In order to assess the effects of ventilation rate, temperature, relative humidity and source of air on bioaerosol levels and dust with particle size < 10 um, a total of 120 pigs housed in 12 pens in two separate rooms were used. Pigs averaged 30 kg initially and the trials were discontinued when 20% of the pigs were marketed. A six-stage Andersen sampler and a light scattering particle counter were used to determine bioaerosols and respirable dust (0.1-10 um), respectively. Total bioaerosols were assessed using Trypticase Soy Agar, Potato Dextrose Agar was used for fungal aerosols and Baird-Parker Agar used for isolation of Staphylococcus aureus. Moulds amounted to less than 1% of total microoganisms. Gram positive bacteria made up 72% of the bacterial isolates. Respirable dust was not correlated with respirable bioaerosols. Ventilation rate (2, 5 or 8 changes per hour) did not affect bioaerosol level or respirable dust. Total bioaerosols were significantly reduced (P<0.05) in higher temperatures only. Relative humidity did not influence total bioaerosols but in one series respirable bioaerosols were significantly (R = 0.53) (P<0.05) correlated with RH. Total bioaerosols were not different in outside air or attic air.