Final - October 10, 2014 

1

Significant New Alternatives Policy Program 

Refrigeration and Air Conditioning Sector 

Risk Screen on Substitutes in Refrigerated Transport 

Substitute: Carbon Dioxide (CO2 or R-744) 

This risk screen does not contain Clean Air Act (CAA) Confidential Business Information (CBI) 

and, therefore, may be disclosed to the public. 

1. INTRODUCTION 

Ozone-depleting substances (ODS) are being phased out of production in response to a series of 

diplomatic and legislative efforts that have taken place over the past two decades, including the Montreal 

Protocol and the Clean Air Act Amendments of 1990 (CAAA). The U.S. Environmental Protection 

Agency (EPA), as authorized by Section 612 of the CAAA, administers the Significant New Alternatives 

Policy (SNAP) Program, which identifies acceptable and unacceptable substitutes for ODS in specific 

end-uses based on assessment of their health and environmental impacts. 

EPA’s decision on the acceptability of a substitute is based on the findings of a screening assessment of 

potential human health and environmental risks posed by the substitute in specific applications. EPA has 

already screened a large number of substitutes in many end-use applications within all of the major ODSusing sectors including: refrigeration and air conditioning, solvent cleaning, foam blowing, aerosols, fire 

suppression, adhesives, coatings and inks, and sterilization. The results of these risk screens are presented 

in a series of Background Documents that are available in EPA's docket. 

The purpose of this risk screen is to supplement EPA’s Background Document on the refrigeration and air 

conditioning sector (EPA 1994) (hereinafter referred to as the Background Document). This risk screen 

evaluates the potential use of carbon dioxide (CO2 or R-744) in the refrigerated transport end-use, 

specifically marine containers, trailers, trucks, and direct drive applications. Table 1 presents the 

composition of the proposed substitute, including the estimated concentrations of impurities, which may 

be present in the proposed substitute. 

Table 1. Composition of CO2

Constituent Chemical Formula CAS Number Concentration 

(Weight Percent)a

Carbon Dioxide (R-744) CO2 124-38-9 ≥ 99.9% 

Potential Impurities 

Water H2O 7732-18-5 ≤ 10 ppm 

Air N/A 132259-10-0 ≤ 250 ppm 

Hydrocarbons N/A N/A ≤ 50 ppm 

High Boiling Residue N/A N/A ≤ 5 ppm 

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

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

 Section 3: Atmospheric Assessment 

 Section 4: Volatile Organic Compound Assessment 

 Section 5: Potential Health Effects 

 Section 6: Discussion of End-Use Scenarios 

 Section 7: Flammability Assessment 
Final - October 10, 2014 

2

 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 

CO2 is recommended for SNAP approval for the refrigerated transport end-use, specifically marine 

containers, trailers, trucks, and direct drive applications. 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, R-502, and 

HCFC-22, as well as other commonly used refrigerants. In addition, CO2 contributions to volatile organic 

compound (VOC) emissions are not a concern as CO2 is exempt under CAA regulations (40 CFR 

51.100(s)). No significant toxicity or asphyxiation risks to workers, end-users, or the general population 

are expected. It is expected that procedures identified in the material data safety sheet (MSDS) for CO2

and good manufacturing practices will be adhered to, and 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, servicing, and 

disposal of refrigerated transport equipment using CO2. In addition, it is expected that CO2 monitors will 

be installed inside refrigerated transport containers and container doors will be open during end-use to 

prevent significant exposure to CO2 in the case of catastrophic leakage. Because units are to be installed 

in locations with adequate space or ventilation (i.e., outdoors) as discussed in greater detail in Section 9, 

significant toxicity risk to end-users is also unlikely. Additional safeguards are also provided by 

adherence to industry standards including American Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE) Standards 15,1

 34,2 Air Conditioning, Heating, and Refrigeration 

Institute (AHRI) Standards 700,3

 1110,4 Compressed Gas Association (CGA) Standard G-6,5 and UL 

Advisory on New CO2 Systems. 

