southwest metropolitan mexico city children are repeatedly exposed to high levels of a complex mixture of air pollutants , including ozone , particulate matter , aldehydes , metals , and nitrogen oxides . 
 we explored nasal cell 8-hydroxy-2'-deoxyguanosine ( 8-ohdg ) , a major mutagenic lesion producing g-->t transversion mutations , using an immunohistochemical method , and dna single strand breaks ( ssb ) using the single cell gel electrophoresis assay as biomarkers of oxidant exposure . 
 nasal biopsies from the posterior inferior turbinate were examined in children in grades one through five , including 12 controls from a low - polluted coastal town and 87 mexico city children . 
 each biopsy was divided for the 8-ohdg and dna ssb assays . 
 there was an age - dependent increase in the percentage of nasal cells with dna tails > 10 microm in mexico city children : 19 + /- 
 9% for control cells , and 43 + /- 
 4 , 50 + /- 
 16 , 56 + /- 
 17 , 60 + /- 
 17 and 73 + /- 
 14% , respectively , for first through fifth graders ( p < 0.05 ) . 
 nasal ssb were significantly higher in fifth graders than in first graders ( p < 0.05 ) . 
 higher levels ( 2.3- to 3-fold ) of specific nuclear staining for 8-ohdg were observed in exposed children as compared to controls ( p < 0.05 ) . 
 these results suggest that dna damage is present in nasal epithelial cells in mexico city children . 
 persistent oxidative dna damage may ultimately result in a selective growth of pr eneoplastic nasal initiated cells in this population and the potential for nasal neoplasms may increase with age . the combination of 8-ohdg and dna ssb should be useful for monitoring oxidative damage in people exposed to polluted atmospheres.imagesfigure 1figure 2figure 3figure 4