Significant New Alternatives Policy Program
Refrigeration and Air Conditioning Sector
Risk Screen on Substitute for CFC-12, CFC-13, R-13B1, and R-503 in Very Low Temperature Refrigeration and Non-Mechanical Heat Transfer
                          Substitute: Ethane (R-170)
This risk screen is restricted to very low temperature (VLT) refrigeration and non-mechanical heat transfer (thermosiphon) applications covered under UL 471: Commercial Refrigerators and Freezers.
                                       
This risk screen does not contain Clean Air Act (CAA) Confidential Business Information (CBI) and, therefore, may be disclosed to the public.
1.	INTRODUCTION
Ozone-depleting substances (ODS) are being phased out of production in response to a series of diplomatic and legislative efforts that have taken place over the past two decades, including the Montreal Protocol and the Clean Air Act Amendments of 1990 (CAAA).  The U.S. Environmental Protection Agency (EPA), as authorized by Section 612 of the CAAA, administers the Significant New Alternatives Policy (SNAP) Program, which identifies acceptable and unacceptable substitutes for ODS in specific end-uses based on assessment of their health and environmental impacts.  

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

The purpose of this risk screen is to supplement EPA's Background Document on the refrigeration and air conditioning sector (EPA 1994) (hereinafter referred to as the Background Document). This risk screen evaluates the potential use of ethane (R-170) as a substitute for CFC-12, CFC-13, R-13B1 and R-503 in very low temperature (VLT) refrigeration, specifically in VLT freezers, and in non-mechanical heat transfer (thermosiphon). VLT refrigeration systems require maintaining temperatures at approximately -80°C (-176°F) or lower. This type of equipment requires extremely reliable refrigeration cycles to maintain low temperatures and must meet stringent technical standards that do not typically apply to refrigeration systems (EPA 2013). Table 1 presents the composition of the proposed substitute. 

                         Table 1. Composition of R-170
                                  Constituent
                               Chemical Formula
                                  CAS Number
                                 Concentration
                              (Weight Percent)[a]
                                    Ethane
                                 C2H6; CH3CH3
                                    74-84-0
                                    >99%
	[a] Negligible amounts of impurities may be present.


Section 2 of this report 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
Ethane is recommended for SNAP approval for use in very low temperature freezers and non-mechanical heat transfer (thermosiphons), included under Underwriters Laboratory (UL) Standard 471: Commercial Refrigerators and Freezers. EPA's risk screen indicates that the use of the proposed substitute will be less harmful to the atmosphere than the continued use of CFC-12, CFC-13, R-13B1, and R-503. No significant asphyxiation risks to personnel or toxicity risks to workers, personnel, or the general population are expected.  Ethane's contributions to volatile organic compound (VOC) emissions is not significant, as ethane is exempt under CAA regulations (40 CFR §51.100(s)). It is expected that procedures in the material safety data sheet (MSDS) for ethane and good manufacturing practices will be adhered to. Additionally, it is expected that the appropriate safety and personal protective equipment (PPE) (e.g. protective gloves, tightly sealed goggles, protective work clothing, and suitable respiratory protection in case of leakage or insufficient ventilation) consistent with Occupational Safety and Health Administration (OSHA) guidelines will be used during manufacture, installation and servicing, and disposal of very low temperature freezers using ethane.  Because units are to be installed in locations with adequate space or ventilation in accordance with the VLT freezer manual for ethane, as discussed in greater detail in Section 9, significant toxicity or flammability risk to workers is also unlikely. 
Additional safeguards, including specified refrigerant concentration limits (RCL) for ethane, are also provided by adherence to industry standards including American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 15, National Fire Protection Association (NFPA) 497, NFPA 68, Underwriters Laboratory (UL) Standard 471, and all federal, state and local laws regarding the storage and use of flammable gases. In addition, the submitter's recent UL certification for this equipment provides additional safeguards.