3. ATMOSPHERIC ASSESSMENT 

This section presents an assessment of the potential risks to atmospheric integrity posed by the use of CO2 

in refrigerated transport. The ozone depletion potential (ODP), global warming potential (GWP), and 

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

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

shorter atmospheric lifetime than refrigerants used in refrigerated transport, such as CFC-12, R-502, and 

HCFC-22. In addition, CO2 also has lower climate impact and a shorter atmospheric lifetime than those 

predicted for other substitutes examined in the Background Document, as well as commonly utilized 

 

1 Safety Standard for Refrigeration Systems ASHRAE Standard 15 establishes safeguards for life, limb, health, and property and 

prescribes safety requirements. 

2 Designation and Safety Classification of Refrigerants ASHRAE Standard 34 is establishes a uniform system for assigning 

reference numbers, safety classifications, and refrigerant concentration limits to refrigerants. Safety classifications based on 

toxicity and flammability data are included. 

3 AHRI Standard 700: Standard for Specifications for Refrigerants establishes purity specifications, to verify composition, and to 

specify the associated methods of testing for acceptability of refrigerants regardless of source (new, reclaimed, and/or 

repackaged) for use in new and existing refrigeration and air conditioning products within the scope of AHRI. 4 AHRI Standard 1100: Performance Rating of Mechanical Transport Refrigeration Units establishes a single set of requirements 

for the testing and rating of cooling capacities and electric standby cooling capacities for mechanical transport refrigeration units 

employed forced-circulation air-coolers. 

5 CGA Standard G-6: Carbon Dioxide provides information on production, storage, transportation, uses and safe handling of 

carbon dioxide in its gaseous, liquid and solid (dry ice) forms. 
Final - October 10, 2014 

3

refrigerants, R-404A and HFC-134a. Furthermore, the submitter anticipates using recycled CO2 in 

refrigerated transport units which is typically captured, recovered, and processed from industrial CO2

emission sources (e.g., power plants); therefore, any releases of recycled CO2 are not considered to 

contribute as additional atmospheric CO2 and atmospheric impacts are considered to be neutral (Carrier 

2013a). 

Table 2. Atmospheric Impacts of CO2 Compared to Other Refrigerated Transport Refrigerants 

Refrigerant Ozone Depleting Potential 

(ODP)a

Global Warming Potential

(GWP)b

Atmospheric Lifetime in 

Years (ALT) 

CO2 0 1c 5-200b,d

CFC-12 0.82 10,900 100b

R-502e 0.2 4,650 NAf

HCFC-22 0.04 1,810 12a

R-404Ag 0 3,920 NAh

HFC-134a 0 1,430 14b

a WMO 2010 Scientific Assessment Report (2011). 

b IPCC 4th Assessment Report (Forster et al. 2007) c

 If the substitute is obtained via recycling, the GWP is considered to be zero, as atmospheric impacts are considered to be neutral. d The CO2 response function used in the IPCC 4th Assessment Report is based on the revised version of the Bern Carbon Cycle Model using a 

background CO2 concentration value of 378 ppm. The decay of a pulse of CO2 with time t is given by 

where a0 = 0.217, a1 = 0.259, a2 = 0.338, a3 = 0.186, t1 = 172.9 years, t2 = 18.51 years, and t3 = 1.186 years. (Forster et al. 2007) e R-502 is a blend consisting of HCFC-22 (49%) and CFC-115 (51%). 

f

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

the ALT for CFC-115 is 1,700 years (IPCC 4th Assessment Report [Forster et al. 2007]). g R-404A is a blend consisting of HFC-143a (52%), HFC-125 (44%) and HFC-134a (4%). 

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

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

4. VOLATILE ORGANIC COMPOUND (VOC) ASSESSMENT 

CO2 is not a volatile organic compound (VOC) according to CAA regulations (40 CFR 51.100(s)) due to 

its negligible photochemical reactivity. Therefore, environmental impacts from the release of CO2 as a 

VOC is not a concern. 

5. POTENTIAL HEALTH EFFECTS 

To assess potential health risks from exposure to the proposed substitute in refrigerated transport, EPA 

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

different scenarios. To protect end-users and workers from the potential dangers of a catastrophic leak 

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

reduce the risks of acute toxicity, asphyxiation, and flammability hazards in occupied spaces (ASHRAE 

2010b). ASHRAE Standard 15 implements the aforementioned standard, requiring that “the concentration 

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

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

hypoxia No Observed Adverse Effect Level (NOAEL) and exposure limits, as an additional, conservative 

limit to ensure that significant toxicity and asphyxiation risks, do not occur. 

