Indoor exposure to "outdoor PM10": assessing its influence on the relationship between PM10 and short-term mortality in U.S. cities.

BACKGROUND

Seasonal and regional differences have been reported for the increase in short-term mortality associated with a given increase in the concentration of outdoor particulate matter with an aerodynamic diameter smaller than 10 μm (PM(10) mortality coefficient). Some of this difference may be because of seasonal and regional differences in indoor exposure to PM(10) of outdoor origin.

METHODS

From a previous study, we obtained PM(10) mortality coefficients for each season in seven U.S. regions. We then estimated the change in the sum of indoor and outdoor PM(10) exposure per unit change in outdoor PM(10) exposure (PM(10) exposure coefficient) for each season in each region. This was originally accomplished by estimating PM(10) exposure coefficients for 19 cities within the regions for which we had modeled building infiltration rates. We subsequently expanded the analysis to include 64 additional cities with less well-characterized building infiltration rates.

RESULTS

The correlation (r = 0.71 [95% confidence interval = 0.46 to 0.86]) between PM(10) mortality coefficients and PM(10) exposure coefficients (28 data pairs; four seasons in each of seven regions) was strong using exposure coefficients derived from the originally targeted 19 National Morbidity, Mortality, and Air Pollutions Study cities within the regions. The correlation remained strong (r = 0.67 [0.40 to 0.84]) when PM(10) exposure coefficients were derived using 83 cities within the regions (the original 19 plus the additional 64).

CONCLUSIONS

Seasonal and regional differences in PM(10) mortality coefficients appear to partially reflect seasonal and regional differences in total PM(10) exposure per unit change in outdoor exposure.