Heavy Metals Pollution in Street Dusts of Tehran and Their Ecological Risk Assessment

IntroductionStreet dusts are suspended particle matters in the air from different natural and anthropogenic sources that may fall down because of their size and density. The growth of population, industrial activities and vehicles, has recently increased urban systems pollution. Therefore, it has recently been focused on the assessment of street dust contamination as a major source of urban environment pollution, all over the world. Heavy metals in street dusts are among major environmental pollutants that may be originated either from anthropogenic sources such as high traffic loads, industrial activities, building erosions, corrosion of tires and car parts, mineral activities and fossil fuel burnings or from natural sources such as local soil. During recent years huge amounts of atmospheric dust have entered Tehran, crossing national western borders of Iran and settled on the surfaces of urban areas.Assessing heavy metal contents of street dusts which can make serious human health risks seems to be important in this highly populated city. In this study, fifty street dust samples were collected from streets of southern and eastern parts of Tehran, the capital of Iran. The concentrations of nine metals (i.e., Cu, Cr, Pb, Ni, Cd, Zn, Fe, Mn and Li) were determined in samples. Probable natural or anthropogenic sources of metals were identified using multivariate statistical analyses. Enrichment factors and Hakanson’s ecological risk assessment method were also applied.Materials and MethodsFifty locations were selected for sampling. The distribution of sampling points is as follows: ten samples from each Resalat, Baghery and Hengam streets, five samples from Esteghlal Street which are main streets in eastern parts of Tehran. Fourteen samples were collected from the South terminal of Tehran which is located in one of the high polluted areas in the south of Tehran and the last sample was collected from the campus of Iran University of Science and Technology. Dust samples were collected during summer under dry and stable weather conditions, using broom and suction from the edge of pathways in streets and terminal. The samples were kept in special bags and were taken to laboratory. In the laboratory, the samples were air dried at room temperature until they were constantly weighted. Thereafter, they were passed through sieving sets of numbers 10, 35, 60 and 230 (63 µm). The particles with sizes lower than 63 µm can easily become suspended and emitted in the air. Therefore, their entrance into human’s respiratory system and causing serious effects are more probable than larger ones. Dust particles with the size of less than 63 µm were subjected to chemical analyses. Dust samples were acid digested (HCl, HNO3, HClO4). The metal contents of the samples were determined using Atomic Absorption Spectrometer (Buck Scientific model 210 VGP model).By using of Hakanson’s method and RI the ecological risks of each sampling location were calculated. Cluster and principal component analyses were used to identify probable sources of pollutants. In addition, possible effects of human activities on the concentrations of heavy metals were evaluated, using enrichment factor (EF).