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

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

of chronic worker exposure were evaluated using workplace guidance levels (WGL). Risks from potential 
Final - October 10, 2014 

4

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

guidance levels (EGL). 

Table 3 lists the relevant toxicity limits and RCL for CO2, and is followed by Table 4, which provides an 

explanation of each toxicity limit. EPA’s approach for identifying or developing these values is discussed 

in Chapter 3 of the Background Document. 

Table 3. Exposure Limits for Carbon Dioxide 

Proposed 

Substitute 

WGL 

(Long-term Exposure) 

ppm 

EGL 

(Short-term Exposure) 

ppm 

Refrigerant Concentration 

Limit (RCL) 

ppm 

CO2 5,000a (8-hour OSHA PEL) 30,000a (15-minute ACGIH STEL) 

40,000a (30-minute NIOSH IDLH) 40,000b

a OSHA PEL, ACGIH TLV, and NIOSH IDLH available at: http://www.cdc.gov/Niosh/npg/npgd0103.html b ASHRAE (2010b) 

Table 4. Explanation of Exposure Limit-Related Terminologya

Organization Definition 

OSHA Occupational Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health 

ACGIH American Conference of Governmental Industrial Hygienists 

Exposure Limit Definition Explanation 

Short-Term Exposure 

RCL Refrigerant Concentration 

Limit 

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

toxicity, asphyxiation, and flammability hazards in normally occupied, 

enclosed spaces. The RCL for each refrigerant is the lowest of the 

Acute-Toxicity Exposure Limit (ATEL), Oxygen Deprivation Limit (ODL), 

and Flammable Concentration Limit (FCL). Determination assumes full 

vaporization with no removal by ventilation, dissolution, reaction, or 

decomposition and complete mixing of refrigerant in the space to which 

it is released. 

STEL Short-Term Exposure Limit A 15-minute TWA exposure that should not be exceeded during a 

workday, even if the 8-hour TWA is within the TLV–TWA, set by 

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. 

Long-Term Exposure 

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

a All information in this table taken from EPA (1994) except where otherwise noted. 
Final - October 10, 2014 

5

According to the MSDS, exposure to CO2 may be hazardous if inhalation, skin contact, or eye contact 

with the proposed substitute occurs at sufficiently high levels. CO2 is moderately irritating to the 

respiratory system and can cause rapid suffocation and/or symptoms of asphyxiation when present in 

concentrations high enough to significantly lower oxygen concentrations below 19.5 percent by volume 

(e.g., headaches, ringing in ears, dizziness, drowsiness, nausea, vomiting, depression of all senses, and 

unconsciousness). Damage may occur to the lungs, cardiovascular system, and central nervous system 

and under some circumstances of over-exposure (i.e., oxygen levels fall below 6 percent by volume), 

death may occur. Concentrations of CO2 of 4 percent by volume (i.e., 40,000 ppm) may cause headache, 

dizziness, increased blood pressure, or uncomfortable dyspnea (shortness of breath) within a few minutes. 

Concentrations of CO2 of 6 percent by volume (i.e., 60,000 ppm) may cause effects such as hearing and 

visual disturbances, alterations to heartbeat, headache and dyspnea in a few minutes, with more severe 

effects at higher concentrations (EPA 2000). 

If excess quantities of CO2 are inhaled, person(s) should be immediately removed and exposed to fresh 

air. In accordance with the MSDS, EPA further recommends that if breathing is difficult or irregular, 

person(s) be given oxygen or artificial respiration, provided a qualified operator is present, and medical 

attention be sought. Rescuers should not attempt to retrieve victims of exposure to CO2 without adequate 

PPE. At a minimum, a self-contained breathing apparatus (SCBA) should be worn. 

Exposures of CO2 to the skin may cause frostbite or burns. In the case of dermal exposure, the MSDS for 

CO2 recommends that person(s) immediately wash the affected area with water for at least 15 minutes 

while removing all contaminated clothing to avoid irritation; if frostbite occurs, bathe (do not rub) the 

affected area with lukewarm, not hot, water. If water is not available, cover the affected area with a clean, 

soft cloth. Alternatively, if the fingers or hands are frostbitten, warm the affected area by placing it in the 

armpit; gently exercise the affected part while being warmed, and seek medical attention immediately. 

