This paper describes a new, highly compact and lightweight monitor that directly measures personal exposure resolved into four microenvironments. The device is the Total-Isolated-by-Microenvironment-Exposure (TIME) monitor. The monitor can identify electronically a subject's microenvironment and measure the time and the concentrations of air pollutants while the subject is in that microenvironment. A sensor electronically directs the air it samples from a microenvironment to a sampling collector (sorbent tube) uniquely associated with each of the four microenvironments: indoors-nonoccupational, indoors occupational, in-transit, and outdoors. Over a period of 24-hours, the TIME monitor measures exposures in the four microenvironments and, by adding the concentrations for the four components, the monitor provides a measure of total daily exposure. (Copyright (c) 1994 Princeton Scientific Publishing Co. Inc.)