In general, air samples are collected using small pumps and air sample cassettes. Sampling time depends on the test method used and the contaminant being evaluated. Typical air sampling times per sample vary as follows: bacteria - 2 minutes; mold - 10 minutes; VOC's - 15 minutes; asbestos - 90 minutes; silica - 2 to 4 hours; soot - 90 minutes; formaldehyde - 60 minutes.