Clothing and shoes should be cleaned thoroughly before reuse. Exposures of CO2 to the eyes could cause 

eye irritation. In case of ocular exposure, the MSDS for CO2 recommends that person(s) immediately 

flush the eyes, including under the eyelids, with copious amounts of water for 15 minutes. 

EPA’s review of the human health impacts of this proposed substitute is contained in the public docket 

for this decision. The potential health effects of CO2 can be minimized by following the exposure 

guidelines and the ventilation and PPE recommendations outlined in the MSDS for CO2 and this risk 

screen. 

6. DISCUSSION OF END-USE SCENARIOS 

CO2 has been proposed for use as a refrigerant in refrigerated transport units. As provided by the 

submitter, the maximum and typical charge sizes and typical and minimum container sizes of the 

refrigerated transport units proposed for use with CO2 systems (i.e., marine containers, trailers, and 

trucks) are listed in Table 5.

Table 5. Typical and Maximum Charge Sizes and Typical and Minimum Container Sizes of CO2

Refrigerated Transport Units 

Refrigerated 

Transport Unit 

Typical Charge Size 

(kg) 

Maximum Charge Size 

(kg) 

Typical Container 

Size (m3) 

Minimum Container 

Size (m3) 
Final - October 10, 2014 

6

To evaluate potential end-use exposures to CO2 in refrigerated transport units, this risk screen examines a 

scenario in which personnel are located inside the refrigerated transport unit containing goods. A 

reasonable worst-case scenario for this assessment would consider the catastrophic leak of refrigerant 

occurring while the container doors are open because EPA expects the refrigerated container door to be 

open while personnel are present inside the container at end-use. However, the limitations of the box 

model used for this assessment do not allow consideration of this scenario; calculating the concentration 

of CO2 in the container as it exchanges with the warm, outside air through the open door would require 

more complex computational fluid dynamic modeling. 

Given these limitations, the risk screen evaluates a more unlikely scenario, in which it is assumed that the 

full charge of the refrigerated transport unit is emitted over the course of one minute into the refrigerated 

container with the door closed. Although CO2 is denser than air (specific gravity of CO2 relative to air is 

1.52 [air = 1]), a vertical concentration gradient is not assumed, because when the full charge is released, 

the presence of ventilation and the high pressure operating conditions of the refrigerant are expected to 

disperse the proposed substitute evenly and immediately throughout the refrigerated transport container. It 

was assumed that an individual is present at the start of the leak and remains in the container until the 

charge is completely released. 

Furthermore, this risk screen evaluates a refrigerated transport unit with a ventilation rate of 25 m3

/hr, a 

lower ventilation rate appropriate for less perishable produce (perishable items require a higher 

ventilation rate up to 60 m3

/hr) (Brecht et. al 2000). Hence, based on suggested ventilation rates for 

refrigerated transport units for various types of produce, this analysis conservatively assumes a ventilation 

rate of 25 m3

/hr.6 

Because refrigerated transport units can be installed in a wide range of end-uses with varying charge sizes 

and container sizes, the analysis in this risk screen evaluates all possible combinations provided by the 

submitter (i.e., the minimum and maximum charges sizes within the typical and minimum container 

sizes). Table 6 details the end-use modeling assumptions used in Sections 8 and 9. 

Table 6. End-Use Scenario Model Assumptions 

Parameter Assumptions 

End-Use Area Refrigerated Transport Container 

Container Size (m3) [ ] 

Charge Size (g) [ ] 

Ventilation Rate (m3/hr) 25a

Length of Release (minutes) 1 

Vertical Concentration Gradient No a Brecht et. al (2000); Hamburg Süd (2010) 

7. FLAMMABILITY ASSESSMENT 

CO2 is classified by ASHRAE Standard 34 as an A1 refrigerant, and is considered to be nonflammable 

(ASHRAE 2010b). Based on this classification, manufacture, servicing, and use of CO2 is not expected to 

present a flammability risk. 

