Objectives The aim of the study was to detect single-strand breaks in deoxyribonucleic acid (DNA) in mononuclear blood cells of fire fighters exposed to o-nitroanisole and other substances released into the environment during an accident in a chemical plant.Methods The level of DNA single-strand breaks in mononuclear blood cells was detected by alkaline elution. The results for 16 fire fighters who worked in a contaminated area for about 8 h and two references groups (one of fire fighters who had not worked in the contaminated area and one of persons without any apparent occupational exposure to genotoxic substances).Results The mean normalized elution rate (nER) 19 d after the accident was slightly but statistically significantly (P