Radon and Thoron in the Environment - Aerosols: Measurement, Dosimetry, and Health Effects - page 540

Epidemiologic follow-up studies of 11 underground mining groups show a lung cancer dose
response increasing with increasing radon concentration. Some homes can attain the concentrations
that existed in historic mines, and, thus, country radon surveys and residential risk estimates are
now common. The global residential measurements to date show that the current residential con-
centration averages 55 Bq m −3 (about 1.5 pCi L −1 ). The U.S. average is estimated as 46 Bq m −3 (about
1.2 pCi L −1 ). There are now over 80 worldwide residential epidemiologic studies, which include 23
case control studies, initiated to estimate residential risk directly and not rely on risk projection
models from the underground studies. The presence of 220 Rn (thoron), often unaccounted for in the
gas measurements, may affect the calculated risk estimates for homes.
The residential case control studies support lung cancer risk estimates similar to the under-
ground miner studies. The higher exposures in mines permit features of the lung cancer response to
be examined. For example, miners irst exposed at older ages have a lower lung cancer response per
unit exposure, and lung cancer risk diminishes with time since end of exposure.
21.2 SOURCES OF RESIDENTIAL AND OCCUPATIONAL RADON EXPOSURE
The concentration of radon in the volume into which it diffuses depends on the source strength
(parent 226 Ra concentration), the radon emanation fraction from the source, the volume, and the
ventilation rate. Underground miners work in a relatively enclosed space close to the rock source;
therefore, radon concentrations can be very high. In uranium mines, the rock (ore) concentration
of uranium and 226 Ra is much higher than typical soil concentrations and radon levels in mines can
be extraordinarily high, but depend upon the local air exchange rate. Radon gas is quite soluble in
water and in some non-uranium mines water-bearing 222 Rn can produce high concentrations. An
example of this are the Canadian Fluorspar mines (deVilliars, 1966; deVilliars et al., 1971; Morrison
et al., 1988, 1998; Villeneuve et al., 2007). High radon concentrations in drinking water in homes
with private wells can have elevated indoor air concentrations from the gas release during water use.
Average residential concentrations depend on typical 226 Ra concentrations in the underlying soil
and radon low into the dwelling. Residential concentrations are generally about a factor of 2 higher
than outdoor concentrations. Radon concentrations in multistory apartments or ofices are generally
lower than single-family residences because of smaller source terms due to more rugged founda-
tions or higher ventilation rates.
Most countries with public health programs have conducted comprehensive indoor radon sur-
veys in homes. In epidemiologic studies, large numbers of radon measurements have been made in
homes of lung cancer cases and controls to estimate lung cancer risk for the populations involved.
21.3 UNITS FOR RADON EXPOSURE
The miner studies rely on historic radon decay product measurements made by collecting a short-
term (minutes) iltered air sample and alpha counting the ilter at speciied times following col-
lection (NCRP, 1988). In the Czech mines only radon gas measurements were made. The decay
product measurements were reported in working levels (WLs). Cumulative exposure was calculated
in working level months (WLM) and is equal to WL times the total number of hours worked in
mines divided by 168 h in a work month. The WL was originally deined as the potential energy
released from a radon concentration of 100 pCi L −1 (3700 Bq m −3 ) in 1 L of air in equilibrium with
its short-lived decay products. This potential alpha energy release is equal to 1.28 × 10 5 MeV L −1 and
usually rounded to 1.3 × 10 5 MeV L −l . This potential energy quickly became the de facto deinition
of the WL.
In practice, the decay products are never in equilibrium with the parent radon and UNSCEAR
(2006) adopted an equilibrium factor of 40% for indoor environments. The radon gas concentration
times 0.4 is the equilibrium equivalent concentration (EEC) and is the equilibrium gas concentra-
tion that gives the same potential energy as the nonequilibrium mixture.