8. ASPHYXIATION ASSESSMENT 

The risk of asphyxiation was investigated for CO2 in refrigerated transport units. In this section, risk of 

asphyxiation is assessed in three ways: 1) modeling the oxygen concentration under the charge sizes and 

 

6Ventilation rates range from 0 to 60 m3/hr depending on the type of good that is transported (Hamburg Süd 2010). 
Final - October 10, 2014 

7

container sizes specified by the submitter for marine containers, trucks, and trailers, 2) performing a 

threshold analysis of minimum charge size needed to cause an asphyxiation risk in the container sizes 

specified by the submitter and 3) performing a threshold analysis of the typical container size needed to 

cause an asphyxiation risk with the charge sizes specified by the submitter. This analysis does not 

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

be exposed, such as open container doors, or even openings at the bottom of container doors that allow air 

to flow in and out. This analysis does not assume a vertical concentration gradient. If the proposed 

substitute passes the screening analysis with these restrictive assumptions in place, it can be reasonably 

assumed that no risks of asphyxiation will be present under real-world conditions. The results of the 

asphyxiation assessment are summarized in Table 7a below. 

Table 7a. Asphyxiation Assessment – Worst Case Scenarioa 

Refrigerated 

Transport 

Container 

Scenario Charge 

Size (g) Container Size (m3) Percent Oxygen 

Concentrationb

Marine 

Containers 

Worst-Case 1a – 

Minimum Container Size/Typical Charge Size 20 

Worst-Case 1b – 

Typical Container Size/Maximum Charge Size 20 

Trailers 

Worst-Case 2a – 

Minimum Container Size/Typical Charge Size 20 

Worst-Case 2b – 

Typical Container Size/Maximum Charge Size 20 

Trucks 

Worst-Case 3a – 

Minimum Container Size/Typical Charge Size 17 

Worst-Case 3b – 

Typical Container Size/Maximum Charge Size 19 

Bold font indicates modeling results. 

 a Cells highlighted in green are the scenarios with acceptable oxygen concentrations 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.7

 Based on the worst-case scenario modeling assumptions, CO2 in refrigerated 

transport units does not present a significant risk of asphyxiation. The oxygen concentration inside a 

refrigerated container following the release of [ ] is not anticipated to fall below 17 percent, which is well 

above the concentration that would cause a risk of asphyxiation. Because this modeling does not take into 

account any ventilation which is likely to occur, such as open container doors or openings at the bottom 

of container doors that allow air to flow in and out, as mentioned above, the actual asphyxiation risk to 

personnel is likely to be even smaller than modeled. 

Table 7b. Asphyxiation Assessment – Charge and Container Sizesa 

Scenario Charge Size 

(g) 

Container Size 

(m3) 

Percent 

Oxygen 

Concentrationb

Bold font indicates modeling results. 

 

7 Twelve percent oxygen in air is the NOAEL for hypoxia (ICF 1997). 
Final - October 10, 2014 

8

 a Cells highlighted in green are the scenarios in which the charge and container sizes evaluated in this risk screen are acceptable according 

to the threshold analysis. 

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

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

concern would exist were determined. Within the smallest container size of [ ], as shown under Threshold 

Analysis 1a, the charge size of the CO2 refrigeration unit would have to be at least [ ] for asphyxiation to 

be a concern, [ ]. Using the largest charge size of [ ] unit, as shown under Threshold Analysis 2b, the 

volume of the container would have to be [ ] or smaller for asphyxiation to be of concern, [ ]. 

To further protect against an asphyxiation risk, end-users should ensure container doors are open while 

personnel are inside to allow released CO2 to diffuse into the outside air. Leak prevention, detection, or 

mitigation devices should be installed and the ventilation requirements outlined in the MSDS for CO2

should be closely followed. If these guidelines are followed, a catastrophic leak of CO2 is not anticipated 

to result in a significant asphyxiation risk. 

9. END-USE EXPOSURE ASSESSMENT 

This section presents estimates of potential personnel end-use exposures to CO2 in refrigerated transport 

units. [ ]. Personnel exposure to the proposed substitute at end-use (e.g., personnel located inside truck 

cab or refrigerated trailer) is not expected to be a significant concern because of established equipment 

design. 

