Air pollution is a major environmental risk for human health. The mutagenicity of polar organic extracts collected from São Paulo city was recently investigated. São Paulo has a mix of vehicle types using a variety of fuels including several gasoline blends and alcohol. Higher levels of acetaldehyde and formaldehyde were found in the atmosphere of São Paulo. As a reactive electrophile, acetaldehyde reacts with nucleophilic groups forming adducts. In this work we have investigated cell viability, lipid peroxidation and DNA damage in IMR-90 cells treated with acetaldehyde (1 mM to 1 M) for 3 h. The incubation of the cells with acetaldehyde caused significant cell death evaluated with MTT.  Moreover, acetaldehyde caused lipid peroxidation in a dose dependent manner as monitored by the quantification of malondialdehyde (MDA) using HPLC coupled to fluorescence detection. To evaluate the genotoxicity of acetaldehyde we performed the Comet assay. DNA migration was assessed by fluorescence microscopy. A significant amount of comet formation was observed in the cells treated with the aldehyde in comparison with the control. Conclusively, we confirmed that acetaldehyde may be important causative of the mutagenicity of São Paulo's atmosphere extracts. Possible carcinogenesis due to chronic acetaldehyde exposure needs further investigation.