 3. 	ATMOSPHERIC ASSESSMENT
This section presents an assessment of the potential risks to atmospheric integrity posed by the use of ethane in VLT freezers. The ozone depletion potential (ODP), global warming potential (GWP), and atmospheric lifetime (ALT) of the proposed substitute are presented in Table 2.  
The proposed substitute is substantially less harmful to the ozone layer, has a lower climate impact, and a shorter atmospheric lifetime when compared to CFC-12, CFC-13, R-13B1, and R-503. In addition, ethane also has a lower climate impact and shorter atmospheric lifetime than other utilized VLT refrigerant substitutes, such as R-508B and HFC-23. 
   Table 2. Atmospheric Impacts of Ethane Compared to Other VLT Refrigerants
                                 Refrigerant 
                       Ozone Depleting Potential (ODP)[a]
                           Global Warming Potential
                                   (GWP)[b]
                      Atmospheric Lifetime in Years (ALT)
                                    Ethane
                                       0
                                     5.5 c
                                    0.17 d 
                              R-13B1 (Halon 1301)
                                     15.9
                                      7,140
                                      65[b]
                                    CFC-12
                                     0.82
                                    10,900
                                    100[b]
                                    CFC-13
                                       1
                                    14,420
                                    640[b]
                                     R-503
                                     0.599
                                    14,560
                                    - NA[e]
                                    R-508B
                                       0
                                    13,396
                                     NA[f]
                                    HFC-23
                                       0
                                    14,760
                                      270
[a] WMO 2010 Scientific Assessment Report (2011)
[b] IPCC 4th Assessment Report (Forster et al. 2007), unless otherwise noted.
[c] Global Cooling Inc. (2012)
[d] Gonzalez et al. (2011)
[e] R-503 is a blend consisting of CFC-13 (60%) and HFC-23 (40%). The ALT for CFC-13 is 640 years and the ALT for HFC-23 is 270 years. 
[f] R-508B is a blend consisting of HFC-23 (46%) and perfluoroethane (PFC-116) (54%). The ALT for HFC-23 is 222 years and the ALT for PFC-116 is 10,000 years. 
4. 	VOLATILE ORGANIC COMPOUND (VOC) ANALYSIS
Ethane is exempt from the definition of volatile organic compound (VOC) under CAA regulations (40 CFR 51.100(s)).  Therefore, environmental impacts from the release of ethane as a VOC are not a significant concern.

5.	DISCUSSION OF END-USE SCENARIOS MODELED
Ethane is proposed as a secondary heat transfer fluid in the thermosiphon of a very low temperature (VLT) freezer. The VLT freezer uses a Stirling refrigeration system coupled to a thermosiphon to cool the interior space (see Figure 1). The thermosiphon is a hermetic copper tube and condenser attached to the Stirling machine's cold head and does not penetrate the Stirling machine. The working medium in the Stirling engine is helium gas which does not undergo phase change. The thermosiphon, containing ethane, carries out the thermal transport from the interior of the freezer to the cold acceptor of the Stirling (Berchowitz and Kwon 2012).
  Figure 1. Main components of VLT freezer with Stirling Refrigeration System
                                       
                         Source: Global Cooling (2012)
The submission states that the VLT freezer thermosiphon will contain a maximum charge size of 100 grams (Global Cooling 2012). UL Standard 471, however, states that commercial refrigeration units containing flammable refrigerants can have a charge size of up to 150 grams. Both charge sizes, 100 grams and 150 grams, are modeled in this risk screen. 
To represent a reasonable worst-case scenario, it is assumed that a catastrophic leak of refrigerant occurs while the VLT freezer is installed at the end-use. In these locations, the freezers are typically clustered side by side, spaced 25.4 cm (10 in) apart and in laboratory rooms with 3.0 m (10 ft) ceilings (Carnegie Mellon University 2007). A typical laboratory module has a minimum width of approximately 10 ft, but will vary in length based on the number of modules configured in a row, typically ranging up to an ideal length of approximately 33 ft long (National Institutes of Health 2008). Therefore, the analysis in this risk screen conservatively assumes that the VLT freezer is installed in one laboratory module that is 3.0 m (10 ft) wide, 3.0 m (10 ft) long, and 3.0 m (10 ft) high. This space is assumed to have an effective volume of 28 m[3] (1,000 ft[3]), with an effective floor area of 9.3 m[2] (100 ft[2]) (i.e., excluding the space filled by the freezers and other items in the laboratory). 
Under the worst case scenario, the full charge of the unit is assumed to be emitted over the course of one minute into the laboratory with 4 air changes per hour (ACH) (E.O. Lawrence Berkeley National Laboratory 2013). A vertical concentration gradient is not assumed because ethane has approximately the same density as air (specific gravity of ethane relative to air is 1.0378 [air = 1]), and, therefore, when the full charge is released, the proposed substitute is expected to disperse evenly and immediately. It was assumed that an individual is present at the start of the leak and remains in the room until the charge is completely released. Table 3 details the assumptions in the models throughout this risk screen (i.e., in Sections 7, 8, and 9).
                  Table 3. End-Use Scenario Model Assumptions
Parameter
                                  Assumption
Refrigeration Unit
                                  VLT Freezer
Room 
                              Laboratory Module 
Charge Size (g)
                                100a and 150[b]
Length of Release (minutes)
                                       1
Effective Room Size (volume - m[3])
                              28 (1,000 ft[3])[a]
Room Ventilation (ACH)
                                     4[c]
 Vertical Concentration Gradient
                                      No
             [a] Global Cooling Inc. (2012)
             [b] UL 471
             c E.O. Lawrence Berkeley National Laboratory (2013)

