Risk Analysis for CO2

, HFC-152a, and 

Hydrocarbon Refrigerants in MACs

Erin Birgfeld, U.S. EPA

Earth Technologies Forum

April 23, 2003
Risk Assessment of Alternative MAC 

Systems

• Identify and characterize risk

• Identify risk mitigation strategies

• Provide reliable information for industry and 

government
EPA MAC Risk Analysis Team

• Erin Birgfeld, MPH, Risk Analysis

• Dave Godwin, P.E., Engineering

• Stephen Andersen, Ph.D.
Government/Industry Partnership

• U.S. EPA

• Australian 

Greenhouse Office

• DENSO

• Toyota

• DaimlerChrysler

• Japan METI and EA

• Fiat

• Underwriters 

Laboratories

• U.S. Army, CECOM

• Delphi

• General Motors

Industry participation and expertise critical for a successful project 
Current Status: Where are we now?

• Request industry to provide any existing data or 

risk analysis information to EPA

• Consult with industry and gov’t representatives

• Summarize and consolidate information

• Plan a strategy and draft risk assessment
Convene Meeting(s) of Government and 

Industry Experts

• Review technical information and inputs to risk 

assessment

– Assure all reasonable aspects are considered 

• Gain insights into national and regional perspectives

• Define additional research that may be needed
Outcome

• Risk assessment report published by year end

– Issued jointly by gov’t and industry

• Significant New Alternatives Policy (SNAP) 

Program to review and act on promising 

alternatives

• Industry decisions on new systems
Risk Assessment Overview

• Compare likely exposure to levels where a 

hazard exists

• SNAP sponsored a fault tree analysis (FTA) for 

134a and other non-flammable refrigerants

– Estimates frequency of occupational and passenger 

exposures to refrigerants

– Performed to assess toxicity issues
Exposure to

vehicle occupants

in U.S.

Exposure to

vehicle occupants

3,600/yr

150 million

vehicles

4

1.59

occupants

per vehicle

1.5 x 10 /yr -5 5

Sudden large

leak in pass.

compartment

11

Occupant

in passenger

compartment

12

Exposure due to

sudden large

leak

1 x 10 /yr -8

3 x 10 /yr -4

4 x 10 /yr -5

Exposure due to

sudden medium

leak

1 x 10 /yr -6

Sudden

medium leak

in passenger

compartment

6

Occupant

in passenger

compartment

7

Doors and

windows closed

8

Air blower on

9

Air

control in

recirculation

mode

10

Refrigerant mixed

with breathing air

2 x 10 /yr -4

0.04

0.9 0.95 0.2

0.17

Exposure due to

leak following

recharge

1 x 10 /yr -6

Leak in pass.

compartment

following servicing

13

Occupant

in passenger

compartment

14

Doors and

windows closed

15

Air blower on

16

Air

control in

recirculation

mode

17

Refrigerant mixed

with breathing air

1 x 10 /yr -4

0.3

0.95 0.99 0.2

0.19

Exposure due to

leak caused by

collision

8 x 10 /yr -6

Leak in pass.

compartment

caused by collision

18

Occupant

in passenger

compartment

19

Occupant

survives

collision

20

9 x 10 /yr -6

0.99 0.9

22.5 million

vehicles

serviced

1

Exposure to

service technicians

in U.S.

Exposure to

service technicians

per vehicle

135,000/yr

0.006/yr

0.001/yr 0.005/yr

Sudden large

leak in service

equipment

3

Sudden large

leak in vehicle

system during

servicing

2

The result of several initiating and/or 

contributing events through “and” or “or” gate with roles of occurrence either yearly or per 

dmeand

Numbers of components, serve as multipliers

“And” gate multiplies the rate of occurrence of 

the incoming branches

“Or” gate adds the rates of occurrence of the 

incoming branches

Contributing events with rules of occurrence 

per demand. Conditional on the prior initiating 

and contributing events having taken place.

Initiating events with yearly rates of occurrence

Fault Tree Logic Sympols

Fault Tree Analysis (Jetter, 2001)

• Definitions
Conclusions of FTA by Jetter et al.

• Most exposures are within acceptable limits

• Engineering can mitigate exposure 

– Reducing Charge Size

– Reconfiguring cooling system

– Increasing airflow/intake
Next Generation Alternatives

• CO2 

– High pressure systems

– Low toxicity but causes narcotic and other effects at higher 

concentrations

• HFC 152a 

– Toxicity is low and well characterized (similar to 134a, R-12)

– Flammability concerns

• Hydrocarbons (Propane, Isobutane)

– Low toxicity

– Flammability concerns
Proposed Strategy for Risk Analysis

• Refine FTA to address high pressure CO2

systems and system using flammable refrigerants

– passengers and technicians

• Identify safe upper bound exposure levels for 

short and long term exposures to CO2(Ongoing)

– Medical literature: NASA/Navy research

– Consider sensitive groups
Proposed Strategy for Risk Analysis (con’t)

• Computerized Fluid Dynamic Modeling (CFD) 

conducted by Dr. Neal Blackwell, Army CECOM

– Work funded through AAPPSO

• Assess magnitude of flammability and CO2

hazards without expensive testing

– Quality inputs/assumptions critical

– Industry input necessary
Conclusion and Next Steps

• Goal is to provide a comprehensive unbiased 

risk assessment for alternative systems

• Further input from industry and other interested 

parties on risk assessment strategy

• Draft risk assessment paper by July 15, SAE’s 

Alternate Refrigerant Symposium
Contacts

• Erin Birgfeld

(202) 564-9079

birgfeld.erin@epa.gov

• Dave Godwin

(202) 564-3517 

godwin.dave@epa.gov