The authors conducted this prospective study at the Shahdara industrial area of Delhi, India. They examined the effects of indoor and outdoor air pollutant levels on respiratory health in 394 children aged 7 to 15 years. The majority of children had a history of respiratory problems, including cough (62.7%), sputum production (24.4%), shortness of breath (32.0%), wheezing (25.6%), common cold (44.4%), and throat congestion (43.1%). The association of indoor and outdoor air pollutant levels showed that outdoor SO2 and NO2 was significantly higher than indoor SO2 and NO2 levels, whereas the mean indoor level of suspended particulate matter (SPM) was significantly higher than outdoor SPM level. Indoor SPM level also was significantly higher in homes of children with a history of respiratory illness than homes of children having no history of respiratory illness. Results suggest that both indoor and outdoor particulate exposure may be important risk factors in the development of respiratory illness in children.