![](media/image1.jpeg){width="8.686805555555555in"
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Report of the 14--15 August 2007\
U.S. EPA Workshop on

**HFC-152a\
Secondary Loop Vehicle A/C Systems**

*October 30, 2007*

Dr. Alberto Ayala, California Air Resources Board;

Dr. Stephen O. Andersen, Director of Strategic Climate Projects, US EPA
Climate Protection Partnerships Division;

Ward Atkinson, Chair, Interior Climate Control Committee, Society of
Automotive Engineers, and President, Sun Test Engineering;

James Baker, Senior Staff Research Scientist, Delphi Corporation;

Lisa Bendixen, Vice President, ICF International;

Dr. Neal Blackwell, Senior Project Engineer, US Army;

David D. Doniger, Climate Center Policy Director, Natural Resources
Defense Council;

Dave Godwin, Environmental Engineer, Stratospheric Protection Division,
US EPA;

Gary Hansen, Vice President, Red Dot;

Elvis Hoffpauir, President, Mobile Air Conditioning Society Worldwide;

Paul Hughes, Low Emission Vehicle Manager, California Air Resources
Board;

Dr. Carloandrea Malvicino, Head, Thermal Systems Department, Centro
Ricerche Fiat;

Nha Nguyen, International Policy and Harmonization, US National Highway
Traffic Safety Administration;

Sally Rand, Director High GWP Voluntary Industry Partnerships, Climate
Change Division, US EPA;

Alberto N. Reginaldo, MAC Principal Engineer, Toyota Technical Center;

John Rugh, Senior Engineer, MAC Fuel Use Reduction Task, US National
Renewable Energy Laboratory;

Fred Sciance, Manager of Global Climate Issues, General Motors;

Leland Shields, Member, SAE Fire Safety Committee and President, Leland
E. Shields, Inc.;

Scott Stone, Senior Attorney, Institute for Government and Sustainable
Development; and

Kristen Taddonio, Manager, Strategic Climate Projects, US EPA; and

Mark Zima, Engineering Manager, Delphi Corporation.

Peer reviewed by Hagen Deyhle (Porsche), Thomas Drees (Volkswagen), Hans
Fernqvist (Volvo), Reynaldo Forte (EPA), William Hill (General Motors),
James Jetter (EPA), Stephen Lepper (Ford), Jean Lupinacci (EPA), John
Meyer (Visteon), Roberto Monforte (Fiat), Erhard Moser (BMW), Wolfgang
Rilling (DaimlerChrysler), Paul Risch (Audi), and Helen Tope (Energy
International Australia).

**Table of Contents**

1.  1.  
    2.  
    3.  
    4.  

2.  

3.  1.  

    2.  

    3.  1.  
        2.  
        3.  

4.  

5.  1.  1.  

    2.  1.  

    3.  

6.  1.  
    2.  

7.  

8.  1.  1.  
        2.  

    2.  

    3.  

    4.  

    5.  

Executive Summary [1](#executive-summary)Key Findings---Health and
Safety, Climate, and Air Quality Benefits
[3](#key-findingshealth-and-safety-climate-and-air-quality-benefits)Key
Findings---Flammability Risk [4](#key-findingsflammability-risk)Key
Findings---Public and Private Safety Regulations and Standards
[5](#key-findingspublic-and-private-safety-regulations-and-standards)Key
Findings---Service [5](#key-findingsservice)Annotated Review of HFC-152a
Commercialization
[5](#annotated-review-of-hfc-152a-commercialization)Climate and Air
Quality Benefits of HFC-152a MACs
[6](#climate-and-air-quality-benefits-of-hfc-152a-macs)Energy Efficiency
Is Key to Good LCCP [7](#energy-efficiency-is-key-to-good-lccp)HFC-152a
Is an Efficient, Low-GWP Option
[7](#hfc-152a-is-an-efficient-low-gwp-option)Comparing the Climate
Impacts of Alternative MAC Systems With Green-MAC-LCCP©
[8](#comparing-the-climate-impacts-of-alternative-mac-systems-with-green-mac-lccp)
How LCCP Is Calculated for MACs [8](#how-lccp-is-calculated-for-macs)
How to Use the Model [9](#how-to-use-the-model) Preliminary Results
[9](#preliminary-results)Flammability Risk of HFC-152a
[10](#flammability-risk-of-hfc-152a)Mitigation Strategy to Reduce
Flammability Risk
[11](#mitigation-strategy-to-reduce-flammability-risk)Mitigation
Adequacy [11](#mitigation-adequacy) Fault Tree Findings From HFC-152a
Risk Assessments
[11](#fault-tree-findings-from-hfc-152a-risk-assessments)Safety-Mitigated
Secondary Loop HFC-152a Systems\
Relative to Other Vehicle-Related Risks
[14](#safety-mitigated-secondary-loop-hfc-152a-systems-relative-to-other-vehicle-related-risks)
Compared to Vehicle Fuel Flammability Risks
[14](#compared-to-vehicle-fuel-flammability-risks)Importance of SAE
Standards to A/C Safety
[15](#importance-of-sae-standards-to-ac-safety)Safety and Training
Measures for MAC Service
[16](#safety-and-training-measures-for-mac-service)Unique Fittings and
Labels, and Service Equipment and Tools
[16](#unique-fittings-and-labels-and-service-equipment-and-tools)Service
Technician Safety Precautions
[17](#service-technician-safety-precautions)Model State Legislation to
Safely Allow HFC-152a Systems
[17](#model-state-legislation-to-safely-allow-hfc-152a-systems)Appendices
[21](#appendices)SAE New and Revised HFC-152a Documents
[21](#sae-new-and-revised-hfc-152a-documents) Design Requirements
[21](#design-requirements) Service Sector [21](#service-sector)HFC-152a
Chemical and Physical Properties
[22](#hfc-152a-chemical-and-physical-properties)HFC-152a Flammability
Properties [24](#hfc-152a-flammability-properties)Flammable Refrigerant
Fault Tree Analysis
[25](#flammable-refrigerant-fault-tree-analysis)Annotated CalTech
Glossary on Explosion Dynamics
[30](#annotated-caltech-glossary-on-explosion-dynamics)

# 1. Executive Summary

The Challenge and Approach, Tiger Team Members, Consensus Process,
Critical Technical Background.

All passenger vehicles and light trucks manufactured with air
conditioning (A/C) worldwide currently use refrigerant hydrofluorocarbon
134a (HFC-134a) in direct expansion systems.

European Union (EU) regulations require air-conditioned vehicles sold in
EU countries to use refrigerants with global warming potentials (GWP)
less than 150 beginning in 2011 for new type vehicles and in all
vehicles by 2017. GWP is an index comparing the climate impact of an
emission of a greenhouse gas relative to that of emitting the same mass
of carbon dioxide (CO~2~).[^1] The calculated GWP of any particular
substance depends on the changing condition of the atmosphere, evolving
carbon sinks, and updated laboratory measurements. Because the GWP
values can change over time, policymakers specify the values from a
particular referenced report for purposes of regulations. For the U.S.
greenhouse gas inventory, the United Nations Framework Convention on
Climate Change, and the Kyoto Protocol, the GWP is fixed permanently at
the value in the Intergovernmental Panel on Climate Chance (IPCC) Second
Assessment Report (1996).[^2]

The predominant refrigerant in today's A/C systems, HFC-134a, has a
100-year IPCC Second Assessment Report GWP of 1300 and is therefore
being phased out under the EU legislation described above. HFC-152a has
a 100-year IPCC Second Assessment Report GWP of 140 and is therefore
allowed by the EU regulations. Environmental authorities in California
and 12 other states are considering similar regulations that may soon
control or prohibit HFC-134a. The EU regulation does not take into
account indirect emissions of greenhouse gases from fuel consumption
required to power and transport the additional weight of A/C systems.

Vehicle manufacturers prefer a single global refrigerant that satisfies
regulatory authorities in all global markets.

Systems satisfying the EU regulation using CO~2~ (CO~2~ -- R-744), and
HFC-152a (HFC-152a -- R-152a) have been engineered and tested, and
component and systems suppliers have announced their commercial
availability. BMW and other German vehicle manufacturers have announced
that they plan to offer CO~2~ systems in the EU. Hydrocarbons (HCs, e.g.
propane -- R-290) refrigerants are commercially available and widely
used outside North America in small refrigerators, refrigerated display
cases, and in other applications, and would technically satisfy the EU
regulation. However, no automotive supplier recommends or offers HC
systems for vehicles. In addition, the industry is reviewing other
recently announced synthetic-blend refrigerants (typically with at least
one HFC ingredient) that would meet the GWP 150 limit.

"Direct expansion" or "primary loop" A/C systems place the condenser in
a location of high air flow directly behind the vehicle front grill
inlet to reject heat and place the refrigerant-to-air in the air
distribution system. The evaporator is inside the passenger or engine
compartment providing cool air to the passengers (see figure 1). Breach
of components in the dash would discharge refrigerant directly into the
passenger compartment. Breach of under-hood components would discharge
refrigerant to the engine compartment. The average charge size of
refrigerant for a direct expansion A/C system is as little as 500 grams,
with a typical charge of 650 to 800 grams for single evaporator systems.
A portion of refrigerant charge may remain in a breached system
dissolved in the approximately 100--150 grams of lubricant (typically a
polyaklylene glycol (PAG) oil) and coating components.

Secondary loop A/C systems locate a refrigerant-to-liquid evaporator
(called a chiller) in the engine compartment, rather than an evaporator
in the air distribution system. The refrigerant chills coolant (likely
an antifreeze/water mix) that is circulated by an electric pump to a
heat exchanger that cools the air in the passenger compartment (see
figure 2). The secondary loop A/C system prevents the refrigerant from
entering the air distribution system and the passenger compartment in
the event of a leak or collision.

![](media/image2.png){width="3.1166666666666667in"
height="2.1666666666666665in"}

***Figure 1: Direct expansion A/C system Figure 2: Secondary loop A/C
system***

The length of the refrigerant plumbing (tubing/hose) may be shortened
depending on the chiller location, which, together with the properties
of R-152a, significantly reduces the system refrigerant charge. Because
all components containing refrigerant are located in the engine
compartment, air flow around and under the engine dissipates any
refrigerant leaks without significant infiltration into the passenger
compartment. The average charge size of refrigerant for a secondary loop
A/C system can be about half that of a direct expansion A/C
system---about 250 or 400 grams for vehicles that would otherwise use a
500 or 600 gram direct expansion system, respectively.