This end-use exposure assessment examines a scenario in which personnel are located inside the 

refrigerated transport unit containing goods; such scenarios are expected to occur while the container 

doors are open. When the container door is opened, a stack effect exists from the presence of the denser, 

cold air coming into contact with the warm, moist air outside. This effect will cause the colder air to flow 

outside of the container and become replaced by the warm, outside air through infiltration. Therefore, 

EPA expects that if a release of the proposed substitute occurs while personnel are present inside the 

refrigerated transport container (e.g., during loading and unloading of refrigerated goods), the container 

doors would be open, increasing the air exchange rate inside of the unit and also resulting in diffusion of 

any CO2 to flow towards the outside of the container.

To simulate a worst-case scenario, the catastrophic release of the entire charge of the unit into the 

refrigerated container was modeled. The estimated exposures are conservative, as the analysis assumes 

that a catastrophic release occurs while the refrigerated transport container doors are closed and also does 

not consider other variables that would reduce the levels to which individuals would be exposed. This 

analysis also conservatively compares the modeled exposures to the STEL for CO2, which represents the 

maximum concentration limit at which a worker should be exposed over 15 minutes. 

 
Final - October 10, 2014 

9

Table 8a. End-Use Exposure - Worst Case Scenarioa,b 

Refrigerated 

Transport 

Container 

Scenario Charge 

Size (g) 

Container Size 

(m3) 

15-minute TWA 

End-Use Exposure

(ppm)c

Marine 

Containers 

Worst-Case 1a – 

Minimum Container Size/Typical Charge Size 55,900 

Worst-Case 1b – 

Typical Container Size/Maximum Charge Size 27,700 

Trailers 

Worst-Case 2a – 

Minimum Container Size/Typical Charge Size 61,300 

Worst-Case 2b – 

Typical Container Size/Maximum Charge Size 37,600 

Trucks 

Worst-Case 3a – 

Minimum Container Size/Typical Charge Size 155,800 

Worst-Case 3b – 

Typical Container/Maximum Charge Size 75,300 

a Bold font indicates modeling results. b Cells highlighted in green are the scenarios with acceptable exposure levels, i.e., below the STEL, given various modeling assumption 

options. 

c

 15-min STEL for CO2 is 30,000 ppm; see Table 3 for more information. 

In Worst-Case Scenarios 1a and through 3b in Table 8a, the catastrophic release of CO2 from marine 

containers, trailers, and trucks with various charge sizes was modeled. The modeling results indicate that 

the 15-minute STEL is likely to be exceeded for all scenarios except for marine containers in a typical 

container size and maximum charge size (Worst-Case Scenario 1b). 

Because the worst-case scenarios presented in this assessment reflect an emergency situation, it is also 

relevant to compare potential end-use exposures to the 30-minute Immediately Dangerous to Life and 

Health (IDLH) value for CO2 (i.e., 40,000 ppm), which represents the concentration at which personnel 

should be able to escape the container within 30 minutes without experiencing escape-impairing or 

irreversible health effects. For each scenario presented in Table 8a, the 30-minute TWA exposure is 

slightly less than the corresponding 15-minute TWA. Therefore, the scenario that does not exceed the 15-

minute STEL also does not exceed the IDLH (i.e., Worst Case Scenario 1b). Although the 15-minute 

STEL is projected to be exceeded for trailers with a typical container size and maximum charge size 

(Worst-Case Scenario 2b), the IDLH is not likely to be exceeded. The other scenarios that exceed the 

STEL also exceed the IDLH, (i.e., scenarios 1a, 2a, 3a and 3b. However, because the modeling was done 

using a conservative worst-case scenario where the refrigerated transport container door is closed, and 

because normally personnel would be exposed in a situation where the door is open with significantly 

higher ventilation levels, personnel exposure to the proposed substitute at end-use is not expected to reach 

the modeled concentrations shown in Table 8a. Therefore, end-use exposure is not considered to be a 

significant risk. 
Final - October 10, 2014 

10

A threshold analysis was performed to determine the combinations of charge sizes and container sizes 

with a 25 m3

/hr ventilation rate that would avoid end-use exposures above the STEL for CO2 (i.e., 30,000 

ppm). The results from the threshold analyses are used to establish guidelines for choosing an effective 

container size, charge size, and ventilation rate such that use of CO2 in refrigerated transport units does 

not present risk to personnel at end-use. The results of the threshold analyses on charge and container 

sizes are presented in Table 8b. 

