The concentration of radon daughters suspended in air is measured in working levels (WL) or equilibrium equivalent concentration (EEC). Human exposure to radon daughters depends on the concentration at a specific location and the time that the person spends at that location; it is measured in working level months (WLMs). The risk increases as the exposure in WLM increases. Numerical estimates of risk have been reviewed extensively by the committee on health risks of exposure to radon (BEIR VI) (NRC, 1999) and International Commission on Radiation Protection (ICRP, 1981; 1991; 1993).
The risk of skin cancer in radon-rich atmospheres is not associated with inhalation or other intake of radioactivity. It is related to the plate out of radon daughters on the skin. The dose to sensitive cells in the skin depends on the concentration of radioactivity on the surface skin and the ability of alpha particles from 218Po and 214Po to penetrate to the location of the sensitive cells. Previous studies of that process have used a nominal value of 70 mm as the depth of the sensitive basal cells at the base of the epidermis. Recent measurements have indicated large variation in the depth of the basal cells, and this could lead to higher doses than previously expected (Eatough, 1997). We have assessed the pathway to determine whether the projected risk of skin cancer is comparable with the risk of lung cancer posed by the same exposure in WLM.
The dose delivered by an alpha particle as it passes through tissue depends on the stopping power, dE/dx. Values of that quantity have been complied by the National Institute of Standards and Technology and can be downloaded directly from its Web site (http://physics.nist.gov/PhysRefData/contents-radi.html). Those data can be used to compute the dose as a function of depth for radioactivity uniformly deposited on the surface. Figure B.1 shows the results of the computation for 218Po (6 MeV alpha particle) and 214Po (7.7 MeV alpha particle). The data clearly illustrate that the alpha particle from 214Po will not reach basal cells at a depth of more than 50 µm.
The epidermis is not a flat organ located uniformly below the skin. There are undulations that are responsible for large variations in depth. Eatough has published distributions of epidermal thickness in various locations on the body (Eatough, 1997). We have simulated those data as a lognormal distribution (median = 50, σg = 1.5) to represent the epidermal thickness on the exposed face of a person in Figure B.2.
The next step is to determine the amount of radioactivity that will plate out on the skin for a given concentration of radon daughters suspended in the air. Denman et al. (2003) and Sevkova et al. (1978) have reported measurements of plate out on exposed skin. We have adopted a value of 2 (Bq/m2)/(Bq/m3) which is consistent with the reported results. Combining those results and assuming a steady-state condition with equal concentrations of 218Po and 214Po on the skin, we obtain a result of 200 mSv/WLM for the effective dose to the skin.