County air quality monitoring data was used in conjunction with concentration-
response factors for several health effects to estimate the added
health risks to individuals posed by exposures to particulate matter.
Scorecard's analysis assumes that EPA's new PM-2.5 air quality standard defines
the threshold at which PM-related health effects begin to occur in a exposed
population. Exposures below the standard are assumed to be safe, although there
is considerable scientific debate about whether there is an identifiable
threshold for the health effects of PM.
ESTIMATING PM EXPOSURES POSING HEALTH POTENTIAL HEALTH RISKS
To estimate the extent of PM exposures in exceedance of the presumed threshold
for PM effects, Scorecard ascertains whether 1998 monitored PM concentrations in
an area exceed the new annual PM2.5 standard. Because 1998 monitoring data is
for PM10, the PM2.5 standard of 15 ug/m3 was converted into a PM10 equivalent by
assuming a PM2.5/PM10 ratio of 0.5 for California and a ratio of 0.6 for the
rest of the United States. Annual average PM10 concentrations were calculated
for every county with monitoring data in the US (approximately 500 counties,
including just over 50% of the US population.) Excess PM10 exposure above the
health effects threshold was calculated by subtracting the PM10-equivalent
standard of 30 ug/m3 in California and 25 ug/m3 for the rest of the United
States from observed annual average PM concentrations.
CONCENTRATION-RESPONSE FACTORS USED IN RISK ESTIMATION
To estimate the added health risks associated with PM10 exposures,
concentration-response functions for specific health effects related to PM10
were multiplied by the amount of PM exposure above the health threshold. The
concentration-response functions estimate the change in the number of cases of
each health effect associated with a unit change in annual PM concentrations
(increase in adverse health effect/1 ug/m3 change in PM10 concentration).
Concentration-response functions were obtained from the peer-reviewed scientific
literature, based on the recent analysis of the health benefits of reducing
particulate matter air pollution by Ostro and Chestnut. Central estimates of
these health effect functions were used to calculate the excess risk associated
with incremental changes in PM concentrations. This methodological approach
assumes that concentration-functions are linear down to a threshold at the PM
standard. Excess health risk associated with PM exposure was calculated by
multiplying exceedance levels by the concentration-response function.
CONCENTRATION-RESPONSE FACTORS FOR HUMAN HEALTH EFFECTS ASSOCIATED WITH PM10
Annual risk factor given a 1 ug/m3 change in annual average PM10
Mortality risk: 1.80E-05
Chronic bronchitis risk: 6.10E-05 [for population 25 years and older]
Respiratory hospital admissions: 8.40E-06
Cardiac hospital admissions: 3.00E-06
Emergency room visits: 2.40E-04
Asthma symptom days: 5.80E-02 [for 4.7% of population with asthma]
Restricted activity days: 5.80E-02 [for population 18 years and older]
Days with acute respiratory symptoms: 1.68E-01
Children with bronchitis: 1.60E-03 [for population under age 18]
Scorecard's methodology for assessing the health risks of PM exposures is based
Ostro, B. and L. Chestnut, Assessing the Health Benefits of Reducing Particulate
Matter in the United States. Environmental Research, Section A-76,
pp. 94-106 (1998).
A similar risk estimation methodology can be found in:
US Environmental Protection Agency, Review of the National
Ambient Air Quality Standards for Particulate Matter, Policy Assessment
of Scientific and Technical Information, OAQPS Staff Paper. Office
of Air Quality Planning and Standards, Research Triangle Park, NC.