This study evaluated changes in respiratory health associated with daily changes in fine particulate pollution (PM10). Participants included a relatively healthy school-based sample of fourth and fifth grade elementary students, and a sample of patients with asthma 8 to 72 yr of age. Elevated PM10 pollution levels of 150 micrograms/m3 were associated with an approximately 3 to 6% decline in lung function as measured by peak expiratory flow (PEF). Current day and daily lagged associations between PM10 levels and PEF were observed. Elevated levels of PM10 pollution also were associated with increases in reported symptoms of respiratory disease and use of asthma medication. Associations between compromised respiratory health and elevated PM10 pollution were observed even when PM10 levels were well below the 24-h national ambient air quality standard of 150 micrograms/m3. Associations between elevated PM10 levels, reductions in PEF, and increases in symptoms of respiratory disease and asthma medication use remained statistically significant even when the only pollution episode that exceeded the standard was excluded. Concurrent measurements indicated that little or no strong particle acidity was present.