Except for the chiller (replacing the conventional evaporator in the
direct expansion A/C system), the refrigerant circuit of the secondary
loop system utilizes components similar to HFC-134a systems. HFC-152a
systems have operating system parameters, such as pressures, comparable
to HFC-134a systems.

Use of the chilled water/antifreeze mix in secondary loop systems also
eliminates the necessity of large refrigerant charges for vehicles
currently having two or more refrigerant evaporators (SUVs, passenger
vans, high-end passenger vehicles, limousines, and advanced hybrid
vehicles requiring battery and inverter cooling).

The U.S. Environmental Protection Agency (EPA) has proposed that
HFC-152a refrigerant be listed under the Significant New Alternatives
Policy (SNAP) Program as acceptable for new vehicle A/C systems, as long
as systems are designed to avoid, in the event of a collision or
equipment failure, concentrations in the passenger cabin free space
above the 3.7% lower flammable limit (LFL) for more than 15 seconds. EPA
also has proposed to list R-744 (CO~2~) systems as acceptable under SNAP
with proper safety features to avoid unsafe CO~2~ concentrations in the
passenger cabin.

## 1.1 Key Findings---Health and Safety, Climate, and Air Quality Benefits 

HFC-152a has comparable toxicity to the current refrigerant, HFC-134a.
Both have a 1,000 parts per million (ppm) Acceptable Exposure Limit
(AEL) and Workplace Environmental Exposure Limit (WEEL) and no Short
Term Exposure Limits (STEL). Neither has a toxicity concern at the
concentrations that can be experienced in the event of a full discharge
into the passenger compartment.

+----------------------+-----------------------+-----------------------+
| **Toxicological      | **HFC-134a**          | **HFC-152a**          |
| Test**               |                       |                       |
+----------------------+-----------------------+-----------------------+
| Acute (LC50) (ppm)   | \>500,000             | 383,000               |
+----------------------+-----------------------+-----------------------+
| Cardiac sensitivity  | 50,000                | 50,000                |
| -- no effect level   |                       |                       |
| (ppm)                |                       |                       |
+----------------------+-----------------------+-----------------------+
| Worker exposure      | 1,000                 | 1,000                 |
| limit (ppm)          |                       |                       |
+----------------------+-----------------------+-----------------------+
| 28-day NOAEC (ppm)   | 50,000                | 100,000 (14-day)      |
+----------------------+-----------------------+-----------------------+
| 90-day NOAEC         | 50,000                | 25,000 (2-year)       |
+----------------------+-----------------------+-----------------------+
| Developmental        | Effect threshold      | Negative              |
| toxicity             | 40,000                |                       |
+----------------------+-----------------------+-----------------------+
| Genotoxicity         | Negative              | Negative              |
|                      |                       |                       |
|                      |                       | (Ames test only)      |
+----------------------+-----------------------+-----------------------+
| Aquatic toxicity     | 450                   | No data               |
| (mg/L)               |                       |                       |
+----------------------+-----------------------+-----------------------+

HFC-152a has a 100-year IPCC Second Assessment Report GWP of 140 and an
atmospheric lifetime of 1.45 years, whereas HFC-134a has a 100-year IPCC
Second Assessment Report GWP of 1,300 and an atmospheric lifetime of
14.6 years (IPCC, 1995).

Life-Cycle Climate Performance (LCCP) is the integrated estimate of
total greenhouse gas emissions resulting from direct refrigerant
emissions and indirect fuel use. A computerized LCCP model has been
developed by a partnership headed by General Motors and including EPA,
the Japan Automobile Manufacturers' Association (JAMA), the U.S.
Department of Energy's (DoE) National Renewable Energy Laboratory
(NREL), and other partners.

NREL has calculated a potential 21% reduction in A/C fuel use for an
HFC-152a secondary loop system on an Opel Astra that incorporates
capacity control to take full advantage of the higher thermal ballast
available in secondary loop systems. An added benefit of a secondary
loop system is that it enables controls that exploit vehicle inertia to
cool the secondary loop fluid and components during deceleration or when
the engine is operating at high energy efficiency (and lowest tailpipe
emissions). In addition, the secondary loop allows extended idle stop
operation in hot weather by drawing on the thermal ballast of that
cooled secondary fluid and components. Idle stop requires additional
components beyond those of the secondary loop A/C system. These
secondary loop HFC-152a systems would increase the fuel efficiency of
all vehicles, with the greatest percentile gains by the most efficient
vehicles and the least percentile gains by the least efficient vehicles,
where A/C fuel use is a smaller part of total fuel consumption.

NREL and EPA estimate that secondary loop HFC-152a systems with capacity
control and extended idle stop vehicle operation can save up to 2.6
billion gallons of fuel per year for the United States alone if
implemented in every vehicle, eliminating 23.2 million metric tons (MMT)
CO~2~ equivalent (CO~2~eq) per year in greenhouse gases.

At \$3.00/gallon, average fuel savings of 11.4 gallons/year/vehicle, and
\$100 to \$150 savings from one less system recharge during the useful
life of the vehicle as a result of longer refrigerant containment prior
to service, the added cost of secondary loop HFC-152a systems in new
vehicles can be rapidly paid back in hot climates. Addition of a fluid
pump and cooler may add to warranty costs.

HFC-152a is a low-cost chemical that no longer has manufacturing patent
restrictions and is used in hundreds of household products today,
including Dust-off^®^ pressurized blowers, hairspray, deodorant, and
other consumer products.

## 1.2 Key Findings---Flammability Risk

While some tests have shown that HFC-152a has limited flammability and
is difficult to ignite when released into engine compartments, it still
requires risk mitigation such as a secondary loop or active detection
and safe discharge if used as a refrigerant in vehicle A/C systems.
HFC-152a therefore is not suitable for use in vehicles not designed for
flammable refrigerants. Current CFC-12 or HFC-134a direct expansion
systems cannot be safely retrofitted for flammable refrigerants. In
Europe the updated EN 378 standard developed for stationary A/C systems
currently bans refrigerants considered flammable in direct expansion
systems but allows them in building systems with secondary loops.

Direct expansion HFC-152a systems with refrigerant-containing components
within the passenger compartment would require active safety systems
that can detect and respond with high reliability to any leakage of
refrigerant into the passenger compartment and require additional safety
mitigation for risk of leakage into the engine compartment from
collision damage or component failure.

Secondary loop HFC-152a systems eliminate the risk of refrigerant
leakage to the passenger compartment and explicitly satisfy the
EPA-proposed criteria that refrigerant concentrations in the passenger
cabin free space not exceed 3.7% for more than 15 seconds.

HFC-152a systems would contribute little to overall vehicle fire risk if
components containing HFC-152a are located within the engine compartment
and are designed to avoid conditions where flammable mixtures can
develop adjacent to potential sources of ignition.

Properly designed HFC-152a secondary loop systems would include
incremental flammability risk similar to commonly accepted automotive
design choices affecting fuel choice and the quantities, containment,
and location of under-hood flammable materials. These risks are
manageable and reducible by modest design changes.

## 1.3 Key Findings---Public and Private Safety Regulations and Standards 

Mobile A/C systems are federally regulated by EPA and the U.S.
Department of Transportation (DoT), and are subject to Society of
Automotive Engineers (SAE) standards. EPA and several states cite
SAE-recommended practices and standards in regulations designed to
ensure refrigerant safety and environmental performance.

The Mobile Air Conditioning Society (MACS) Worldwide is developing new
training programs for servicing HFC-152a mobile A/C systems that will be
based on new SAE standards, including a proposed standard that would
require certification to work on HFC-152a systems.

International cooperation of authorities from governments, companies,
and nongovernmental organizations (NGOs) on introduction of HFC-152a and
other systems can help ensure uniformity and acceptability in fire risk
mitigation and efficiency in expenditure of resources for design,
development, testing, service training, and refrigerant supply.
Cooperation can also promote the choice of system designs and
refrigerants that provide superior life-cycle climate performance.

Development of new SAE standards is underway to document proper design
of refrigerant systems, service equipment, and service procedures for
handling of R-152a, and to ensure continued refrigerant safety and
performance. The standards will be developed in the coming months and
should be referenced by new government regulations.

## 1.4 Key Findings---Service

Member surveys by MACS indicate that flammable substances are already
routinely handled in servicing shops, and that HFC-152a would not be
considered a significant addition to normal operating risks that are
inherently a part of any automotive service job, as long as service
personnel are aware that a flammable refrigerant is in use. The design
of commercially manufactured HFC-152a systems will include development
of service equipment and procedures to minimize flammability risk.

# 2. Annotated Review of HFC-152a Commercialization

> Late 1980s: EPA investigated flammable refrigerants---including HCs,
> HFC-152a, and dimethylether (DME).
>
> Early 1990s: DoE investigated flammable refrigerants and published the
> definitive study by A.D. Little.
>
> 2002: SAE's Alternate Refrigerants Cooperative Research Program
> (ARCRP) evaluated a direct expansion system using HC refrigerant
> (R290).
>
> 2003 Secondary loop systems with HFC-152a were demonstrated at the SAE
> Phoenix Alternate Refrigerant Symposium.
>
> 2004: HFC-152a studies were conducted during the SAE Alternate
> Refrigerants Cooperative Research Program. Also, Red Dot Corporation
> developed and demonstrated a roof top R-152a system. Field trials are
> underway in Australia in cooperation with EPA, the Australia
> Commonwealth EPA, the Victoria EPA, the Australia Coal Council and
> other trade, environmental, and industry advisers.
>
> 2007: Secondary loop systems with HFC-152a were demonstrated at the
> SAE Phoenix Alternate Refrigerant Symposium and the Second European
> Workshop on Mobile Air Conditioning (Orbassano, Italy).

