA Exposure for propane estimated for this risk screen is 10,414 ppm, which exceeds the AEGL-1 value by 414 ppm. Although the calculated end-use exposure is marginally greater than the recommended exposure limit, toxicity is not a concern for multiple reasons. First, the AEGL-1 exposure limit marks the concentration above which "it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure."  Specifically, if the 15-minute TWA were to reach or exceed the AEGL-1, certain irritation or notable discomfort would be experienced, but symptoms would be alleviated upon leaving the affected area and no permanent damage would be sustained. Second, due to the conservative assumptions used in the models, it is unlikely that this worst-case scenario would occur. The two main assumptions used in the End-Use Exposure Assessment (see Section 4) included room size and ventilation rate. In Section 4, the most conservative, yet reasonable, values were chosen for each category. Adjusting either of these assumptions to a value that is slightly less conservative removes the modeled toxicity threat for propane exposure. 

In particular, the modeling used in the risk screen assumes a leak occurs in the storage room of a convenience store with an area of 14 m[2] (volume of 42 m[3]). This area is considerably smaller than the total area of a typical convenience store, which have sales areas of 100-300 m[2] (Armines 2008). If the minimum storage room area is increased to 15 m[2] (volume of 45 m[3]), the exposure becomes 9,719 ppm. The exposure model also conservatively assumed a ventilation rate of 2.5 ACH. 

The average ventilation rate for a typical convenience store in the U.S. is 1.9 ACH. Although this rate is lower than the assumed rate for a storage room within a convenience store, the use of exhaust fans and door ventilators in the storage rooms of convenience stores significantly increases the air exchange rate. In addition, Occupational Safety & Health Administration (OSHA) requires that any storage room containing flammable material must have a ventilation system "designed to provide for a complete change of air within the room at least six times per hour."  Though this storage room will only contain a small amount of propane, higher ventilation rates would further protect against flammability and toxicity concerns. If ventilation was increased in the storage room to 4.5 ACH, the 15-min TWA would be 8,458 ppm. 

Further, as described above, the box-model approach used in this modeling assumes that 95% of the refrigerant leaks evenly into the bottom 0.4 meter of the room, while the remaining 5% of refrigerant mixes evenly in the top breathing zone portion of the room. Once the refrigerant disperses throughout the entire room, exposure will be minimized. Because the ACH and room volume assumptions used in the models are both very conservative, it is unlikely that these conditions would simultaneously exist and therefore the risk of exceeding the toxicity threshold is extremely small. It is strongly recommended, however, that steps to protect against toxicity threat in the space should be taken, as detailed in Section 5, such as ensuring there is adequate ventilation in rooms containing propane ice cream freezers. 

Finally, exposure to the minute quantities of the impurities in the formulation (see Table 9) is also not expected to pose a toxicity threat, given the low toxicity of these substances.  Should catastrophic release from a cabinet located in the main area of a store occur, exposure levels are not expected to pose a toxicity threat as the room volumes in the store would be larger than those used in this model.  Thus, exposure concentrations would be even lower in the main area of the store.
7.4	GENERAL POPULATION EXPOSURE
In the Background Document , the RfC value for propane is 0.9 mg/m[3].  This value was compared to estimated factory releases and on-site releases, resulting in a ratio of exposure concentration to RfC that varies between 1.9 x 10[-][4] to 3.5 x 10[-1], depending on the type of release scenario.  Ratios of exposure concentration to RfC for the proposed substitute's impurities (for which RfCs are available) were even lower.  Since the exposure concentrations for the substances are lower than the RfC values, the proposed substitute is not expected to pose a toxicity threat to the general population.
VOLATILE ORGANIC COMPOUND (VOC) ASSESSMENT
Propane has not been exempted as a VOC under the CAA (40 CFR 51.000).  VOC emissions from the production of retail ice cream cabinets using propane as a refrigerant are controlled through standard industry practices, and as such, emissions from the manufacture of units are likely to be minimal. For units in operation for retail applications, using leak estimates from the Vintaging Model, the annual release rate (including operating, servicing, and disposal leak rates) was calculated to be approximately 10% of the refrigerant charge size. Assuming this release rate, two assessments were performed to compare the annual VOC emissions from the use of propane in self-contained reach-in freezers to other anthropogenic sources of VOC emissions.  The first assessment approximates the annual VOC emissions that would be emitted from the use of propane in all Unilever/Ben and Jerry's U.S. retail ice cream cabinets.  The second assessment approximates the annual VOC emissions if all reach-in freezers in the U.S. were charged with propane.
 
                   Table 10.  VOC Annual Emissions Analysis
                            Set of Sources Assessed
Annual Propane VOC Emissions Compared to VOC Emissions from All Anthropogenic Sources[a] (%)
     All Unilever/Ben and Jerry's U.S. Propane Retail Ice Cream Cabinets
                                 7.5x10[-][7]%
                 All U.S. Self-contained Reach-in Freezers[b]
                                 1.3x10[-][4]%
      [a] Based on 2010 projections calculated using 2008 EPA annual VOC emissions data (EPA 2009) and ICF assumptions.
      [b] Calculations based on 2011 Buildings Energy Data Book (DOE 2011), Energy Savings Potential and R&D Opportunities for Commercial Refrigeration (DOE 2009), and ICF assumptions.

As illustrated in Table 10, emissions from the use of propane under both scenarios are minimal compared to other sources of VOC emissions.  Assuming all Unilever/Ben and Jerry's ice cream cabinets in the U.S. were charged with propane, the resulting VOC emissions in one year would be equal to  approximately 8.5x10[-2] percent of the VOC emissions caused by the generation of electricity used to power small retail food refrigeration equipment, 1.2x10[-3] percent of all the VOC emissions caused by the generation of electricity for all refrigeration and air conditioning applications, or 7.5x10[-][7] percent of all annual anthropogenic VOC emissions in the U.S.  

Moreover, if all reach-in freezers in the U.S. were charged with propane (the second, more conservative scenario), the resulting VOC emissions would be equal to approximately 1.3x10[-][4] percent of all annual anthropogenic VOC emissions in the U.S.  Because these emissions of propane are several orders of magnitude lower than the VOC emissions generated by other anthropogenic emissions, and the VOC emissions from all Unilever/Ben and Jerry's ice cream cabinets is only a small fraction of VOC emissions from the use of electricity in the retail refrigeration sector, the environmental impacts of the incremental VOC emissions from propane releases for small refrigeration cabinets are not considered a threat to local air quality.

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