Genotoxic effects of traffic-generated particulate matter (PM) are well described, whereas little data are available on PM from combustion of biomass and wood, which contributes substantially to air pollution world wide. The aim of this study was to compare the genotoxicity of wood smoke particulate matter (WSPM), authentic traffic-generated particles, mineral PM and standard reference material (SRM2975) of diesel exhaust particles in human A549 lung epithelial and THP-1 monocytic cell lines. DNA damage was measured as strand breaks (SB) and formamidopyrimidine DNA glycosylase (FPG) sites by the comet assay, whereas cell cytotoxicity was determined as lactate dehydrogenase release. The exposure to WSPM generated SB and FPG sites in both cell lines at concentrations from 2.5 or 25 microg/ml, which were not cytotoxic. Compared to all other studied particles, WSPM generated greater responses in terms of both SB and FPG sites. Organic extracts of WSPM and SRM2975 elicited higher levels of SB than native and washed PM at 25 and 100 microg/ml, whereas assay saturation precluded reliable assessment of FPG sites. During a 6h post-exposure period, in which the medium with PM had been replaced by fresh medium, 60% of the DNA lesions generated by WSPM were removed. In conclusion, WSPM generated more DNA damage than traffic-generated PM per unit mass in human cell lines, possibly due to the high level of polycyclic aromatic hydrocarbons in WSPM. This suggests that exposure to WSPM might be more hazardous than PM collected from vehicle exhaust with respect to development of lung cancer.