Aerial dust from 11 commercial pig finishing houses, 4 with mechanical and 7 with natural ventilation, was sampled during an 8-month period. Mass concentrations of up to 38 mg/m3 were correlated with outside temp. and inside relative humidity. The variance of particle size was insignificant. Size, shape and origin of particles from one farm were evaluated. Starch, bran and skin particles were identified with optical microscopy.