Estimating Average Air Toxics Exposure

USEPA Cumulative Exposure Project:
Estimated 1990 Exposure to Air Toxics

Pollution Sources

• Background -- natural sources, long-range transport, resuspension
• Point -- e.g., metal manufacturing, utilities
• Area -- e.g., other manufacturing, dry cleaners, consumer products, roofing
• Mobile -- cars, trucks, buses, farm equipment, boats, planes

Pollutant Dispersion Model

• wind speed and direction
• breakdown of chemicals
• formation of pollutants
• transport within 50 km

Limitations of the Study

• 1990 source data -- things have changed
• many assumptions and simplifications
• least reliable at census tract level
• 1996 source data model due at end of year

Changes -- 1990 Clean Air Act

• 189 Hazardous Air Pollutants regulated
• Maximum Available Control Technology- solvents, body shops, dry cleaners, platers
• Mobile Sources -- cleaner fuels, improved pollution control, E-check
• Consumer Products -- reduced VOCs

Health Effects Benchmarks

• Cancer -- one-in-a-million lifetime risk
• Hazard Ratio: estimated concentration divided by health benchmark
• Limitations: combined effects, limited health data

Cancer Hazard Ratio

estimated concentration ÷ cancer benchmark concentration = cancer risk ratio

e.g. carbon tetrachloride: 0.9 µg/m3 ÷ 0.07 µg/m3 = 13

Highlights of 1990 National Results

• 7 air toxics exceeded cancer benchmarks in all 60,000 U.S. census tracts
• some census tracts had as many as 32 pollutants which exceeded the cancer benchmark
• average of 14 chemicals per census tract which exceeded cancer bench mark

Air Toxics Estimated to Exceeded Cancer Benchmark in All Census Tracts -- 1990

• benzene
• carbon tetrachloride
• chloroform
• ethylene dibromide
• ethylene dichloride
• formaldehyde
• methyl chloride

• suggests pollutants to target for monitoring, compliance and enforcement
• provides baseline for measuring progress
• highlights importance of controlling mobile and area sources
• highlights need for virtual elimination of persistent toxics
• confirms need for stringent air pollution controls, including E-check

Cuyahoga County: Highlights of Results

• 14 pollutants exceeded cancer benchmarks
• background, area and mobile sources major contributors

Cuyahoga County: Comparison of Modeled and Measured Hazardous Air Pollutants (HAP)

• HAP concentrations from 2 Ohio EPA Cleveland air pollution monitoring stations -- 1996 measurements
• USEPA modeled concentrations for the same census tracts -- 1990 source data
• no statistically significant differences between the modeled and the measured concentrations

Comparison of Modeling and Monitoring in Cleveland Table

[TOP]