The influence of outdoor pollution on indoor air quality was studied in downtown Santiago(Bandera street). Carbon monoxide (CO), nicotine, particulate matter, respirable fraction (PM5) and total and carcinogenic polyaromatic hydrocarbons (PAHs) were simultaneously monitored indoors and outdoors in restaurants, offices and other places. The levels of CO changed simultaneously outdoors and indoors (r = 0.89) specially during traffic rush hours, demonstrating the importance of outdoor infiltration into the indoor air quality and masking the contribution of other CO indoor sources. The maximum CO concentrations were over 800% and over 1000% higher indoors and outdoors respectively than the 9 ppm CO National Ambient Air Quality. The PM 5 concentrations were very high and showed no significant differences (p > 0.05) from indoors to outdoors, or between indoor levels in restaurants, offices and other places. Total and carcinogenic PAHs levels were also very high outdoors and indoors, outdoor levels being generally higher than those indoors and no significant; differences (p > 0.05) were found for the indoor levels between restaurants, offices and other places. Nicotine levels showed significant differences (p < 0.05) between indoor and outdoor levels. In addition, great differences (p < 0.05) in indoor levels, were found between offices and restaurants, and offices and other places. Among indoor sources cigarette smoke seems to be a minor source since nicotine concentrations, being 2.3 times higher in restaurants and other places than in offices, do not contribute to enhance significantly PM5 and total and carcinogenic HAPs in the first ones. These results suggest that in downtown Santiago, infiltration might be the main source of indoor pollution. This is supported by two evidences: a) coronene, a tracer of vehicle emissions was found in high concentration indoors and b) in restaurants (in which PAHs emissions might be higher indoor) a correlation coefficient of 0.987 for the indoor and outdoor concentrations of carcinogenic PAHs was found. Furthermore a survey asking for different symptoms and effects probably related to air pollution was made to people working in Bandera and in a rural area located 40 Km from Santiago. The results showed that excluding smoking as a confoundend factor, people working in Bandera showed a significantly greater (p < 0.05) risk of ill effects on their health than people working in the rural area.