Objective To assess the mutagenicity of polycyclic aromatic hydrocarbons(PAHs) extracted from the atmospheric particulates collected from different aerodynamic sizes,seasonal variation and the sampled sites,and to detect the effect of these factors on the mutagenic activity of PAHs.Methods PAHs were extracted from the atmospheric particulates(PM10 and PM2.5) collected from two different sites,business area and industrial area(summer and winter respectively).The mutagenicity of these PAHs samples was evaluated by the Salmonella mutagenicity assay(Ames assay),using Salmonella typhimurium strains TA98 and TA100 with and without metabolic activation(S9).Each sample was assayed in three doses(125,250 and 500?g per plate).Results All samples induced the number of revertants to be higher measured with TA98 with and without S9 mix.The level of specific mutagenic activity measured with strain TA98 was higher than TA100.Metabolic activation with the S9 fraction increased the mutagenicity of the industrial area samples studied for the strain TA98.All groups showed a good dose-response relationship of mutagenicity(P