# 3. Climate and Air Quality Benefits of HFC-152a MACs

LCCP is the integrated estimate of total greenhouse gas emissions
attributable to a particular technology over its lifetime. Worldwide, it
is considered the most comprehensive way to calculate climate impacts.
For mobile A/C (MAC) systems, LCCP is the sum of direct refrigerant
emissions and indirect emissions that result from the energy consumption
due to MAC manufacturing, A/C operation, and end-of-life processing. The
many direct and indirect factors that contribute to MAC greenhouse gas
emissions are listed in box 1 and are illustrated by the figure below.

*Figure 3: Life-Cycle Greenhouse Gas Emissions From MAC systems.*

*Credit: Stella Papasavva, General Motors*

![](media/image8.jpeg){width="1.654724409448819in"
height="0.5090551181102362in"}

## 3.1 Energy Efficiency Is Key to Good LCCP

Energy efficiency is critical to good life-cycle climate
performance---an alternative MAC system should have low indirect
emissions from fuel use and low direct refrigerant emissions. A switch
to an alternative refrigerant would be counterproductive if it resulted
in an increase in tailpipe greenhouse gas emissions greater than the
reduction in greenhouse gas refrigerants. It is especially important
that the alternative refrigerant selected worldwide be energy efficient
because throughout the world, A/C energy use is currently unregulated.
Energy efficiency is not part of fuel efficiency standards and is not
yet fully accounted on fuel mileage labeling.

A/C is responsible for more than 5% of all motor fuel consumption each
year in the United States and for up to 30% of fuel consumed each year
in the hottest, most humid climates.[^3] For example, MAC fuel use in
India is estimated at up to 19.4% of total passenger vehicle
consumption.[^4] In the United States, A/C fuel use makes up the
majority of greenhouse gas emissions that result from MAC systems. In
the United States alone, vehicle air conditioners consume 7 billion
gallons of gasoline every year, equivalent to over 61 MMT of CO~2~.[^5]
(By comparison, HFC-134a refrigerant MAC emissions in the United States
are equivalent to approximately 53 MMT of CO~2~.[^6]) The impact of
vehicle A/C will increase significantly in the future because it is
quickly becoming a standard feature on vehicles sold throughout the
world. A/C is already standard in developed countries, and it is quickly
becoming standard in Brazil, China, India, and all other developing
countries with high growth in motor vehicle ownership.

##  

## 3.2 HFC-152a Is an Efficient, Low-GWP Option

HFC-152a secondary loop A/C systems offer attractive fuel economy
benefits in addition to large reductions in direct refrigerant
emissions. NREL has calculated a potential 21% reduction in A/C fuel use
for an HFC-152a secondary loop system that incorporates capacity control
to take full advantage of the higher thermal ballast available in
secondary loop systems.[^7] An added benefit of a secondary loop system
is that it enables controls that exploit vehicle inertia and allow
extended idle stop operation in hot weather. Idle stop requires
additional components beyond those of the secondary loop A/C system.

NREL and EPA estimate that secondary loop HFC-152a systems with capacity
control and extended idle stop vehicle operation can save up to 2.6
billion gallons of fuel per year for the United States alone when
implemented in every vehicle, eliminating 23.2 MMT CO~2~eq/year in
greenhouse gas emissions. At \$3.00/gallon, average fuel savings of 11.4
gallons/year/vehicle, and \$150 savings from longer refrigerant
containment prior to service, the added cost of secondary loop HFC-152a
systems in new vehicles can be rapidly paid back in hot climates.

## 3.3 Comparing the Climate Impacts of Alternative MAC Systems With Green-MAC-LCCP^©^ 

Past efforts to compare alternative MAC systems have been thwarted by
the lack of a single, globally accepted method to calculate MAC
greenhouse gas emissions. Different researchers used different
assumptions and methodologies, resulting in wide variation in
calculations. To enable an "apples-to-apples" comparison, General
Motors, JAMA, SAE, and EPA teamed up to develop a common methodology to
calculate the life-cycle climate performance of MACs with different
refrigerants. The effort was successful: The Global Refrigerants Energy
& Environmental Mobile Air Conditioning Lifecycle Climate Change
Performance (GREEN-MAC-LCCP)© model is now available from the
partnership and is online
([www.epa.gov/cppd/mac](http://www.epa.gov/cppd/mac)) in cooperation
with the United Nations Environment Programme Division of Technology,
Industry, and Economics (DTIE). This tool lets stakeholders compare
choices in a transparent manner. It is globally peer reviewed and
globally accepted, and is now the gold standard in LCCP credibility.

### 3.3.1. How LCCP Is Calculated for MACs

LCCP provides a holistic approach in estimating all greenhouse gas
contributions emitted during the lifetime of any MAC system, regardless
of refrigerant. The direct and indirect components of the model are
explained as follows:

**Direct Emissions** result from the direct leaks of the refrigerant
into the atmosphere and are evaluated based on the GWP of each chemical
and its mass emitted into the atmosphere. The direct emissions are
expressed in terms of CO~2~eq emissions and calculated based on the GWP
of the refrigerant (see box 1 above for an elaboration).

**Indirect Emissions** result from the energy consumption due to MAC
manufacturing, operation, and EOL, and is reported in terms of CO~2~eq
emissions, considering the carbon content of the fuel utilized in each
process and during vehicle operation (see box 1 above).

### 3.3.2. How to Use the Model

GREEN-MAC-LCCP© consists of many interlinked spreadsheets of data
required to run the model. Most of the input data are fixed based on the
harmonization process. This prevents tampering with the model and makes
sure that comparisons are genuine. Only a small amount of input data is
required to run the model. For each global alternative refrigerant, the
following information is required:

-   Component mass

-   Refrigerant mass and GWP values

-   Leakage rates

-   Coefficient of Performance (COP) and Qe data obtained from bench or
    vehicle tests

The model output provides the LCCP (in terms of CO~2~eq emissions) of
any global alternative refrigerant.

### 3.3.3. Preliminary Results

The GREEN-MAC-LCCP© model has been used to calculate the LCCP of
direct-expansion systems with R-134a, improved R-134a, R-152a, R-744,
and R-744 "with Orifice."[^8] Results are presented in the graph on the
next page. COP and Qe data for an optimized secondary loop HFC-152a
system were not available at the time the model was run. The Mobile Air
Conditioning Climate Protection Partnership is currently obtaining the
information necessary to run the GREEN-MAC-LCCP© model on secondary loop
HFC-152a MACs in order to compare them with other global alternatives.

![](media/image13.pct){width="7.43125in" height="4.740972222222222in"}

# 4. Flammability Risk of HFC-152a

Vehicle fires are a small proportion of the total of vehicle fatal
injuries and property damage (see table 1). One-tenth of one percent
(0.1%) of crashes result in fire, [^9] and most of the 4% of fatalities
in vehicles with fires are due to crash-related trauma.[^10] In
addition, it can be estimated that approximately 0.1% of registered
vehicles are subject to fire in a given year, the vast majority of which
are noncollision fires.[^11] Still, fires are perceived to be disturbing
in a manner that affords them greater vehicle safety attention than
their numbers would suggest.

For further context, it should be understood that the vast majority of
vehicle fire-related injuries and fatalities occur in crashes, and
because fires are usually slow events (relative to the time it takes to
exit a vehicle), fatalities occur primarily in cases of incapacitation
and entrapment. Losses due to property damage are of greater concern in
the heavy vehicle industry, where the vehicles themselves are more
costly. These vehicles more commonly have manufacturer options for fire
detection and suppression systems.

**Table 1: US Vehicle Fire Statistics**[^12]

  ----------------------- ----------------------- -----------------------
  **Condition**           **Number**              **Scale**

  Fatality and Fire       1,077                   3.8% of Fatals
  (2004)                                          

  Crash and Fire (2004)   10,739                  0.1% of Crashes

  Fires (\~98% noncrash,  257,000                 \$1.064 Billion
  2001)                                           
  ----------------------- ----------------------- -----------------------

As a result of the concerns about vehicle fires, research, design,
standards, and regulation continue to foster improvements in vehicle
fire safety. In recent years, the US National Highway Traffic Safety
Administration (NHTSA) has increased the stringency of FMVSS 301,
regulating leakage of gasoline and diesel fuel in collisions. NHTSA is
also evaluating improvements to FMVSS 302 (regulating the flammability
of vehicle interior components). In addition, the National Fire
Protection Association is considering a proposal for a standard designed
to increase the time for fire propagation to the vehicle interior. Each
of these (changes and proposals) can be taken as representative of the
nature of public concern and the direction of intended improvements.

At the same time, with fires a small percentage of the vehicle safety
problem, there is cause for moderation as well. Improvements in vehicle
fire safety may be advanced, for example, by increasing the use of
fire-retardant materials which in themselves may have health risks
worthy of deliberation. It is therefore prudent and necessary to
consider tradeoffs inherent in designs for fire safety with other health
and safety factors.[^13] It is in this context that consideration of
HFC-152a must be considered: While HFC-152a may have an additional
non-zero risk of fire (as compared to the overall existing non-zero
risk), it s use may also provide benefits that far outweigh those risks.

Fire safety research related to HFC-152a usage in MACs with secondary
loop systems has been initiated, and some studies have been published.
Additional studies have been conducted by manufacturers and are
considered proprietary at this time. At least one laboratory study of
the HFC-152a flammability properties concluded that it had potential for
explosivity[^14]; a full-scale vehicle test showed at least moderate
flammability and explosivity.[^15] The potential flammability of the
material warrants careful consideration as to its impact on such
important fire-safety issues as increase in under-hood fuel load;
propagation time to the interior; explosivity in crashes, enclosed
areas, and during service operations; safety of fire service personnel
when fighting vehicle fires; and specific risks in aging vehicles. But
the precise flammability characteristics of the material are not in and
of themselves the critical factor for vehicle safety; rather, vehicle
safety is achieved through the process of risk assessment, mitigation,
design, and standard setting. The EU/EPA project to remove barriers to
the use of all refrigerants satisfying the GWP 150 EC regulation
advocates SAE design, emissions, and service standards to achieve these
objectives.