Table 8b. End-Use Exposure Threshold Analysis – Charge and Container Sizesa,b,c

Refrigerated 

Transport 

Container 

Scenario Charge Size 

(g) 

Container Size 

(m3) 

15-minute 

TWA EndUse 

Exposure

(ppm) 

a Analysis was performed using a 25 m3/hr ventilation rate. b Bold font indicates modeling results. c

 Cells highlighted in green are the scenarios in which the charge and container sizes evaluated in this risk screen are acceptable according to 

the threshold analysis. 

As shown in Threshold Analysis 1a in Table 8b, the maximum charge size that should be installed in a 

refrigerated transport container of [ ], is [ ]. Additionally, the threshold analysis indicates that refrigerated 

transport units with charge sizes of [ ] and [ ], the minimum and maximum charge sizes provided by the 

submitter, should not be installed in containers smaller than [ ] and [ ], respectively, assuming a 

ventilation rate of 25 m3

/hr. As shown in Table 8b, these recommended minimum container sizes vary in 

direct correlation with charge size. The modeling results presented in Table 8a and 8b reflect conditions 

that are assumed to be a worst-case scenario, as described in Section 6.

Infiltration rates through the opening of a refrigerated truck container have been measured to be 

approximately 2,520 m3

/hr, which is roughly 100 times the ventilation rates used in the threshold analyses 

(Bonjour et. al 2011). As previously discussed, EPA expects the refrigerated container door to be open 

while personnel are present inside the container at end-use, increasing the air exchange rate inside of the 

unit and also encouraging any CO2 to flow towards the outside air. Therefore, under a reasonable worstcase scenario where the refrigerated container doors are open, the exposures shown in the modeling 

results are unlikely. 

Because refrigerated transport units using the proposed substitute were modeled using a conservative, 

worst-case scenario where the refrigerated transport container door is closed, personnel exposure above 

the proposed substitute’s STEL of 30,000 ppm is unlikely. However, these potential exceedances 

emphasize the importance associated with proper equipment use, employment of engineering controls to 

limit releases and/or exposures, and the need for workforce training. To prevent exposure and potential 

serious side effects during larger releases of the proposed substitute, EPA recommends the installation of 

CO2 monitors on both ends of the refrigerated container, especially near the refrigeration system unit, and 

ensure the doors to the refrigerated container are open while personnel are inside. In addition, EPA 

recommends that warning labels are present in the refrigerated transport container indicating that CO2

refrigerants are in use and can cause suffocation, injury, or death. An example of this warning label would 

read: “WARNING: Carbon dioxide gas can cause injury or death. Ventilate the area before entering. A 

high carbon dioxide concentration can occur in this area and can cause suffocation.” Training materials 

and an operations manual describing procedures for proper use of the equipment and safety guidelines for 
Final - October 10, 2014 

11

handling CO2, and prior safety training is required for all personnel. These procedures and guidelines 

should be acknowledged and followed to mitigate exposure in case of CO2 leakage.

[ ] 

Personnel exposure to the proposed substitute at end-use (e.g., personnel located inside truck cab or 

refrigerated trailer) is not considered a significant concern because of established equipment design. 

Proper leak protection devices, engineering control requirements, and adherence to the MSDS will further 

ensure that exposure limits are maintained below those described in Table 3. 

10. OCCUPATIONAL EXPOSURE ASSESSMENT 

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

disposal of CO2 refrigerated transport units. To evaluate occupational exposure, EPA has assessed the 

risks associated with refrigerant release scenarios which are assumed to occur accidentally. 

10.1 Occupational Exposure at Manufacture 

EPA expects that the charging of refrigerated transport units occur in a closed system. [ ]. 

As indicated by the submitter, occupational exposure at manufacture is anticipated to occur only during 

the activities listed [ ]. Exposures were measured for all three activities [ ]. These concentrations are well 

below the exposure limits for CO2 as listed in Table 3. Because the expected maximum occupational 

exposure at manufacture is well below levels of concern and typical exposure concentrations are expected 

to be minimal, EPA does not believe that CO2 presents a significant risk to workers during manufacture. 

10.2 Occupational Exposure at Servicing and Disposal 

[ ]. Potential exposures to CO2 can occur during [ ]. 