EPA recognizes that VLT freezers may be placed in a variety of room types with different sizes and ventilation rates. In addition, more than one VLT freezer may be installed in the same space, as the units are fairly small with a volume of 0.78 m3 (27.5 ft[3]) and dimensions of 2.0 m x 0.87 m x 0.91 m (79 in x 34 in x 36 in), which is similar in capacity to a household refrigerator (Stirling Ultracold 2013). When units are installed in smaller, enclosed spaces, there is a higher risk for flammability, asphyxiation, or toxicity concerns. To address these concerns, this risk screen incorporates threshold analyses in addition to the worst-case scenario modeling. The results from the threshold analyses are used to establish guidelines for choosing an effective room size, charge size, and ventilation rate such that use of ethane in VLT freezers does not present risk to end-users or servicing technicians. 
6. 	POTENTIAL HEALTH EFFECTS
To assess potential health risks from exposure to the proposed substitute in VLT freezers, EPA identified the relevant toxicity threshold values for comparison to modeled exposure concentrations for different scenarios. To protect consumers from the potential dangers of a catastrophic refrigerant leak from a VLT freezer, ASHRAE Standard 34 determined refrigerant concentration limits (RCLs) to reduce the risks of acute toxicity, asphyxiation, and flammability hazards in occupied spaces (ASHRAE 2010b). ASHRAE Standard 15 implements the aforementioned standard, requiring that "the concentration of refrigerant in an enclosed space following a complete discharge of a high-probability system shall not exceed the RCL" (ASHRAE 2010a). As such, this risk screen references the RCL in addition to the lower flammability limit, hypoxia No Observed Adverse Effect Level (NOAEL), and exposure limits, as an additional, conservative limit to ensure that significant flammability, asphyxiation, and end-use exposure risks, respectively, do not occur.  
For the occupational exposure analysis, potential risks from chronic worker exposure to ethane concentrations were evaluated by comparison to the workplace guidance level (WGL). In the absence of an established short-term exposure level (STEL), acute exposure guideline (AEGL), or an emergency response planning guideline (ERPG), potential short-term, end-use exposures to ethane were compared to the excursion limit.  Table 4 lists the relevant toxicity limits, followed by Table 5 which describes each of these exposure limits in more detail. EPA's approach for identifying or developing these values is discussed in Chapter 3 of the Background Document. 
                      Table 4. Exposure Limits of Ethane
                Workplace Guidance Limit  (Long-term Exposure)
                                      ppm
                               Excursion Limit  
                             (Short-term Exposure)
                                      ppm
                   Refrigerant Concentration Limit (RCL) ppm
                             1,000[a]  (ACGIH TLV)
                                   3,000[b]
                                   7,000[c]
      [a] ACGIH (2012)
      b Calculated as three times the TLV-TWA. (ACGIH 2004)
      [c]  - 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
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. 
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.
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, chronic exposure to ethane may cause damage to the heart or central nervous system if inhalation, skin contact, or eye contact with ethane occurs. The most likely pathway of exposure is through inhalation. If ethane is inhaled, person(s) should be immediately moved 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. In the case of dermal exposure, the MSDS for ethane recommends that person(s) immediately flush the skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. 
Furthermore, to avoid the risk of static discharges and gas ignition, soak contaminated clothing thoroughly with water before removing it. Any contact with rapidly expanding ethane gas may cause burns or frostbite. If this is the exposure case, the MSDS recommends attempting to warm up the frozen tissues and seeking medical attention. Bathe (not rub) the affected area with lukewarm, not hot, water and consult a physician. 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. 
In case of ocular exposure, the MSDS for ethane recommends that person(s) immediately flush the eyes, including under the eyelids, with copious amounts of water for at least 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 ethane can be minimized by following the exposure guidelines and ventilation and PPE recommendations outlined in the MSDS for ethane and this risk screen.   
7.	FLAMMABILITY ASSESSMENT
ASHRAE Standard 34 classifies ethane as a Class A3 refrigerant. Ethane is flammable when its concentration in the air is in the range of between 3 percent and 12.5 percent by volume (30,000 ppm to 125,000 ppm). In the presence of an ignition source (e.g., static electricity, a spark resulting from a switch malfunction, or a cigarette), an explosion or a fire could occur when the concentration of ethane exceeds its lower flammability limit (LFL) of 30,000 ppm, posing a significant safety concern for workers and personnel if it is not handled carefully. Only refrigerant technicians certified to work with flammable refrigerants should handle VLT units containing ethane during manufacturing, installation, servicing, transportation and disposal. The remainder of this section assesses the flammability risks and summarizes the recommended measures to ensure safe handling and use of the refrigerant during servicing, disposal, and end-use.

