EPA and German scientists examined air from cities with high rate of bronchitis, allergies and asthma and found that airborne particles induce the problem. People living near coal-burning plants and industry, which emit toxic metals such as zinc, copper, tin and copper were more affected by asthma and allergies than those living in nonindustrial, farming areas.
Also Peden DB. Pollutants and asthma: role of air toxics. Environ Health Perspect. 2002. http://www.ncbi.nlm.nih.gov/pubmed/12194888