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 micro m, 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 micro m), 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 microorganisms. 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/h) did not affect bioaerosol level or respirable dust. Total bioaerosols were significantly reduced in higher temperatures only. Relative humidity did not influence total bioaerosols but in one series respirable bioaerosols were significantly correlated with RH. Total bioaerosols were not different in outside air or attic air.