7.1	Flammability Risk during Manufacture
In production facilities or other facilities where large quantities of the refrigerant will be stored, proper safety precautions should be in place to minimize the risk of explosion. As indicated by the submitter, the proposed substitute is purchased and not manufactured on site. However, Global Cooling manufactures the very low temperature freezers and, during the final stage of freezer manufacture, the thermosiphon is filled with the proposed substitute. According to the submitter, the magnitude of release is approximately 0.2 grams per filling and 10 units are manufactured and filled each day. A flow restrictor and two manual valves must be opened for the filling process to take place. A Schrader-type valve is attached to the fill tube (removed after pinch-off) so that the delivery line is sealed and opened only on connection to the fill tube (Global Cooling Inc. 2012). Any release of refrigerant is expected to take place during this filling activity which is assumed to occur in a factory area with proper ventilation rates. 
All ethane storage and transport equipment should be installed with safety devices that minimize the likelihood of catastrophic releases. VLT freezers installed with ethane should be clearly labeled as containing a flammable refrigerant charge. EPA believes that because relevant safety standards, the MSDS for ethane, and OSHA requirements under 29 CFR 1910 (e.g., proper ventilation and storage practices within manufacturing facilities to prevent fire and explosion) are followed by workers, flammability during manufacture is not a concern. 
7.2	Flammability Risk during 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 ethane. Both servicing and end-use of very low temperature freezers charged with ethane are expected to take place in the same location. The submitter indicates that during servicing, the cooling machine is replaced and the proposed substitute is recovered using recovery equipment designed specifically for flammable hydrocarbon refrigerants (i.e., equipped with flammable gas leak detectors and monitors and without potential sources of ignition) with sufficient onboard storage capacity and suitable for the refrigerant in terms of the pressure rating and compatibility of valve seals.  For example, the Caresaver Universal Refrigerant Recovery Unit specified by the submitter has an onboard storage capacity of 250 g, which is sufficient for two VLT freezer units charged with ethane. In this process, a flow restrictor and two valves must be opened for filling. When the charging valve is disconnected, only about 0.02 grams of the proposed substitute is expected to be released. In order to determine the potential flammability risks during servicing or end-use in case of a catastrophic release of refrigerant in the laboratory hallway, concentrations of ethane immediately following a complete release of refrigerant, were compared to the lower flammability limit (LFL) for ethane (30,000 ppm). Table 6 presents the results of the analysis.
                      Table 6. Flammability Assessment[a]
                                   Scenario
                                Charge Size (g)
                          Effective Room Size (m[3])
                      Maximum Instantaneous Concentration
                                   (ppm)[b,c]
                            Reasonable Worst-Case 1
                                      100
                               28 (1,000 ft[3])
                                    2,900  
                            Reasonable Worst-Case 2
                                      150
                               28 (1,000 ft[3])
                                    4,300 
                       Threshold Analysis 1: Charge Size
                                    1,100 
                                28 (1,000 ft[3])
                                    30,000
                       Threshold Analysis 2a: Room Size
                                      100
                                2.7 (95 ft[3])
                                    30,000
                       Threshold Analysis 2b: Room Size
                                      150
                                4.0 (141 ft[3])
                                    30,000
      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 ethane is equal to 30,000 ppm
  [c] Values provided in these columns refer to the concentration in entire room because  a vertical concentration gradient is not assumed. In this scenario, 100% of the charge size leaks. The air flow rate is 4 ACH. 
As demonstrated in Table 6, the maximum instantaneous concentration of ethane in the entire room at a charge size 100 grams would be approximately 2,900 ppm, which is less than 10 percent of the LFL for ethane. At a charge size of 150 grams, the maximum instantaneous concentration of ethane in the room would be approximately 4,300 ppm, which is approximately 14 percent of the LFL for ethane. As such, under the reasonable worst-case scenarios, ethane is not expected to present a significant risk of flammability.
EPA, however, also recognizes that multiple VLT freezers may be used in an individual room, and as such, also performed threshold analyses based on the total charge size (i.e., for all freezers in a given space). The results of the threshold analyses indicate that for flammability to be of concern within an effective room size of 28 m[3], the total charge size would have to be at least 1,100 grams, which is approximately 7 times the maximum charge size of a single VLT freezer. The submitter states that anywhere from five or six to hundreds of VLT freezers are likely to be installed side-by-side in laboratories and other proposed end-use locations. Based on this analysis, to reduce flammability risk, a room with effective volume of 28 m[3] should not exceed 11 freezers with an individual charge size of 100 grams or 7 freezers with an individual charge size of 150 grams; and an individual freezer of 100 grams or 150 grams should not be placed in a room smaller than 2.7 m[3] (95 ft[3]) or 4.0 m[3] (141 ft[3]), respectively.  ASHRAE Standard 34 also more conservatively recommends that the RCL for ethane (8.7 g/m[3] of ethane or 7,000 ppm by volume) should not be exceeded. Based on a room with an effective volume of 28 m[3], this corresponds to a limit of 2 freezers with an individual charge size of 100 grams or 1 freezer with an individual charge size of 150 grams. Thus, EPA believes that when VLT freezer units are installed in spaces that are consistent with manufacturer recommendations and the guidelines of relevant safety standards (e.g., ASHRAE 15 and 34), flammability risk is not a significant concern.
Furthermore, it is extremely unlikely that a laboratory, laboratory hallway, or storage space as small as those determined in the threshold analyses would exist. The submitter states that the freezer units have dimensions of 2.0 m x 0.87 m x 0.91 m (79 in x 34 in x 36 in), meaning that the bottom area of the freezer of 0.80 m[2].The unit, therefore, would cover 91% and 66% of the floor area determined in the threshold analyses for 100 g and 150 g charge sizes, respectively. It is further unlikely that the entire refrigerant charge would accidentally release into the space. The refrigerant is located in the thermosiphon which is a 3/8 inch copper tube that wraps around the 16-gauge stainless steel inner liner of the freezer. A catastrophic release of refrigerant could only occur if the inner liner were punctured sufficiently and at the right place to penetrate the thermosiphon tube. The thermosiphon is placed sufficiently behind the thermal breaker and is encased in polyurethane foam, further reducing the likelihood of such a puncture. End-users, however, should still ensure that there is adequate space (i.e., the effective volume of the room should be larger than 2.7 or 4.0 m[3]), depending on the charge size of the VLT freezer (see Table 6) and ventilation in any room where the VLT freezers are installed to mitigate the risk of fire or explosion resulting from a catastrophic leak during end-use or servicing activities. 