While hydrogen is an undoubtedly explosive gas, its energy and
environmental benefits are so great that enormous research, design, and
standard-setting efforts are being expended on its development in
automotive applications.[^16] Just as in hydrogen fuel development, the
safe development of HFC-152a for MAC can be fostered through appropriate
identification of risks; testing of designs, components, material
properties, and full-scale vehicles; mitigation of identified risks; and
setting appropriate standards (See Appendix VIII-A -- SAE New and
Revised HFC-152a Documents).

# 5. Mitigation Strategy to Reduce Flammability Risk

## 5.1. Mitigation Adequacy

### 5.1.1. Fault Tree Findings From HFC-152a Risk Assessments

EPA and DoE risk assessments conducted in 1991 and 2006 found that the
risks of HFC-152a in direct expansion MAC systems were small and could
be mitigated with various engineering strategies. The secondary loop A/C
system was recommended as one of the most reliable and comprehensive
engineering strategies. Consequently, the risk for a direct expansion
system are not what would be expected for a secondary loop HFC-152a A/C
system, which carries significantly lower risk.

#### a) EPA Risk Assessment (2006)

An EPA risk assessment published in 2006 examined the passenger
compartment risks of primary loop direct expansion HFC-152a systems. The
fault tree analysis focused on passenger and service technician safety.
The conclusion of the EPA risk assessment was that there would be up to
"50 potential events with ignition in the passenger compartment each
year if the entire fleet of passenger vehicles were to use HFC-152a" in
direct expansion systems with comparable charge size and there was no
mitigation for safety.[^17] The analysis predicted that HFC-152a
ignition in the passenger compartment would not necessarily lead to
injury in all cases: "Such exposures might lead to injuries, but not
necessarily in all cases given the limited amount of flammable material
involved."[^18]

The EPA risk assessment recommended safety precautions to avoid even
these unlikely events, stating:

> "There are a number of viable engineering options that reduce the
> amount of charge that could potentially leak into the passenger
> compartment. The results of our modeling indicate that the addition of
> the squib valves/directed release system is one effective strategy for
> mitigating risk for both CO~2~ and HFC-152a systems.
>
> Other mitigation strategies include:

-   Secondary loops to eliminate the possibility of passenger exposure
    by separating the refrigerant from the passenger compartment.

-   Normally closed evaporator isolation valves that allow only a
    fraction of the total charge to be released into the passenger
    compartment in the event of a leak.

-   Close-coupled or hermetically sealed systems that would both serve
    to reduce charge size and decrease the possibility of a leak event.

-   Automatic increases in the air exchange in the passenger compartment
    upon detection of leaks.

-   Automatic venting of refrigerant to the outside upon detection of a
    large leak to the passenger compartment. (The squib valve mitigation
    strategy modeled in the computational fluid dynamic (CFD)
    simulations is one example of this strategy.)"

The fault tree analysis for the 2006 EPA risk assessment did not
calculate the risk of under-hood fires in HFC-152a systems because the
probability of ignition was determined to be very low and the
consequences of any ignition small.[^19] This safety determination is
supported by analytical findings and field experience of HC refrigerant
usage.

> "Available data from Australia, Canada, and other jurisdictions where
> motor vehicles have been \[recharged with HC\]
> refrigerants---generally propane, butane, isobutane or a mixture of
> them---have identified only three or four reports of fires possibly
> caused by the hydrocarbon refrigerant in a fleet estimated to be as
> large as several hundred thousand aged vehicles never designed for
> flammable refrigerants and with an un-quantified mix of aged and
> replacement parts. Hydrocarbons have a lower energy of ignition than
> HFC-152a, a higher energy of combustion, a lower LFL, and a higher
> flammability rating under the American Society of Heating,
> Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 34.
> These flammability characteristics make hydrocarbons such as propane a
> higher risk for ignition and sustained fire than HFC-152a. (See Annex
> III -- Critical Properties of Selected Hydrocarbons and HFC-152a.)
> Consequence modeling for HFC-152a systems indicates the need for
> safety mitigation engineering.
>
> "Engineering mitigation for hydrocarbon systems would necessarily
> require smaller charges, detection of smaller leaks, and a more rapid
> response to prevent potentially dangerous passenger exposures.
> Furthermore, it would probably be much more difficult to reliably
> identify hydrocarbon refrigerant leaks in ambient driving conditions,
> where other hydrocarbon sources are likely, than to identify HFC leaks
> where the only source of HFC is the air conditioning system. EPA has
> not identified a mitigation strategy that would allow for the safe use
> of hydrocarbon refrigerants, and so far, hydrocarbon mobile air
> conditioning advocates have not presented technical evidence that
> hydrocarbons can be safely applied in motor vehicles.
>
> "EPA conducted a global search of available data, including insurance
> and public media reports, and did not find any significant reports of
> fire or ignition from hydrocarbon refrigerants in MAC systems. The low
> number of fire events is further supported by the absence of known
> claims paid by refrigerant manufacturers or insurers and the lack of
> additional insurance premiums for HC use. In addition, as discussed
> above, HFC-152a has a higher flammability limit than hydrocarbons
> which provides an extra margin of safety."

#### b) Department of Energy (DoE Risk Assessment (1991)

A comprehensive DoE (Office of Transportation Technology) risk
assessment conducted in 1991 examined the risks of HFC-152a and two
other flammable refrigerants (propane and cyclopropane) in primary loop
systems. The fault tree analysis conducted in this study looked at
under-hood fire risks along with passenger exposures. It contained event
trees for the occupant compartment and engine compartment (see DoE risk
assessment). The conclusions of the DoE risk assessment is:

> **Conclusions**
>
> "The overall objective of this project was to develop a preliminary
> evaluation of the potential benefits of using a non-inert refrigerant
> in automobile air conditioning systems and to assess the increased
> safety risks associated with this class of working fluids. Three
> non-inert refrigerants were identified that are readily applicable to
> normal automobile practice \-- 2 near drop-ins, HFC-152a and HC-270
> (cyclopropane), and one with higher operating pressures, HC-290
> (propane) \-- and offer potentially significant environmental and
> energy efficiency benefits, at little or no additional costs.
> Flammability is the only hazardous characteristic that was identified
> for any of these three substances. Preliminary risk analyses indicate
> that the fire risk associated with these fluids in automobile air
> conditioning systems is modest, and reducible by modest design
> changes, in comparison to the general levels of fire, property damage,
> and injury risk associated with motor vehicle transportation. Overall,
> the potential benefits are significant relative to the risks, which
> are manageable, given focused efforts to address the issues raised by
> the work in this initial phase of the project."
>
> **Potential Benefits**
>
> "Increased gas mileage, compared to HFC-134a ranging between ½ MPG
> increase for HFC-152a and propane to 1 MPG increase for cyclopropane,
> for a typical compact car whose fuel consumption is 27.5 miles per
> gallon. For such a vehicle driven 15,000 miles per year, with the air
> conditioning operating one half of the time, a 1 MPG increase in fuel
> mileage while the air conditioner operates results in an annual saving
> of approximately 10 gallons of gasoline, reducing annual vehicle
> operating costs by (US) \$12 - \$15 (at 1993 gas prices)." At 2007 gas
> prices the savings are US \$25 - \$30.
>
> "Non-inert candidates HFC-152a, HC-290, HC-270 have less global
> warming impact than HFC-134a -- approximately equivalent to the
> reduced carbon dioxide emissions associated with a 1 to 4 MPG higher
> vehicle fuel efficiency level, depending on the level of refrigerant
> emissions. All three candidates have zero ozone depletion potential,
> as does HFC-134a."
>
> **Fire Risk**
>
> "Based on the results to date the safety of the vehicle occupant with
> HFC-152a, HC-270, HC-290 in conventionally designed systems will
> probably be acceptable, subject to further verification by additional
> field data collection and tests. The estimated injury risk level is
> .35 injuries per year per million automobiles in service. Among the
> bases of this estimate is a field survey of evaporator condition in
> cars that were severely damage by right side "A" pillar impacts. In
> view of the small size of the data sample (a total of nine such
> collision damaged cars were examined), a finite rupture probability
> (5%) was assumed for impacts of this severity level, despite the
> complete absence of either leakage or physical damage to these
> evaporators. A larger sampling needs to be taken to provide a solid
> quantitative basis for estimating the evaporator rupture probability.
>
> "Based on results to date, frontal collisions frequently (in about 1/3
> of severe frontal collisions) result in a significant refrigerant leak
> in conventional system. The range of estimated occurrence of
> refrigerant ignitions and secondary engine compartment fires is high
> enough to be of concern, but there is considerable uncertainty about
> the occurrence of ignition sources.
>
> "The modest set of design modifications to conventional automobile air
> conditioning systems described in Section 5-1 should result in a
> reduction of collision and non-inert refrigerant related fires to less
> than one engine compartment fire per year per million vehicles. This
> is less than one percent of the number associated with collision
> fires, and a negligible fraction of total vehicle fires (currently,
> about one in 20 vehicle fires are caused by collision damage, the
> remainder being caused by mechanical malfunctions, arson, etc.).
>
> "The estimated rate of fire and injury is so low compared to the
> existing levels of vehicle fires, vehicle collisions, and resulting
> injuries and fatalities, that the effect on experience based
> automobile insurance premiums can be expected to be negligible.
>
> "The risk assessment is based on extensive published data on motor
> vehicle collision frequency and distribution and frequency of
> collision damage extent. However, essentially no published data were
> found relating collision damage location and extent to air
> conditioning system rupture. Limited data relating these two were
> developed in this project, consistent with project resources, but more
> extensive data is needed to improve the statistical significance."

## 5.2. Safety-Mitigated Secondary Loop HFC-152a Systems Relative to Other Vehicle-Related Risks

### 5.2.1. Compared to Vehicle Fuel Flammability Risks

NHTSA's FMVSS 301 requires manufacturers of passenger vehicles, light
trucks, and school buses to mitigate fire risk by strengthening and
protecting a vehicle's fuel system so that, in a crash event, the
chances of fuel leakage, and consequently the chances of fire and
occupant injury, will be reduced.[^20]

FMVSS 301 sets three separate limits on fuel spillage from crash-tested
vehicles.

