The goals of this study were (1) to investigate workers' exposure to medium-density fiber (MDF) dust (inhalable dust, particle size),formaldehyde, and volatile organic compounds; (2) to study the possible inflammatory nasal reactions caused by exposure to MDF board dust; and (3) to determine the occurrence of irritative symptoms among exposed workers. Nasal lavage fluid was analyzed for cytokines and nitric oxide/nitrite. and inflammatory cells were counted. The time-weighted average of MDF dust was 1.4 mg/m3 in the workers' breathing zones. MDF board dust was composed mainly of particles exceeding 10 microm in diameter. The MDF board dust released formaldehyde in concentration of about 1000 microg/g when extracted with water for 6 hours at 37 degrees C. The cell counts and cytokine levels of the nasal lavage fluid samples did not show statistically significant differences between the workers exposed to MDF board dust and those exposed to other wood dusts. Nevertheless, two MDF-exposed workers had a considerable increase in the proportion of eosinophils and cytokine levels. Several workers exposed to MDF and wood dusts experienced nasal, eye, and skin symptoms at the end of a work shift. Both exposed groups had significantly more nasal symptoms, although the median dust level was only 1.2 mg/m3, considerably less than the occupational exposure limit for wood dust in Finland. Nasal symptoms were more frequent among workers exposed to MDF board dust and did not correlate with smoking. Our results suggest that the occupational exposure limit of 5 mg/m3 is probably too high for MDF board dust.