 

The submitter estimated background concentrations in the air during [ ]. These concentrations are well 

below the exposure limits for CO2 as listed in Table 3. [ ], occupational exposures greater than the 

maximum anticipated concentration is not likely (Carrier 2013a). 

According to the submitter, the proposed substitute will not be recycled or reclaimed at disposal (Carrier 

2013a); therefore disposal recommendations outlined in the proposed substitute’s MSDS should be 

followed. Occupational exposure may occur at disposal during dismantling of the units. However, 

exposure during disposal is not expected to exceed concentrations anticipated during servicing [ ] and 

therefore occupational exposures greater than the exposure limits for CO2 as listed in Table 3 are not 

likely. 

Training materials and an operations manual describing procedures to reduce leakage during servicing 

and disposal and prior safety training is required for all personnel servicing and disposing of units. 

Because of established equipment design and servicing and disposal practices, exposures to emissions of 

CO2 will be unlikely. Thus, EPA believes that servicing and disposal of a refrigerated transport unit is not 

expected to pose a significant toxicity risk to workers.
Final - October 10, 2014 

12

11. GENERAL POPULATION EXPOSURE 

CO2 is not expected to cause a significant concern to human health in the general population when used as 

a refrigerant in refrigerated transport. The proposed substitute is proposed for use in closed systems, and 

thus, significant releases are not likely. At room temperature, CO2 is a gas, and therefore, releases to 

ground or surface water are not anticipated, as CO2 is expected 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 CO2 concentrated). Should air releases during manufacturing, servicing, or disposal operations 

occur, engineering controls should be used (e.g., carbon absorption scrubbers) to collect CO2 and prevent 

its release into 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 CO2

is not expected to pose a significant toxicity risk to the general population.
Final - October 10, 2014 

13

12. REFERENCES 

AHRI. 2014. Standard 700: Specifications for Refrigerants. Air Conditioning, Heating and Refrigeration 

Institute. 

ANSI/AHRI. 2013. Standard 1110: Performance Rating of Mechanical Transport Refrigeration Units. Air 

Conditioning, Heating and Refrigeration Institute. 

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

Heating, Refrigerating and Air-Conditioning Engineers, Inc. 

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

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

Brecht, P., Hinsch, T., Kader, A., and Thompson, J. 2000. Marine container transport of chilled perishable 

produce. University of California Agriculture and Natural Resources Publications: Oakland, CA. 

Bonjour, J. Ducoulombier, M., and Sartre, V. 2011. Experimental Investigation of the Infiltration Heat Load 

During the Opening of a Refrigerated Truck Body. International Institute of Refrigeration: International 

Congress of Refrigeration. Available online at http://bright-blue.eu/wp-content/uploads/2012/11/artikel392_DucoulombierSartreBonjour_revised_final.pdf. 

Carbon Dioxide Information Analysis Center (CDIAC). 2012. Recent Greenhouse Gas Concentrations. 

Updated February 2012. Available online at http://cdiac.ornl.gov/pns/current_ghg.html. Accessed May 1, 

2012. 

Carrier Corporation. 2013a. CO2 in Refrigerated Transport. Significant New Alternatives Policy Program 

Submission to the U.S. Environmental Protection Agency. July 19, 2013. 

Carrier Corporation. 2013b. Response to Incompleteness Letter for CO2 in Refrigerated Transport. 

November 30, 2013. 

Carrier Corporation. 2014. Response to Incompleteness Letter for CO2 in Refrigerated Transport. July 2, 

2014. 

CGA. 2009. Standard G-6: Carbon Dioxide. 7th Edition. Compressed Gas Association. 

Hamburg Süd. 2010. Reefer Guide. Available online at 

http://www.hamburgsud.com/group/media/sharedmedia/dokumente/brochures/Reefer_guide.pdf 

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

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

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

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

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

H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 
Final - October 10, 2014 

14

ICF International. 1997. Physiological Effects of Alternative Fire Protection Agents - Hypoxic 

Atmospheres Conference. Stephanie Skaggs prepared the proceedings of the conference held May 22, 

1997 in New London, CT. 

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

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

Stratospheric Protection Division. March, 1994. 

U.S. EPA. 2000. Carbon Dioxide as a Fire Suppressant: Examining the Risks. Appendix B. Available 

online at http://www.epa.gov/ozone/snap/fire/co2/co2report.html 

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

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