1)  It requires that no more than 28 grams (1 ounce) by weight of fuel
    be spilled during the time period beginning with the start of the
    impact and ending with the cessation of vehicle motion;

2)  It requires that no more than a total of 142 grams (5 ounces) by
    weight of fuel be spilled during the 5-minute time period beginning
    with cessation of vehicle motion; and

3)  It requires that no more than 28 grams (1 ounce) by weight of fuel
    be spilled during any 1-minute interval in the 25 minute period
    beginning with the end of the 5-minute period.

The test procedure prescribes standardized test methods for estimating
spillage.

In addition, there is a "rollover" fuel spillage test that requires
there be no more than 142 grams (5 ounces) of fuel leakage for the first
5 minutes of testing at each successive 90-degree increment when a
vehicle is rotated along its longitudinal axis. Fuel spillage for the
remaining test is required not to exceed 28 grams (1 ounce) during any
one-minute interval. Because HFC-152a is a pressurized gas, rotating a
vehicle has no impact on discharge rate.

Liquid automotive fuel spills often dissipate far slower than gaseous
spills. Although liquid fuel spills can have different hazards than an
equivalent discharge of a pressurized flammable gas, useful comparisons
can be made.

The 170 grams of gasoline allowed to be spilled during the time period
beginning with the start of the impact and ending 5 minutes after the
cessation of vehicle motion is equivalent to approximately 420 grams of
HFC-152a in terms of theoretical heat of combustion (43 MJ/kg for
gasoline and 17.4 MJ/kg for HFC-152a).[^21]

The heat of combustion of a 350 gram refrigerant charge of HFC-152a is
equal to the heat of combustion of 28 grams of gasoline that is allowed
to be leaked between the time of collision and the vehicle coming to
rest, and is about 20% of the allowed 142 gram leakage in the first five
minutes after the vehicle comes to rest.

A leakage of HFC-152a is less hazardous than the leakage of gasoline
with equivalent heat content in HFC-152a systems designed to prevent the
accumulation of flammable gaseous mixtures and promote safe dispersion
of the gaseous material.

## 5.3. Importance of SAE Standards to A/C Safety

SAE is completing a full set of standards for HFC-152a and CO~2~ to
ensure vehicle life-cycle safety of systems designed to satisfy the EC
regulation prescribing refrigerants with GWP less than 150. However,
there are no SAE standards under development at this time for the
recently proposed low-GWP global alternative refrigerants (GARs). It
will be important to implement design standards such as J-639 and safety
standards such as J-2773 and J-2772 for HFC-152a, CO~2~, or other
candidate refrigerants. Designers of HFC-134a systems will want to
locate components in well-ventilated spaces where any leaks can quickly
disperse. Designers of CO~2~ systems will want to locate components such
that leaks of hot, vaporized lubricant cannot be electrically or
thermally ignited. And for both systems it will be desirable to
implement new regulations requiring that legitimate replacement parts
satisfy SAE and original equipment manufacturer (OEM) safety standards,
and that no counterfeit parts enter the market.

# 6. Safety and Training Measures for MAC Service

The automotive service sector has more than a decade of experience
dealing with a multiple-refrigerant environment, beginning with the
transition from chlorofluorocarbon 12 (CFC-12) to HFC-134a in the early
1990s.

## 6.1. Unique Fittings and Labels, and Service Equipment and Tools

Unique service equipment and tools must be designed for each
refrigerant. Service practices and procedures, system diagnosis, and
repair might have to be modified and adjusted based on the individual
characteristics of each refrigerant.

Ensuring safe, efficient, effective service of mobile A/C systems with
alternative refrigerants begins with the global SAE standards that
address the following critical areas: (1) unique fittings and labels to
help prevent mixing of different refrigerants; (2) refrigerant
identification equipment; (3) leak detection equipment; and (4)
recovery, recycling, and charging equipment. Several new SAE Standards
are designed specifically for HFC-152a (see Appendix A -- SAE New and
Revised HFC-152a Documents).

Industry standards and government regulations require the use of unique
service fittings and labels for each refrigerant. However, fittings can
be defeated with adapters, labels might never be installed, and even
properly affixed labels can fall off or become illegible. For these
reasons, the use of a refrigerant identification device is critical to
prevent the spread of contaminated refrigerant to other vehicles. MAC
systems might contain CFC-12, HFC-134a, any of a number of
hydrochlorofluorocarbons (HCFCs), and/or HFC-containing blends, HCs, or
a mixture of all of the above.

Mixtures of refrigerants must be identified to prevent further spread of
the contaminated refrigerant, potentially damaging other vehicle A/C
systems through use of recovered and recycled refrigerant. Refrigerant
recovery and recycling equipment is designed to recycle only one
refrigerant for reuse. Recycling equipment will not effectively
segregate mixtures of refrigerants into their chemical components.

The use of the identifier is also important to warn a technician that a
hydrocarbon has been introduced into the A/C system. Certain types of
electronic leak detection devices (heated diode or corona discharge), as
well as motors, switches, and controls on recovery and recycling
machines, could serve as ignition sources in the presence of HC
refrigerants. Equipment has been designed to safely recover mixtures
containing HCs.

As with CFC-12 and HFC-134a, a technician must know which alternate
refrigerant is being handled, be trained to handle it safely, and be
equipped with the proper tools, often unique to a single refrigerant.

It should be understood that the refrigerant circuit is only one facet
of mobile A/C service and repair. Systems are becoming more and more
complex, with electronic controls and computers that are integrated with
other vehicle systems.

## 6.2. Service Technician Safety Precautions

General automotive maintenance, service, and repair present a host of
potential hazards that must be addressed to protect the safety and
health of the 680,000 technicians who maintain the 244 million passenger
vehicles and light trucks on U.S. roads today.

It is standard procedure to use lifts and jacks to hold vehicles
weighing thousands of pounds in the air while a technician works below
the suspended vehicle.

Hazards also abound under the hood, including exposed belts, pulleys,
and fan blades (fans may start with ignition off). In addition to these
sources of physical trauma, the under-hood landscape includes numerous
potential sources for burns or shocks.

Another hazard is addressed by Tom Revnyak of Airbag Logic: The safety
of technicians can be seriously jeopardized if an airbag deploys
unexpectedly during vehicle service. Today's vehicles are equipped with
"smart" and dual-stage airbags. Technicians must become familiar with
the hazards posed by accidental deployment, and the necessary
precautions to prevent them. Airbags are located in many areas in
today's vehicle interiors, such as driver, passenger, knee, side-impact
door or seat, and curtain types. Technicians must understand the
complexities involved with each system to ensure maximum safety.

A large variety of power tools, with their inherent dangers, are in use
in all automotive service shops.

Each shop uses a wide variety of solvents, cleaners, and paints that
present safety issues that must be addressed.

Automobile exhaust, gasoline, and chemical fumes may be present in
service bays, so air quality must be addressed. Proper ventilation rates
and appropriate sensors are required to ensure safety in the service
environments.

MACS conducted a survey that indicated that most mobile A/C shops use
flammable gases in their operations. Acetylene was in use in 82% of the
shops, propane in 18%. Open flame operations were conducted in many of
the shops, including brazing (47%), cutting (29%), soldering (14%), and
welding (65%).

The MAC service sector is currently dealing effectively with multiple
refrigerants. Given the proper information, training, and tools, this
sector will successfully service and repair any MAC refrigerant system
chosen by the vehicle manufacturers.

# 7. Model State Legislation to Safely Allow HFC-152a Systems

The following model language is proposed by the Mobile Air Conditioning
Climate Protection Partnership to safely allow HFC-152a vehicle A/C
equipment.

"Mechanical vapor compression refrigeration equipment which is used to
cool the occupied compartment(s) of any on- or off-road motor vehicle
shall be manufactured, installed and maintained with due regard for the
safety of the occupants of the vehicle and the public in accordance with
Society of Automotive Engineers Standard J-639, Safety Standards for
Motor Vehicle Refrigerant Vapor Compression Systems, and other SAE
safety standards now under development, and shall only contain
refrigerant listed as acceptable by the U.S. Environmental Protection
Agency Significant New Alternatives Policy (SNAP) Program."

**Case Studies of Alternative Text**

Three states have revised their laws to allow use of HFC-152a while
still protecting the public against hazardous refrigerants:

**Montana** regulations state: "Aair-conditioning equipment may contain
only refrigerant that has been included in the list published by the
United States Environmental Protection Agency as a safe alternative
motor vehicle air-conditioning substitute for chlorofluorocarbon-12
pursuant to 42 U.S.C. 7671k(c)."

**Wisconsin** regulations provide that "flammable refrigerant" means a
"substance containing butane, propane, mixtures of butane and propane,
or other gaseous hydrocarbons when used or intended for use as
refrigerants in motor vehicles." Flammable refrigerants are banned in
vehicles "whose mobile air conditioners were not designed and
manufactured to use flammable refrigerants."

**Arizona** regulations provide a combined approach. As in Montana, the
regulations provide that refrigerants approved by EPA are acceptable. As
in Wisconsin, the regulations provide that flammable refrigerants must
be used in A/C systems designed for their safe use. In addition, the
regulations provide a set of suggested standards that can be used to
comply with the requirement.

# 8. Appendices

## 8.1. SAE New and Revised HFC-152a Documents

SAE supports introduction of alternate refrigerant HFC-152a with the
following new or revised SAE documents addressing the design and service
requirements.

### 8.1.1. Design Requirements

Revisions for secondary loop (additions to J639)

-   The coolant shall be comparable in toxicity and flammability to
    vehicle antifreeze

-   The coolant specification shall be: xxxxx

    -   The coolant color shall be: xxx

    ```{=html}
    <!-- -->
    ```
    -   The recommended coolant mixture shall be xx % ingredient 1 and
        yy % distilled water

-   The coolant fill identification

    -   The coolant reservoir shall have a cap that incorporates

        -   A snowflake design molded in cap

        ```{=html}
        <!-- -->
        ```
        -   To prevent any coolant contamination from the refrigerant
            loop, the cap shall have a venting device that will release
            any vapor pressure above XXXX in the coolant system

-   The label required for J639 shall also include the coolant type and
    recommended mixture ratio

HFC-152a refrigerant purity and container requirements (new J Doc.)