Catastrophic releases of large quantities of refrigerant, especially in areas where refrigerant is stored, could lead to an explosion in the presence of an ignition source.  For this reason, it is important that only properly trained and certified technicians handle ethane. The submitter has provided safety guidelines for handling ethane, which should be followed. As a further precaution, certification requirements and training programs for technicians that handle ethane should be developed using these guidelines.  During servicing operations, technicians should ensure that proper ventilation is in place through the use of fans (or other mechanical ventilation devices) and portable refrigerant detectors should be used to alert technicians to the presence of flammable gases in the area. In addition, VLT freezers installed with ethane should be clearly labeled as containing a flammable refrigerant. Because relevant safety standards and the MSDS for ethane are followed by service technicians and because ethane refrigeration units will be installed in areas with adequate space and ventilation, EPA believes that flammability during servicing and end-use is not expected to be of concern. 
8.	ASPHYXIATION ASSESSMENT
The risk of asphyxiation for a reasonable "worst-case" scenario was investigated for ethane.  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 reduce the levels to which individuals would be exposed, such as open doors or windows, fans operating, conditioned airflow (either heated or cooled), or even openings at the bottom of doors that allow air to flow in and out. As specified in Section 4, this analysis does not assume a vertical concentration gradient when the proposed substitute is released. 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 reasonable worst-case scenarios that are modeled in this 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 Room[b]
                             Reasonable Worst-Case
                                      100
                               85 (3,000  ft[3])
                                      21
                             Reasonable Worst-Case
                                      150
                               85 (3,000 ft[3])
                                      21
                       Threshold Analysis 1: Charge Size
                                    44,800
                               85 (3,000  ft[3])
                                      12
                       Threshold Analysis 2a: Room Size
                                      100
                               0.19 (6.7 ft[3])
                                      12
                       Threshold Analysis 2b: Room Size
                                      150
                               0.28 (9.9 ft[3])
                                      12
      Bold font indicates modeling results.
         a Cells highlighted in green are the scenarios with acceptable exposure levels  given various modeling assumption options.
         [b] The typical concentration of oxygen in air is considered to be 21 percent (Mackenzie & Mackenzie 1995).
In order for a risk of asphyxiation to occur, the normal concentration of oxygen in air (21 percent) must be reduced to 12 percent. Based on the worst-case scenario modeling assumptions, ethane in a VLT freezer does not present a significant risk of asphyxiation. The concentration of ethane in the air following the release of either charge size does not exceed 0.44 percent, which has an insignificant impact on the normal 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 personnel is likely to be even smaller than modeled. 
The space where a VLT freezer with ethane is installed must allow for adequate room for end-users to access the machine. To further prevent risk of asphyxiation, end-users should ensure that there is adequate space accounting for other appliances, furniture, or storage (i.e., the effective volume of the room should be larger than 0.19 or 0.28 m[3], depending on the charge size of the refrigeration unit [see Table 7] and ventilation (in accordance with the MSDS for ethane) in any room where the VLT freezers are installed. Additionally, adherence to ASHRAE 34, which provides more conservative concentration limits, will ensure that asphyxiation is not a risk. Minimum room sizes according to the RCL stated in ASHRAE 34 can be found in Section 6. Because VLT freezers are unlikely to be installed in spaces where full release of ethane would displace enough oxygen to cause asphyxiation, EPA does not believe that the use of ethane in VLT freezers with either charge size (i.e., 100 grams or 150 grams) poses a significant risk of asphyxiation or impaired coordination to laboratory personnel. 
9.	END-USE EXPOSURE ASSESSMENT
This section presents estimates of potential personnel exposures to ethane during use of VLT freezers.  Personnel exposure analysis was performed to determine acute/short-term laboratory personnel exposures resulting from potential catastrophic leakage of refrigerant from VLT freezers. The analysis was undertaken to determine the 30-minute TWA for ethane, which was compared to the 30-min Excursion Limit for ethane. The results of the end-use exposure assessment are presented in Table 8to assess the risk to personnel. The estimated exposures were derived using conservative assumptions, and represent a worst-case scenario with a low probability of occurrence, 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. 
                    Table 8. End-Use Exposure Assessment[a]
                                   Scenario
                                Charge Size (g)
                          Effective Room Size (m[3])
                     30-min TWA End-Use Exposure (ppm)[b]
                           Reasonable Worst-Case 1 
                                      100
                               28 (1,000  ft[3])
                                     1,600
                            Reasonable Worst-Case 2
                                      150
                               28 (1,000  ft[3])
                                     2,400
                       Threshold Analysis 1: Charge Size
                                      187
                               28 (1,000  ft[3])
                                     2,993
                       Threshold Analysis 2a: Room Size
                                      100
                                15 (530 ft[3])
                                     2,988
                       Threshold Analysis 2b: Room Size
                                      150
                                23 (810 ft[3])
                                     2,923
      Bold font indicates modeling results. 
      [a] Cells highlighted in green are the scenarios with acceptable exposure levels given various modeling assumption options.
      b See Table 4 for more information.
      
