A rough estimation of the amounts of tobacco smoke components taken up by active and passive smoking suggests that, in the case of passive smoking, gas phase constituents in ETS are of greater relevance than particle-bound substances. Since this aspect is of importance for the risk evaluation of passive smoking, it was decided that it should be investigated further in a series of exposure studies with human volunteers. The ETS exposure conditions were characterized by measuring tobacco smoke components such as carbon monoxide (CO), nitrogen oxides (NOx), nicotine, formaldehyde, tobacco-specific nitrosamines (N-nitrosonornicotine (NNN), 4-methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), benzo(a)pyrene (BaP) and particulate matter in the air of the exposure room. The biomonitoring covered carboxyhemoglobin (COHb), thioethers and mutagenic activity in urine. These parameters were compared to those observed after controlled active smoking. It was found that urinary thioether excretion increased in non-smokers after extremely high ETS exposure. This effect could be attributed to gas phase ETS components. Urinary mutagenicity was not measurably increased in non-smokers under these conditions. This indicates that in passive smoking, as opposed to smoking, the gas phase might be more important in terms of possible effects than the particulate matter. It would, therefore, be misleading to make extrapolations based on the burden of smoking to establish the burden of passive smoking.