Monitoring gas-phase pollutants is essential to understand exposure patterns and to establish a link between exposure and health. Measurement of the low concentrations found outdoors or in indoor living space normally requires large, expensive instruments that use electrical power. In this study, colorimetric passive diffusion tubes, normally used to monitor high concentrations of airborne contaminants in the workplace for sampling periods of a few hours, were evaluated to measure much lower concentrations of the same pollutants for periods of up to 1 wk. These tubes are small, inexpensive, and require no electrical power. Responses of diffusion tubes for carbon monoxide (CO), hydrogen sulfide (H2S), nitrogen dioxide (NO2), sulfur dioxide (SO2), and benzene were studied. Low pollutant concentrations measured with passive diffusion tubes matched reasonably well with true concentrations for all pollutants except NO2. These results suggest that passive diffusion tubes can provide an inexpensive, unobtrusive, and effective method to monitor low pollutant concentrations. Passive diffusion tubes may be particularly useful in surveys where the spatial variability in concentrations is high and where the cost of traditional monitoring instruments is a concern.