Under the reasonable worst-case scenario described in Section 5, catastrophic releases of ethane from a VLT freezer with a charge size of 100 grams and 150 grams were modeled. The modeling indicates that the maximum anticipated personnel exposure concentration for ethane with a 150-gram charge size is 2,400 ppm, within the range of the 30-min Excursion Limit of 3,000 ppm. However, the maximum 100-gram charge size indicated by the submitter is below the 30-min Excursion Limit.  Because the modeling was done using a conservative laboratory module size and the submitter reported the maximum charge size for the VLT freezer to be 100 grams, personnel exposure to the proposed substitute at end-use is not considered a significant risk. To mitigate risk during end-use, EPA recommends that for a minimum room volume of 28 m[3], the charge size not exceed 112 grams, and for a maximum charge size of 150 grams, the VLT freezer not be installed in a room of volume less than 38 m[3]. To minimize the risk of exposure, EPA recommends that refrigeration units be installed in rooms with adequate space and/or the ventilation and engineering control (e.g., local ventilation such as a fan or vents on doors to adjoining rooms) outlined in the MSDS.  To prevent exposure and potential serious side effects during larger releases, EPA recommends that the charge size for an ethane VLT freezer does not exceed the RCL (7,000 ppm) and proposes to require compliance with UL 471. 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 ethane. As indicated by the submitter and discussed in Section 5.1, the proposed substitute is purchased and not manufactured on site. The charging of ethane VLT freezer units occurs at the final stage of freezer manufacture. 
 To ensure that use of the proposed substitute in VLT freezers does not pose an unacceptable risk to workers during servicing and disposal, occupational exposure modeling was performed using a box-model approach. For a detailed description of the methodology used for this screening assessment, the reader is referred the occupational exposure and hazard analysis described in Chapter 5 of the Background Document. Estimates of refrigerant release per event for various release scenarios and data on number of events in 2010 were obtained from the Vintaging Model. For the VLT freezer end-use, the release per event was conservatively assumed to be 0.25 percent of the equipment charge during servicing and 35 percent of the equipment charge during disposal. Additionally, EPA is proposing to allow intentional venting or release of ethane as a refrigerant during installation, servicing, maintenance and disposal from very low temperature refrigeration equipment and non-mechanical heat transfer equipment. If venting does occur, EPA recommends that it is done in a well-ventilated area (e.g., outdoors). Therefore, scenarios in which 100% of the equipment charge was released during servicing and disposal were also considered. The release rate per event was multiplied by the number of events estimated to occur over a workday. These assumptions result in approximately eight potential exposure events per day. For disposal, it was conservatively assumed that 10 units are disposed during an 8-hour work day.  The modeled exposure concentrations were compared to the estimated short-term occupational exposure limits at servicing and long-term exposure limits at disposal. Ethane does not have an established STEL or AEGL value so the short-term modeling results for ethane were evaluated using the calculated 30-min Excursion Limit (3,000 ppm). 
10.1 	Occupational Exposure during Manufacture
Ethane is produced around the world for use in a wide variety of end-uses. As indicated in the SNAP submission, however, the proposed substitute is not manufactured by the submitter. Where manufacture occurs in the United States, the MSDS for ethane should be referenced and proper engineering controls and PPE used. Ethane is not expected to pose a risk to workers during manufacture when the engineering controls and PPE recommendations referenced in the MSDS for ethane are followed. Engineering controls should include local ventilation (e.g., chemical hoods) for standard manufacturing procedures so workers can avoid physical contact with the refrigerant and to achieve emission control. 
Exposure to the refrigerant is expected to take place during filling of the thermosiphon, which occurs at the final stage of manufacture of the freezer. Less than five minutes is required to fill and pinch-off the filling tube. Therefore, the total fill process time in one day is approximately 50 minutes, assuming 10 units are manufactured in one day. According to the submitter, the magnitude of release is approximately 0.02 g per filling. (Global Cooling 2012) The filling activity is assumed to occur in a factory area with proper ventilation rates and carried out using the process described in Section 5.1. The Threshold Limit Value-Time Weighed Average (8-hour TWA) for ethane is 1,000 ppm for an 8-hour work day (ACGIH 2012).  Assuming an 8-hour work day in factory room with volume 5,667 m[3], workers would be exposed to approximately 0.2 grams of isobutane per day, or a concentration of 3.5 x 10-5 ppm, which is approximately 3.5 x 10-6 percent of the 8-hour recommended exposure limit for ethane. This amount of exposure is insignificant in comparison to the 8-hour TWA. Additionally, because of established equipment design and installation practices, significant exposures to ethane will be unlikely. Thus, EPA does not believe that manufacturing, including charging, of VLT freezers with ethane presents a significant concern to workers. 
10.2	Occupational Exposure during Servicing
As indicated by the submitter, when ethane VLT freezers are serviced, the refrigerant will need to be removed using recovery equipment designed for hydrocarbon refrigerants.  The total time to replace a cooling machine would be one hour. It takes ten minutes to extract the proposed substitute and it takes less than five minutes to fill and pinch-off the filling tube. Exposure to the proposed substitute is only expected to occur when the charging valve is disconnected. (Global Cooling 2012) 
For servicing of the VLT freezers, it was conservatively assumed that the release per event was 0.25 percent of the equipment charge and eight events of servicing a unit occur in an 8-hour work day. The maximum 30-minute time weighted average (TWA) exposure for ethane was estimated for the servicing exposure scenarios (see Table 9). The modeling results indicate that the maximum anticipated occupational exposure concentrations for ethane are very low with either a 100-gram or 150-gram charge size for both 0.25% and 100% release of refrigerant during servicing. Because ethane does not have short-term exposure limits and the exposure concentrations modeled are significantly small, occupational exposure to the proposed substitute during servicing is not considered a significant risk to workers. Further, the estimated exposures were derived using conservative assumptions, and represent a worst-case scenario with a low probability of occurrence. These types of systems are typically serviced in well-ventilated areas by trained personnel using proper industrial hygiene techniques and PPE. 
EPA is proposing to allow intentional venting or release of ethane as a refrigerant during installation, servicing, maintenance and disposal from very low temperature and non-mechanical heat transfer equipment. However, both servicing and end-use of VLT freezer units are expected to take place in the same room. Servicing is likely to be done by a service technician using refrigerant recovery equipment with an onboard storage capacity that is sufficient for more than one VLT freezer unit charged with ethane, as described in Section 7.2. Given that the recovery equipment is efficient and capable of servicing multiple units in the end-use location and it is cost-effective to recover the ethane refrigerant, EPA does not expect venting to occur frequently. 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. If a technician is servicing a bank of these VLT freezer units charged with ethane and venting occurs frequently, EPA recommends the use of this safety equipment to prevent the concentration of ethane in the room from exceeding its RCL.  
           Table 9. Occupational Risk Assessment during Servicing[a]
                                   Component
                   30-minute TWA Occupational Exposure (ppm)
                            Charge Size: 100 grams
                   30-minute TWA Occupational Exposure (ppm)
                            Charge Size: 150 grams
                        30-min Excursion Limit[b] (ppm)
                                       
