This study assessed the association between respirable particulate air pollution (PM10) and changes in the pulmonary function (FEV1, FEV1/FVC, and FVC) of smokers with mild to moderate airflow limitation. Spirometric data of Salt Lake City participants in The Lung Health Study were used from two screening visits 10 to 90 days apart after an initial screening visit, which included spirometry. We analyzed differences in pulmonary function (delta FEV1, delta FEV1/FVC, and delta FVC) for participants between the two spirometry visits. Significant associations between changes in pulmonary function and PM10 were observed. delta FEV1 and delta FEV1/FVC were inversely associated with changes in PM10. Although these associations were small, explaining only about 2 to 3% of the variance in delta FEV1, they were consistently negative and statistically significant (p < or = 0.01). On average, an increase in PM10 equal to 100 micrograms/m3 was associated with a marginal decline in FEV1 equal to approximately 2%. Associations between delta FVC and PM10 were consistently negative, but they were not statistically significant. No consistent or statistically significant associations between delta FEV1, delta FEV1/FVC, or delta FVC with changes in daily temperatures were observed. The effect of PM10 was greater for men than for women. The effect was nearly the same for those with nonspecific airway hyperresponsiveness (AHR), based on methacholine inhalation challenge testing, versus those without AHR. We conclude that in current smokers PM10 possibly has a small transient negative effect on lung function that is not entirely obscured by their smoking habit.