-   30-pound containers shall have the male version of the HFC-152a low
    side quick couple service fitting

Coupled flexible refrigerant hose assemblies

-   Modification of J2064 or new document

Design criteria for coolant system fittings and connections

### 

### 8.1.2. Service Sector

Technician training/certification

-   This document shall address:

    -   Technician service procedures patterned after SAE J2211 --
        Recommended Service Procedure for the Containment of HFC-134a

        -   This document covers the technician refrigerant recovery/
            recycling procedures when servicing HFC-134a mobile A/C
            systems and identification of excess noncondensable gases
            (NCGs).

-   Possible certification program requirements for a service technician
    to handle flammable substances

Recovery/recycling equipment

-   New document with guidelines for refrigerant removal and charging
    developed from J2788

-   This document shall address:

    -   Service equipment that recovers/recycles HFC-152a and addresses
        the equipment certification to handle a flammable refrigerant

        -   Prototype equipment has been developed and commercial
            equipment is currently under development

-   "Standard of Purity for Recycled HFC-152a for use in Mobile Air
    Conditioning Systems"

-   "Retest of Refrigerant Cylinder" -- refrigerant containers used with
    recovery and recovery/recycle equipment must be inspected every 5
    years to assure their safety

Equipment service hose and fittings

-   New document(s) patterned after J2196 and 2197

Leak detection tools

-   Trace dye requirements

    -   Potential revision of J2297, 2798, and J2299 or new document(s)

-   Electronic detection

    -   Potential revision of J2791 or new document

Technician use of leak detection equipment

-   Revision of J1628 addressing technician procedures to identify
    HFC-152a leakage using electronic leak detectors

-   Revision of J2298

Review of SAE J1771 -- Criteria for Refrigerant Identification Equipment
for Use With Mobile Air Conditioning Systems for Use With HFC-152a
Refrigerant

## 8.2. HFC-152a Chemical and Physical Properties[^22]

> IUPAC Name: 1,1-difluoroethane
>
> CAS Number: 75-37-6
>
> Chemical Formula: CH~3~CHF~2~
>
> Molecular Weight: 66.05 g/mole
>
> Boiling Point: --24.0^o^ C
>
> Critical Temperature: 113.3^o^ C
>
> Critical Pressure: 4.52 MPa
>
> Lower Flammability Limit: 4.8 vol%
>
> Upper Flammability Limit: 17.4 vol%
>
> Heat of Combustion: 17.4 MJ/kg
>
> ASHRAE Safety Group: A2
>
> Atmospheric Lifetime: 1.4 years
>
> Ozone Depletion Potential: 0
>
> GWP (IPCC 2^nd^ Assessment): 140
>
> Volatile Organic Compound: No

+-----------------------------------------------------------------------+
| Kyoto Protocol Regulatory GWPs for Hydrofluorocarbons (1995 Second    |
| Assessment)[^23]                                                      |
+-----------------------------------------------------------------------+
|   -----                                                               |
| --------- --------------- ----------- ---------- ---------- --------- |
|   Species        Chemical        Lifetime    GWP (Time                |
|                  formula         (years)     Horizon)                 |
|                                                                       |
|                                                                       |
|                                       20 years   100 years  500 years |
|                                                                       |
|   H                                                                   |
| FC-23         CHF~3~          264         9,100      11,700     9,800 |
|                                                                       |
|                                                                       |
|  HFC-32         CH~2~F~2~       5.6         2,100      650        200 |
|                                                                       |
|   HFC-41         CH~3~F          3.7         490        150        45 |
|                                                                       |
|                                                                       |
|  HFC-43-10mee   C~5~H~2~F~10~   17.1        3,000      1,300      400 |
|                                                                       |
|                                                                       |
|  HFC-125        C~2~HF~5~       32.6        4,600      2,800      920 |
|                                                                       |
|                                                                       |
|  HFC-134        C~2~H~2~F~4~    10.6        2,900      1,000      310 |
|                                                                       |
|                                                                       |
|  HFC-134a       CH~2~FCF~3~     14.6        3,400      1,300      420 |
|                                                                       |
|   HFC-152a       C~2~H~4~F~2~    1.5         460        140        42 |
|                                                                       |
|   HFC-143        C~2~H~3~F~3~    3.8         1,000      300        94 |
|                                                                       |
|   H                                                                   |
| FC-143a       C~2~H~3~F~3~    48.3        5,000      3,800      1,400 |
|                                                                       |
|                                                                       |
|  HFC-227ea      C~3~HF~7~       36.5        4,300      2,900      950 |
|                                                                       |
|   H                                                                   |
| FC-236fa      C~3~H~2~F~6~    209         5,100      6,300      4,700 |
|                                                                       |
|                                                                       |
|  HFC-245ca      C~3~H~3~F~5~    6.6         1,800      560        170 |
|   -----                                                               |
| --------- --------------- ----------- ---------- ---------- --------- |
+-----------------------------------------------------------------------+

For thermophysical properties of saturated liquid refrigerant and
saturated vapor HFC-152a, see:

> American Society of Heating, Refrigerating and Air-Conditioning
> Engineers, 2001 ASHRAE Handbook, Fundamentals, I-P Edition, page
> 20.21. ASHRAE, Atlanta, GA. 2001.

## 8.3. HFC-152a Flammability Properties

Flammability Limits: LFL = 4.2%; upper flammable limit (UFL) =
17.8%[^24]

> Tested according to ASTM E681-01
>
> "Standard Test Method for Concentration Limits of Flammability of
> Chemicals" (15,000V ignition source with electrodes at 6.3 mm)

Minimum Ignition Energy (MIE) \~= 0.35 mJ at 11% concentration in
air[^25]

Tested according to ASTM E582-88

> "Standard Test Method for Minimum Ignition Energy and Quenching
> Distance in Gaseous Mixtures" (high voltage capacitive discharge with
> electrode gap of 2.0--2.3 mm).

Electrical ignition of combustion at 530mJ and above arcing in relays

Tested with 20-liter stainless steel vessel

Relays without covers or with holes and slots in covers

Wire disconnections under load

No electrical ignition from components due to current commutation

HVAC blower, alternator, and starter stalled for 5 minutes

Ignition of HFC-152a refrigerant/oil mixture sprays in 4 to 59 seconds

Tested according to ASTM D 3065-72

> "Standard Test Method for Flammability of Aerosol Products, Part
> Three: Closed Drum Test," (55 gallon drum on its side)
>
> 95% R-152a liquid with 5% PAG
>
> 99% R-152a liquid with 1% PAG
>
> 99% R-152a vapor with 1% PAG

Ignition of HFC-152a refrigerant/oil mix on hot surface at 500° C

No ignition of HFC-152a refrigerant alone (up to 850° C)

Sprayed onto 0.5-inch thick, 8.0 inch x 8.0 inch heated vertical steel
plate

Steady state gas-phase flame extension at up to 0.07 grams/second

With nozzle diameters = 0.043 and 0.072 for HFC-152a

> 95% to 100% liquid R-152a with corresponding PAG
>
> 75% to 100% vapor R-152a with corresponding PAG
>
> 75% vapor & 95% liquid R-134a with corresponding PAG

Limited range stable liquid-phase jet flame at 0.3 to 1.5 grams/second

With nozzle diameters = 0.043 and 0.072 for HFC-152a

> 95% to 100% liquid R-152a with corresponding PAG
>
> 75% to 100% vapor R-152a with corresponding PAG
>
> 75% vapor & 95% liquid R-134a with corresponding PAG

No significant flame impingement on materials

No ignition of coolant overflow container at 100° C after 10 seconds jet
flame

If ignited, plastic overflow container will burn when empty

> 95% to 99% R-152a vapor with corresponding PAG
>
> 95% to 99% R-152a liquid with corresponding PAG

Frictional Spark Ignition of Refrigerant Spray

> Sparks from 1018 steel on grinding wheel did not ignite liquid or gas
> jets as tested

95% R-152a liquid and 5% corresponding PAG

95% R-152a vapor and 5% corresponding PAG

Ignition Potential of Cigars and Cigarettes

No ignition observed from cigarettes, little cigars, and regular-size
cigars as tested with continuous aspiration, intermittent aspiration,
and without aspiration

11% R-152a in air

## 8.4. Flammable Refrigerant Fault Tree Analysis

*Source: Fault Trees Summary from 2006 EPA Risk Assessment*[^26]

*3.1 Assumptions and Inputs to Fault Trees*

Fault Tree Analysis (FTA) is a methodology that graphically presents the
sequences and combinations of failures that lead to a particular
outcome. It utilizes a "top-down" approach, starting with the effect and
seeking potential causes. This technique analyzes the potential
combinations or sequences of events, focusing on the most credible means
by which an undesired event may occur. However, the analysis does not
cover those causes of system failure that cannot be envisioned by risk
analysts or that are not considered credible. FTA can consider effects
of human error, equipment failure, and operating and maintenance
procedures.

FTA provides insight on failure sequences and serves as a guide for
answering questions on which system is more likely to be a source of
trouble. Fault trees display the relative significance of the cause of a
particular event and the importance of one top event as opposed to
others. Not all fault trees are quantified, and absolute accuracy in the
quantification is often secondary to the identification of failure
sequences and the determination of the effectiveness of various
mitigation measures, particularly in daily safety management activities.

In the analysis, fault trees were constructed to specifically address
the adverse health effects associated with exposure to CO~2~ and the
potential for flammability associated with release of HFC-152a. Fault
trees were prepared for the release cases shown in the high-level fault
tree given below. Each of the letters below a box indicates a more
detailed fault tree that has been developed for the event in that box.

