For many years industrial hygiene sampling was done using a mechanical sampling pump. For many organic vapors, the air would be pumped through a charcoal tube. The pump needs to be calibrated before and after the sampling period. If the flow rate of the pump is too high, the vapor to be sampled will break through the sampling tube and be lost. Often two sections of charcoal are used. Then two analysis would be needed to obtain reliable results. This method is expensive and cumbersome, especially for monitoring personal exposure. In 1976 the company Advanced Chemical Sensors was founded by Laurence D. Locker, (Ph.D., M.I.T.) to develop improved methods to measure exposure to toxic vapors regulated by U.S. occupational safety laws. The technique of passive monitoring was developed to eliminate the need for a mechanical pump to collect the air sample. The pump is replaced by a solid adsorbent separated from the ambient air by a porous membrane. This device acts like a pump, but with no moving parts. The cost saving and the ease of use gives this method wide application to measure employee exposure and room concentrations of many chemicals.