[en] During the past five years there has been increasing interest in monitoring the individual exposure of persons to radon and, more importantly, the radon progeny attached to aerosol particles. Several devices have been proposed. These include the following: passive detectors based on nuclear track emulsions and track etching foils; semi-active systems in which the aerosols are collected electrostatically and detected by scintillator/film combinations or track etching; and active systems based on the collection of aerosols on a filter, and integrating measurement of the alpha flux from the filter by TLD or track films. All these systems have drawbacks, which have been minimized in a small active system based on alpha particle track etching which was originally developed at ORNL in 1969. Since then it has undergone substantial laboratory and field testing, and has been subjected to various improvements. The system is briefly discussed