**Fault Trees for HFC-152a and CO~2~ Release With Injury Potential**

![](media/image14.png){width="4.865277777777778in"
height="1.5868055555555556in"}

The full set of fault trees covers a range of operating speeds and A/C
modes (the details for the various speeds consider situations where the
A/C is on as well as where the A/C is off), collision scenarios, and
repair scenarios. The structures of the original fault trees were
revised as the modeling determined likely refrigerant concentrations in
the passenger compartments for various scenarios. For instance, certain
fault tree branches for CO~2~ systems depicting long intervals with no
outside air were deleted when it was determined that the peak
concentration would exceed 6% for one or two minutes---thus a
concentration of concern exists whether the occupants are in the vehicle
for just a few minutes or for a full hour. The consequence modeling has
shown that if the release is not mitigated, there can be unacceptable
CO~2~ exposures or the buildup of flammable HFC-152a concentrations
within the passenger compartment in the event of a hole (in the
evaporator) equal to or greater than the area of the fixed orifice tube
opening. Thus, the actual frequency of the various events is not
critical to determining the need for mitigation as it is clear that any
moderate release while in full recirculation mode is undesirable.
Specific inputs to the fault trees and the fault trees themselves are
given in Annex VI -- Fault Tree Analysis (attached).

*3.1.1 Ignition Probabilities for HFC-152a*

For an energy source to cause ignition, it must either exceed the
minimum ignition energy of the material, or be hot enough to cause
auto-ignition. A literature review was conducted to identify sources
with the potential to ignite propane and methane. The ignition energy of
HFC-152a is slightly higher than that of propane and methane. Therefore,
this information was used to extrapolate which ignition sources could
cause HFC-152a to ignite within its flammability limits.

Propane is flammable in a concentration range in air of 2.2% to 9.5 %
v/v. Propane has a minimum spark ignition energy of around 0.25 mJ in
air at 1 atm pressure, compared to methane at 0.29 mJ.[^27] Since the
energy required to ignite the two materials is similar, any source that
ignites methane is likely to ignite propane.

Ignition sources can be categorized into four main types, including
electrical, open flames, hot surfaces, and friction. Table 5 summarizes
the main sources known to ignite propane, identified during the
literature review.

Potential Ignition Sources

  ----------------------------------- -----------------------------------
  **Type of Ignition Source**         **Sources With Potential to
                                      Ignite**

  Electrical                          Arc, electrostatic spark, high-load
                                      electrical switching, damaged
                                      electrical components

  Open Flame                          Match/lighter, gas pilot, fuel
                                      heater, open fire

  Hot Surface                         Heater element

  Miscellaneous                       Frictional spark
  ----------------------------------- -----------------------------------

More detailed information on each type of ignition source is shown
below.

-   A study by the Arthur D. Little Company for the Gas Research
    Institute investigated ignition by electrical circuits in a
    stoichiometric mixture of methane/air.[^28] While ignition did not
    occur for a typical electric wall clock, vehicle radio, CB, or a
    25-watt incandescent lamp, ignition did occur for 100-watt
    incandescent lamps, traffic lights, toasters, space heaters, clothes
    dryers, and arc welding.

-   The same study also discussed ignition by automotive electrical
    components in a 7% methane in air environment. This testing
    concluded that ignition is not caused by any normal operation of the
    electrical system (i.e., by the alternator, starter relay,
    distributor, and spark plug in housing). Ignition only occurred when
    either a loosened starter wire or a broken ignition wire was used.

-   Ignition of propane mixtures by electrostatic sparks from human
    bodies was reported in a number of sources. Common movements such as
    walking across a carpet, getting out of a vehicle, or removing a
    garment can generate and induce enough charge on the body for any
    subsequent discharge to have sufficient energy to ignite flammable
    gases and vapors. The charging characteristics of clothing, shoes,
    rugs, and carpet play a critical role in determining the likelihood
    of producing a spark that could ignite flammable vapors and gases.
    However, another source tested ignition of a stoichiometric mixture
    of natural gas and air by a spark generated from a human body.[^29]
    The lowest voltage at which ignition was obtained was 6.0 kV with a
    corresponding energy of 1.7 mJ. This is 4.3 times the minimum
    ignition energy of natural gas which is 0.39 mJ. This suggests that
    when sparks are the potential ignition source, more energy is
    required than for some other ignition sources.

-   Ignition by cigarettes is not considered a credible event based on
    ignition testing with 7% methane in air. Three brands were tested to
    cover a wide range of chemical tobacco treatment and curing which
    could affect the burning rate and local temperatures. Both a puffing
    and a nonpuffing cigarette did not cause ignition. However, the
    match or lighter used to light the cigarette could cause
    ignition.[^30] One reference[^31] discusses ignition of propane by
    heated surfaces and also by frictional impact between tungsten
    carbide-tipped steel and sandstone. A coal-cutting pick striking
    sandstone also ignited propane-air mixture when the temperature on
    impact exceeded 1400^o^ C. These hot surfaces are not found inside
    the passenger compartment and therefore were not included as
    ignition sources in the fault trees.

-   Underwriters Laboratories found that there was a very low
    probability of ignition from a leaked hydrocarbon refrigerant from
    household appliances. There are few readily available ignition
    sources that would occur at the same time as a leak in the space
    where the fuel/air mixture is between the LFL and the UFL.

Considering all of the above, the fault trees only consider open flames
(matches, butane lighters) as potential ignition sources in the vehicle
passenger compartment. Static electric sparks were not considered
credible ignition sources both because research shows that the minimum
spark energy generated from a human body was four times higher than the
minimum ignition energy of methane. Further, a seated passenger is
highly unlikely to generate a spark since we would not expect a buildup
of charge while the occupant is sitting in the vehicle.

It should also be noted that these past results also support the
decision to limit the risk analysis to leaks within the passenger
compartment. In the future it is possible that other credible sources of
ignition, such as electric heating elements, could be added to the
passenger compartment. The fault tree does not consider any new ignition
sources that are not currently in standard vehicle designs.

*3.2 FTA Results*

The detailed fault trees are presented in Annex VI -- Fault Tree
Analysis, along with descriptions of their quantification. The overall
results for the different scenarios are shown below. The fault trees for
each refrigerant (HFC-152a and CO~2~) both assume that the full fleet of
US passenger vehicles is using that refrigerant.

Since HFC-152a scenarios require ignition to have the hazardous exposure
occur, the frequencies are lower than for the corresponding CO~2~
scenarios. However, the occurrence of a CO~2~ exposure will not always
result in adverse consequences. Such consequences will depend on the
health of the individual, the exact air flow patterns in the specific
vehicle, whether a window is opened or outside air is introduced through
the ventilation system, etc. The CFD modeling for CO~2~ predicted only
very short duration exposures to high concentrations.

Furthermore, all predicted frequencies are considered to be conservative
(upper range estimates of worst-case scenarios).

**Projected Unmitigated Leak Scenario\
Occurrence Frequencies for HFC-152a Systems**[^32]

  ----------------------------------- -----------------------------------
  Scenario                            Frequency of Exposure to Large Leak
                                      With Ignition Source Present

  Technician Exposures While          1,600/yr
  Servicing                           

  Passenger Compartment Exposures     50/yr

  Stationary/Low-speed Exposures      2.5/yr

  Exposures While Moving              50/yr

  Exposures While in Defrost Mode,    0.09/yr
  Heater on                           

  Exposures Due to Collision          0.4/yr
  ----------------------------------- -----------------------------------

The fault tree analysis identified a number of credible pathways
resulting in undesirable risk from refrigerant released due to equipment
failure or collision. Routine or collision failures of the evaporator
are not likely, but the risk of this occurring becomes significant when
applied across the full fleet of passenger vehicles.

Therefore, mitigation strategies to limit or eliminate the amount of
charge that can leak into the passenger compartment are necessary. In
addition, the total risk resulting from a leak into the passenger
compartment is dominated by the risk that a driver is startled by a
sound from the refrigerant release. It was estimated that 50% of drivers
were startled and that 10% of these drivers lost control and had an
accident. This scenario accounts for more than 90% of the "exposures" in
the HFC-152a systems and for about 40% in the CO~2~ systems. Because
there is no information on whether these systems are likely to rupture
with a large and potentially startling noise, conservative estimates
were selected. It is likely that the actual occurrence of these events
would be much lower than calculated. The chance of an undesired exposure
while servicing vehicles containing these refrigerants depends not only
on the number of vehicles a given service technician or shop handles,
but also on their experience and training.

However, with proper mitigation (see the 2006 EPA Risk Assessment's
Annex V -- Critical Findings of MACS Meeting on August 28, 2003 for
suggestions), the frequency of these exposures can be reduced
dramatically. The estimated exposures are distributed across several
hundred thousand technicians. Interestingly, the chance of exposures for
technicians does not vary considerably by refrigerant.

## 8.5. Annotated CalTech Glossary on Explosion Dynamics

California Institute of Technology, Explosion Dynamics Laboratory
(Sponsored by Sandia National Laboratory and the DoE (SC))

Some of the language used to describe explosions is very precise and
scientific; other terms, including the very nature of an explosion
itself, are quite ambiguous. The following list of terms is not intended
to be exhaustive but simply to provide an explanation for some of the
most commonly used concepts.

**Autoignition temperature (AIT)** The temperature that a fixed volume
of fuel-oxidizer mixture must be heated to before an explosion will take
place without an external ignition source, i.e., spark or flame.

**Burning speed** The speed with which a smooth (laminar) flame advances
into a stationary mixture of reactants. Burning speeds in hydrocarbon
fuels mixed with air are typically less than 0.5 meters/second (m/s).
The burning speed is a function of the concentration of the fuel,
temperature, and pressure of the mixture.

**Explosion** There is no fixed definition of an explosion. Events that
are described as explosions include a rupturing water boiler, a flash of
light created by an electrical short circuit, detonation of a high
explosive, deflagration of a tank containing an explosive fuel-air
mixture, or the shock wave, fireball, and debris cloud produced by a
thermonuclear detonation.

**Explosion limits** Explosion limits usually refer to the range of
pressure and temperature for which an explosive reaction at a fixed
composition mixture is possible. The reaction is usually initiated by
autocatalytic (sometimes called self-heating) reaction at those
conditions, without any external ignition source.

**Expansion ratio** Ratio of burned gas volume to initial volume for a
low-speed (constant pressure) flame. Expansion is responsible for
flame-induced flow.

**Fire** This is a flame that is produced over a stationary fuel source
such as a liquid hydrocarbon pool or solid such as wood.

**Flame** This is a thin zone of combustion in which diffusion plays a
dominant role. Flames in HC fuels and air are less than 0.1 mm thick for
stoichiometric mixtures.

