The authors assessed the relation between sulfur dioxide and other air pollutants (black smoke, carbon monoxide, nitrogen dioxide, and ozone) and daily emergency room admissions for chronic obstructive pulmonary disease in Barcelona, Spain, during 1985-1986. Barcelona is a Mediterranean city of 1.7 million inhabitants that has air pollution levels lower than standard values. A weak but statistically significant association between emergency room admissions and levels of sulfur dioxide, black smoke, and carbon monoxide was observed. Daily emergency room admissions for chronic obstructive pulmonary disease increased by 0.02 and 0.01 for each microgram of sulfur dioxide and black smoke per cubic meter, respectively, and 0.11 for each milligram of carbon monoxide per cubic meter, after adjusting for meteorologic and temporal variables. Similar estimations were obtained after controlling for the autocorrelation effect by means of time series analysis. In addition, when the data were stratified by season, the effects remained in summer. This strengthened the conclusion that the relation observed in winter, spring, and throughout the study period was indeed real. After truncating the data according to sulfur dioxide or black smoke levels, the authors still found effects for these pollutants at levels below the air quality guideline of 100 micrograms/m3 established by the World Health Organization. The consistency of these results with other recent toxicologic and epidemiologic observations suggest that current air pollution standards do not totally protect public health with a margin of safety in specific places.