Air quality was assessed in 2 commercial swine finishing barns, one naturally ventilated and the other mechanically ventilated. The concentrations of inhalable dust IDC) and respirable dust (RDC), bioaerosols, carbon dioxide, and ammonia as well as particle size distribution, air temperature, relative humidity inside the barns were measured for 41 consecutive weeks covering three batches of pigs in each barn. The mean IDC, RDC, total viable particles, and respirable particles in the naturally ventilated barn were 2.19 mg/m3, 0.10 mg/m3, 6.0 x 1^4 and 9.8 x 10^3 respectively. The two barns did not differ significantly in any of the above parameters or in particle size distribution. The IDC and RDC inside the two barns correlated significantly with the weight of the pigs. The IDC also correlated with the outside air temperature, and the RDC correlated with the temperature difference between the outside and inside air. Significant but small correlation was found between the total forming units inside the barns and the weight of the pigs.