The frequency of micronuclei (MN) in cells of the nasal mucosa, oral mucosa and in lymphocytes was evaluated for 25 students in anatomy classes exposed to formaldehyde (FA) over an 8-week period. Each student served as his or her own control. The time-weighted average concentration (TWA) of formaldehyde in anatomical laboratories and in students' dormitories was 0.508 +/- 0.299 mg/m3 and 0.012 +/- 0.0025 mg/m3, respectively. A higher frequency of micronuclei was observed in nasal and oral exfoliative cells after formaldehyde exposure (3.85 +/- 1.48 vs 1.20 +/- 0.676 and 0.857 +/- 0.558 vs 0.568 +/- 0.317, paired-t test: P < 0.001 and P < 0.01, respectively). No significant increase in the frequency of lymphocyte micronuclei was found after formaldehyde exposure (P > 0.05). The present study shows that nasal mucosa cells exposed through respiration are the chief target of FA-induced genotoxic effects.