                                 0.25% Release
                                 100% Release
                                 0.25% Release
                                 100% Release
                                       
                                    Ethane
                                      0.35
                                      74
                                      0.53
                                      212
                                    3,000 
[a] Cells highlighted in green are the scenarios that are deemed to be acceptable given various modeling assumption options.
[b] Calculated as three times the TLV-TWA. ACGIH (2004)
10.3	Occupational Exposure during Disposal
According to the submitter, at disposal of the VLT freezer, the proposed substitute is intended to be recovered using a Caresaver Universal Refrigerant Recovery Unit and recycling of the proposed substitute will be a decision taken by the gas supply companies who receive the recovered proposed substitute (Global Cooling 2012). Disposal of VLT freezers is expected to occur with limited frequency at disposal facilities and with limited duration of exposure to the refrigerant charge. Potential exposures to the refrigerant during recovery and disposal are expected to occur during activities related to draining the refrigerant from the machine unit into cylinders (e.g., connecting of pipes). 
Two disposal scenarios were analyzed: the first scenario assumed 35 percent of the equipment charge was release per event, and the second scenario assumed 100 percent was released per event. The latter scenario accounts for the possibility that venting of the entire charge of ethane could occur during disposal if EPA finalizes a rule that would allow venting of ethane in these end uses. It was conservatively assumed that 10 events of disposing a unit occur in an 8-hour work day for the first scenario while the second scenario assumes that a catastrophic leak of the entire refrigerant charge only occurs for one unit, based on the assumption that the assembly line is shutdown following this event. The modeling indicates that 8-hour worker exposure concentrations for ethane will at no point exceed 40 percent of the long-term exposure limits for ethane during disposal of VLT freezers with either a 100-gram or 150-gram charge size. Table 9 displays the maximum 8-hr occupational exposure levels for each charge size for both 35% and 100% release of refrigerant during disposal.  Because modeled exposure levels in every scenario are significantly lower than the exposure guidelines for ethane, occupational exposure to the proposed substitute during disposal is not considered a toxicity threat.  Further, the estimated exposures were derived using conservative assumptions, and represent a worst-case scenario with a low probability of occurrence. These types of systems are typically disposed of by trained personnel using proper industrial hygiene techniques.  

