Daily exposure of uranium mining personnel to 222rn daughters has established the need for a device that will continually monitor an individual's exposure. Such a device has been built and tested with promising results. The device is an electronic dosimeter using both a solid-state detector and circuitry, and is a personal instrument worn throughout the working shift. Since the instrument is in close proximity to the miner and measures continually, it provides a more accurate account of total cumulative exposure, and avoids the errors caused by the present technique of spot checking the environment. The system permits the measurement of cumulative exposures to airborne radon progeny, expressed in units of working level hours.