**Flame acceleration** Rapid increase in flame speed due to generation
of large and small eddies---turbulence---as flow ahead of flame passes
over objects or through orifices.

**Flame speed** The speed with which a flame, possibly turbulent,
appears to move relative to a stationary observer. The flame speed can
be much larger than the burning velocity due to expansion of the
combustion products, instability, and turbulent deformation of the
flame.

**Flame Stretch** Measure of the rate at which the area of a propagating
flame surface is changing due to curvature of flame surface and strain
(gradients in velocity) in flow ahead of the flame. Units of reciprocal
time.

**Flame Thickness** Characteristic width of flame. One simple estimate
is based on the ratio of the thermal diffusivity to the fame speed.

**Flammability limits** A fuel-air mixture is flammable when combustion
can be started by an ignition source. The main factor is the proportions
or composition of the fuel-air mixture. A mixture that has less than a
critical amount of fuel, known as the Lean or Lower Flammability Limit
(LFL), or greater than a critical amount of fuel, known as the rich or
Upper Flammability Limit (UFL), will not be flammable.

**Flash point** This is the minimum temperature at which the vapor above
a liquid fuel will first support a combustion transient or \"flash\".
The legal description of flammable is used for all liquids with a flash
point less than 100 deg C, and the term combustible is used for liquids
with a flash point in excess of 100 deg C.

**Fuel-air mass ratio** This is the ratio of the mass of fuel to the
mass of air in the reactants. The fuel-air ratio is a method of
measuring the composition of a potentially flammable mixture.

**Heat of combustion** The ideal amount of energy that can be released
by burning a unit amount of fuel. This is between 45 and 50 MJ/kg for
kerosenes.

**Minimum Ignition Energy** This is the lowest possible energy that will
result in the ignition of a flammable mixture by an electrical
discharge. The minimum ignition energy depends on the composition of the
mixture and can be as low as 200 microJoules for many common HC fuels.

**Overpressure** The pressure in excess of the ambient value that is
created by the explosion process. The peak overpressure associated with
deflagrations inside closed vessels can be as high as 10 times the
initial pressure.

**Partial pressure** The pressure created by one component of a gas
mixture. The partial pressure of fuel vapor in a well-mixed ullage over
a liquid fuel layer is equal to the vapor pressure of the liquid under
those conditions.

**Pool Fire** The flame over a puddle or pool of liquid fuel. The heat
released by the combustion of the vapor fuel supplies the energy to
vaporize the liquid.

**Quenching** The cessation of combustion due to either heat transfer
and mass transfer to the surface or aerodynamic effects such as strain
fields and rapid mixing.

**Stoichiometric ratio** The proportions of fuel and oxidizer that will
result in optimal combustion are known as a stoichiometric ratio. The
optimal ratio is determined by finding the amount of air that will
result in the products of the combustion reaction containing only water
and CO~2~ with no left over oxygen.

HFC-152a Technical Literature Database (CD)

A CD of HFC-152a Technical Literature is available from EPA upon
request. To request a CD, send your name and address to Kristen Taddonio
by e-mail at Taddonio.kristen@epa.gov, or call 202-343-9234.

[^1]: GWPs are determined as the ratio of the time-integrated radiative
    forcing arising from a pulse emission of 1 kilogram (kg) of a
    substance relative to that of 1 kg of CO~2~, over a fixed time
    horizon (typically selected by policymakers as 100 years).

[^2]: See: FCCC/SBSTA/2004/8,
    http://unfcc.int/resources/docs/2004/sbsta/08.pdf.

[^3]: Andersen, Hovland, and Rugh (2004). Significant Fuel Savings and
    Emission Reductions by Improving Vehicle Air Conditioning. NREL,
    Boulder, CO.

[^4]: Chaney, L.; Thundiyil, K.; Chidambaram, S.; Abbi, Y.P.; Andersen,
    S. "Fuel Savings and Emission Reductions From Next-Generation Mobile
    Air Conditioning Technology in India." Eighth Vehicle Thermal
    Management Systems Conference Proceedings; May 20--24, 2007. Paper
    No. C640/085/2007.

[^5]: EPA. Retrieved August 30, 2007, from http://www.epa.gov/cppd/mac/;
    conversion of 7 billion gallons of gasoline into CO~2~-eq done at
    http://www.usctcgateway.net/tool/.

[^6]: EPA (April 15, 2007). Inventory of U.S. Greenhouse Gas Emissions
    and Sinks: 1990--2005. Washington, DC. Pages 2--28 and 2--29.

[^7]: Capacity controls for direct expansion systems can capture some of
    these fuel saving benefits.

[^8]: The orifice is a fixed expansion device. It is typically simpler
    and cheaper than a thermostatic expansion valve or other devices
    able to regulate the expansion pressure drop depending on the loop
    working conditions. Normally the orifice is selected to guarantee
    the best performance in a specific working point (e.g. severe summer
    conditions), with the consequence that the system is likely less
    efficient at other operating conditions (e.g. mild temperature
    conditions).

[^9]: Ray, R., Ahrens, M., "Vehicle Fire Data: Different Sources,
    Different Goals, Different Conclusions?" SAE 2007-01-0877.

[^10]: Ibid and Davies, B. T., Griffin, L. I., "A Clinical Evaluation of
    the Death Investigations for 206 Decedents Who Died in Passenger
    Vehicles That Experienced Post-Crash Fires," NHTSA docket number
    98-3588-170, 2002.

[^11]: Shields, L., Scheibe, R., Thomas, T., "Vehicle Design for Fire
    Safety and Evaluation of Design Trade-offs," SAE 2007-01-0879.

[^12]: Ray, R., Ahrens, M., "Vehicle Fire Data: Different Sources,
    Different Goals, Different Conclusions?" SAE 2007-01-0877.

[^13]: Shields, L., Scheibe, R., Thomas, T., "Vehicle Design for Fire
    Safety and Evaluation of Design Trade-offs," SAE 2007-01-0879.

[^14]: Heckenberger, T., "Flammability Properties of R152a versus
    Hydrocarbons," SAE 2005-01-2042.

[^15]: Korb, T., Colwell, J., "R152a Refrigerant Concentration and
    Ignition Tests in Full-Scale Vehicle Passenger Cabins," Prepared for
    SAE, February 2007.

[^16]: Scheffler, G., DeVaal, J., Kissel, G., Schneider, J., Veenstra,
    M., Chang, T., Warner, N., Chernicoff, W., "Developing Safety
    Standards for FCVs and Hydrogen Vehicles," SAE 2007-01-0436.

[^17]: Blackwell, N., Bendixen, L. and Birgfeld, E. (September 2006)
    Risk Analysis for Alternative Refrigerant in Motor Vehicle Air
    Conditioning. EPA, Washington, DC, p. 35.

[^18]: Ibid.

[^19]: Blackwell, N., Bendixen, L., and Birgfeld, E. (September 2006)
    Risk Analysis for Alternative Refrigerant in Motor Vehicle Air
    Conditioning. EPA, Washington, DC, p. 63.

[^20]: This standard applies to vehicles with gross vehicle weight
    ratings (GVWR) of 4,536 kg (10,000 pounds) or less and includes
    frontal, rear, and side impact tests. In the frontal impact test,
    the test vehicle is driven forward into a fixed barrier at 48 km/h
    (30 mph). In the rear impact test, a 1,814 kg (4,000 pound) barrier
    moving at 48 km/h (30 mph) is guided into the full width of the rear
    of a stationary test vehicle. In the side impact test, a 1,814 kg
    (4,000 pound) barrier moving at 32 km/h (20 mph) is guided into the
    side of a stationary test vehicle.

[^21]: Some authorities list the actual heat of combustion of R-152a as
    6.3 MJ/kg, versus the theoretical 17.4. At 6.3 MJ/kg., the fire risk
    of a full discharge of 350 grams of HFC-152a would be one-third of
    the allowed gasoline spill prior to the vehicle coming to rest, and
    less than 10% of the allowed gasoline spill in the first five
    minutes after collision.

[^22]: Calm, J.M., and Hourahan, G.C., "Refrigerant Data Update,"
    Heating/Piping/Air Conditioning Engineering, 79 (1), 50--64, January
    2007, with the exception that the GWP is the IPCC 2^nd^ Assessment
    100-year value.

[^23]: Climate Change 1995, The Science of Climate Change: Summary for
    Policymakers, and Technical Summary of the Inter-Governmental Panel
    on Climate Change Working Group I Report, p. 22, Cambridge
    University Press.

[^24]: Determination of Refrigerant Lower Flammability Limits in
    Compliance with Proposed Addendum p to Standard 34, David P. Wilson
    and Robert G. Richard, ASHRAE Transactions 2002, Volume 108, Part 2,
    p.746--747.

[^25]: International Symposium of Hazards, Prevention and Mitigation of
    Industrial Explosions, Christian Michelsen Research AS, Bergen,
    Norway, 26--28 June 1996.

[^26]: The complete fault tree summary is available in Blackwell, N.,
    Bendixen, L., and Birgfeld, E. (September 2006) Risk Analysis for
    Alternative Refrigerant in Motor Vehicle Air Conditioning. EPA,
    Washington DC.

[^27]: The Investigation and Control of Gas Explosions in Buildings and
    Heating Plant, R.J. Harris, ISBN 0-419-13220-1, 1983.

[^28]: Ignition sources of LNG \[liquefied natural gas\] vapor clouds,
    Arthur D. Little for Gas Research Institute (GRI-80/0108), January
    1982.

[^29]: The ignition of natural gas by spark discharges from the body,
    Wilson, P., The Institute of Physics Conference Series, No. 66, pp.
    21--27, 1983.

[^30]: Ignition sources of LNG vapor clouds, Arthur D. Little for Gas
    Research Institute (GRI-80/0108), January 1982.

[^31]: Further Studies of the Ignition of Gases by Transiently Heated
    Surfaces, D.P. Cutter, Combustion and Flame 33, pp. 85--91, 1978.

[^32]: These values are consistent with unpublished fault tree estimates
    values being used by US and European vehicle manufacturers in
    evaluating new low-GWP substance blends proposed as GARs.