           Table 10. Occupational Risk Assessment during Disposal[a]
                                   Component
                    8-Hour TWA Occupational Exposure (ppm)
                            Charge size: 100 grams
                    8-Hour TWA Occupational Exposure (ppm)
                            Charge size: 150 grams
                    8-Hour Long Term Exposure Limits (ppm)
                                       
                                  35% Release
                                 100% Release
                                  35% Release
                                 100% Release
                                       
                                    Ethane 
                                      18
                                      52
                                      27
                                      78
                                    1,000b
      [a] Cells highlighted in green are the scenarios that are deemed to be acceptable given various modeling assumption options.
         b ACGIH (2012)

Although anticipated occupational exposures are well below the exposure limits for ethane, the recommendations for proper engineering controls and PPE in the MSDS for ethane should be followed. Adequate ventilation should always be established during any use, handling, or storage of ethane. Engineering controls should include vapor-in air detection systems and local exhaust ventilation during use of ethane to prevent dispersion of the proposed substitute throughout the work place. In addition, an eye wash and safety shower should be near the manufacturing facility and locations where isobutane is stored and ready for use.  In general, use of PPE selected in accordance with OSHA 29 CFR 1910.132, 29 CFR 1910.133, 29 CFR 1910.134, and ANSI Z88.2 is recommended, such as splash goggles, mechanically-resistance gloves and metatarsal shoes (i.e., steel-toed boots) when handling cylinders, chemically-resistant gloves when handling the gas mixture (e.g., butyl rubber, chlorinated polyethylene, or neoprene), and protective clothing (Natural Refrigerants 2008). Because proper handling and disposal guidelines are followed in accordance with good industrial hygiene and manufacturing practices and the MSDS for ethane, there is no significant risk to workers during the manufacturing, installation, servicing, and disposal of ethane.

11. 	GENERAL POPULATION EXPOSURE
General population exposures to ambient air, surface water and solid waste to releases of ethane are not expected when manufactured for use and used as a refrigerant in VLT freezers. The proposed substitute is proposed for use in a thermosiphon that is hermetically sealed by a pinch-off tube. Thus, significant releases are not anticipated. At room temperature, ethane is a gas and, therefore, releases to ground or surface water are not anticipated, as ethane is anticipated to dissipate into the atmosphere upon release to outside air (i.e., because natural ventilation rates would be higher and there is no enclosed space to keep ethane concentrated). Should air releases during manufacturing operations occur, engineering controls should be used (e.g. carbon absorption scrubbers) to collect ethane and prevent the release of ethane 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 ethane is not expected to pose a significant toxicity risk to the general population. 

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