A study was undertaken in a plant producing oriented strand board (OSB) from aspen and balsam wood, bonded by methylene diisocyanate (MDI) and phenol formaldehyde. A group of 127 production workers in the plant was compared to 165 oil workers from the same geographic area. Measurements of MDI ranged from 6 to 33 micrograms/m3 (0.001-0.003 ppm), of respirable dust ranged from 0.05 to 0.5 mg/m3, and of formaldehyde were 0.05 ppm or less. The ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC) was significantly lower among the OSB workers compared to the oil workers, and this was more pronounced for ex-smokers and current smokers. A number of respiratory symptoms suggestive of airway reactivity were significantly more common among the OSB workers. It was known that changes to reduce worker exposure had been made in the plant before the study, and it is unclear whether the health effects documented were the result of these low levels or if previous, probably higher levels were responsible.