This work evaluated the performance of four 40-mm threaded respirator canisters. These canisters were tested in laboratory experiments simulating environments similar to those encountered during structural overhaul firefighting operations. The same smoke generation and delivery system used in previous work (254-20C5-M-12750) was used in this study. This time, only 40-mm threaded canisters were tested, as these canisters are purported to be most easily interchangeable with SCBA facepieces currently used by North American firefighters. Three CBRN canisters, from MSA, 3M, and Scott, were tested along with a multi-gas canister similar in appearance to the above but without CBRN certification (3M FR- 64040). These tests were not intended to define contaminant breakthrough time. Rather, the testing protocol was designed to evaluate the effectiveness of these canisters over the 30-minute period reported by firefighters as a typical period in which overhaul activities would be performed. This report includes data and analysis of contaminant concentrations for 12 tests, in which a large matrix of contaminants were simultaneously examined in both the challenge smoke ("chamber air") and in air filtered by the respirator canister ("filtered air"). Originally, we proposed conducting 6 tests, completed in August; in November, we performed additional testing where hydrogen chloride was added to the analytical matrix while removing naphthalene, benzene and total hydrocarbon. All tests included examining CO; NO2; SO2; respirable dust; aldehydes including formaldehyde, acrolein and glutyraldehyde; and cyanide. The challenged and filtered-air concentrations for contaminants considered to be respiratory irritants were aggregated into an irritant index to determine how efficient the air purifying respirator (APR) canisters would protect a firefighter against the multiple contaminants in smoke. This index was computed using 15- and 30-minute occupational exposure limits. When the index exceeded unity, a firefighter would be at risk of respiratory irritation if the smoke was inhaled. In all cases, the challenge concentration irritant index exceeded unity, ranging from 2.9 to 26. For all 12 test cases, the APR canister reduced the overall irritant index to levels below unity, indicating that these canisters would provide protection for firefighters working in overhaul environments. Note: in some tests, levels of carbon monoxide were higher than recommended for persons wearing APRs; these canisters do not protect against carbon monoxide. Firefighters must rely on direct reading warning to indicate high CO levels, indicating the need to leave the area if wearing an APR, as these air purifying respirator canisters would be inappropriate. These APR canisters performed better than previous multi-gas cartridges in our initial studies. Additional work needed to interpret the importance of the free radical penetration; but a much larger firefighter study focused on field measurements would be better suited for this additional investigation.