The composition of exhaust emissions from two-stroke chain saw engines was studied. The emissions of exhaust were sampled and analyzed under controlled laboratory experiments. The compounds sampled were those considered primarily responsible for acute health effects--hydrocarbons, aldehydes, nitrogen oxides and carbon monoxide. Exposure to tetramethyllead, dibromoethane and polycyclic aromatic hydrocarbons also was monitored. The results revealed no significant differences in the exhaust emissions from seven different chain saws. Heavily worn-out chain saws do not emit increased amounts of exhaust. A lean fuel-air mixture increases the emission of aldehydes and nitrogen oxides, whereas a rich mixture increases emission of carbon monoxide and hydrocarbons. Based on these new data on the composition of two-stroke chain saw exhaust emissions, operator exposure to chain saw exhaust was evaluated under various logging situations. Exposure measurements revealed no difference in average levels of exposure between logging in the presence or in the absence of snow. The felling operation, however, results in high exposure levels of short duration--especially when the operation is performed while there is deep snow on the ground. (This operation excludes limbing and bucking into lengths.) This is judged to be the main cause of the discomfort experienced by loggers. Average exposure levels for loggers engaged only in felling are twice those for cutters who also perform limbing, bucking and manual skidding of the timber, since these latter operations involve considerably lower exposure. Typical average levels of exposure are as follows: hydrocarbons, 20 mg/m3; benzene, 0.6 mg/m3; formaldehyde, 0.1 mg/m3; and carbon monoxide, 20 mg/m3.