For genotoxic carcinogens, exposure assessment may be achieved by measurements of the extent of covalent interaction (adduct formation) that has occurred between the carcinogen and macromolecules such as DNA, haemoglobin and albumin. Adducts for many carcinogens have been found in supposedly unexposed populations. This signifies either that endogenous processes contribute to this DNA/protein modification, or that there are exogenous exposures to these carcinogens that were not previously recognised. Notable examples where background genotoxic modification has been found include damage caused by low molecular weight alkylating agents and hydroxyl radicals. The significance of the existence of these adducts to genotoxic risk is as yet unknown.