PM2.5 pollution is one of the most important concerns involved in the evaluation of the health effects of air pollution. The mutagenic activity of ambient outdoor air has been widely reviewed in the literature; however, there are little data specifically regarding PM2.5. The mutagenic properties of fine PM are generally higher than those of coarse PM. PM2.5 organic extracts are generally mutagenic, especially in winter, in urban environments, or in proximity to the chief emission sources of air pollutants. Combustion reactions and the atmospheric reactions of gaseous pollutants with hydrocarbons result in a large number of dispersed DNA-reactive compounds. Biological assays could indicate the effects of the complex environmental mixture on biological systems and might indicate the ability of known or unknown compounds in a mixture to interact with DNA-producing mutation processes. This approach is useful in evaluating the cancer risk associated with air pollution. The accumulated data could better support evaluations of the global genotoxic burden for urban populations and could improve precautionary measures taken to reduce environmental pollution. Future studies need to be carried out on nitro-substituted compounds and on the ultrafine particles that are responsible for most of the mutagenic effects. Moreover, different biological assays may support Salmonella assay results as well as supply biological evidence that helps to elucidate carcinogenic pathways.