There is a need for economical and effective methods to assess practices that reduce gas emissions. The purpose of this project was to compare gas concentrations measured using semi-continuous sampling with dedicated gas analyzers versus concentrations obtained using 50 L bags filled over 24-h. Sampling was done in 2010 and 2011 from four biofilters (flat-bed with old media, A-frame, two flat beds with either 10 cm or 5 cm birch mulch) treating air from swine nurseries. Concentration ratios and percent reductions were calculated and compared using both sampling methods. The NH3, H 2S, CO2, CH4 and N2O concentration data indicated that the 24-h bag sampling concentrations and percent reductions tracked semi-continuous sampling concentrations and percent reductions well. Most NH3 concentrations differences (24-h bag sample minus semi-continuous) were within ±2 ppm of the semi-continuous concentrations, H2S concentration differences within ±200 ppb, CH4 concentration differences within ±10 ppm and N 2O concentration differences within ± 75 ppb. Ratio medians for NH3, CO2 and N2O were close to one, between 0.95 and 1.09. H2S ratio medians varied from 0.61 in 2010 to 1.68 in 2011. Semi-continuous percent reduction results indicate that the four biofilters were able to reduce NH3, H2S and CH4 emissions. Mean percent NH3 reductions of three out of four biofilters ranged from 53 to 86%, percent H2S reductions ranged from 41 to 74%, percent CH4 reductions of three out of four biofilters ranged from 8 to 39%. One biofilter reduced N2O concentrations 17 to 22% while three biofilters generated N2O by 8 to 81%.