3M Formaldehyde Monitor 3720 with Prepaid Analysis uses a bisulfite-impregnated filter and is analyzed by colorimetry. It can be used for 8 hour TWA sampling to meet the OSHA requirement of +/-25 percent accuracy at 0.5 - 5.0 ppm. These compact monitors clip easily to shirt lapel collar or pocket and don't interfere with employee activities Monitors collect certain organic vapor type airborne contaminants using the scientific principle of diffusion. A sorbent pad in the base collects and holds the organic gases and vapors until desorbed for analysis in a laboratory. Monitors with Prepaid Analysis can be mailed unit to the 3M-approved labratory listed on the package for analysis. Monitors have an 18-month shelf life.