UNITS FOR RADON EXPOSURE

partly stem from the mining industry and can be confusing. 1 WL (working level) is the activity of air containing 100 pCi of radon in equilibrium with its daughters per litre of air; 1 WLM (working level month) is the dose received at 1 WL for 170 hours (1 month with 21.25 eight hour shifts). The EPA considers a dose of 1 WLM equivalent to 5,000 mrems to the bronchial epithelium or to a whole-body dose of 300 mrem. The generally accepted lung cancer risk (see Cohen's paper and the BEIR report cited there) is 0.003 per WLM. For measurements of radon alone (not its daughters), 1 WL corresponds to 200 pCi/l of radon. Thus a measured radon concentration of 1 pCi/l will give the bronchial epithelium a dose of 0.147 mrem per hour spent in that air (usually less than 24 hours/day) and the rest follows by proportionality.