This paper describes a new version of the measurement of the calibration factor, K, between radon activity concentration and track density. The use of Solid State Nuclear Track Detectors (SSNTDs) is one of the most convenient techniques to assess the radiation level of -activities in the environment. Exposed plastic films are chemically and electrochemically etched in an alkali solution and the -tracks are evaluated under an optical microscope. The detailed procedure for this study and the caliabration of the etched films for conversion of track density to radon exposure in (Bq·m–3) are given in this paper. It was found the experimental and theoretical values of K were 1.37 and 1.27 (track·cm–2·kBq–1·h–1·m3), respectively, for plastic detectors CR-39.