Radon dose conversion factors
Radon in Air
Typical average Radon concentration in open air near the ground is 10 Bq/m3. As a rule, the Radon concentration in buildings is higher and depends on the construction materials. The typical Radon concentration in buildings is 40 Bq/m3.
For making calculations from “radon concentration” to “effective dose”, “conversion (transition) dose coefficients” are used. The International Commission on Radiation Protection (ICRP) recommends corresponding conversion dose coefficients for Radon. For the population, living in buildings (dwellings) 7000 hours per year, the conversion dose coefficient for Radon recommended by ICRP is:
==> Conversion dose coefficient (inhalation) = 0.017 mSv/y for 1 Bq/m3.
Using this conversion dose coefficient, it can be calculated, that for one year, a person living in building with average annual Radon concentration of 60 Bq/m3, would take annual effective dose of 1 mSv (60 x 0.017 = 1.02). If the Radon concentration is 40 Bq/m3 – typical average Radon concentration in buildings – the annual effective dose would be 0.68 mSv (40 x 0.017 = 0.68).
Radon in Groundwater
Radon in drinking water may give doses of concern by ingestion, especially in young children. Through showers and washing machines, radon in household water is also released to the indoor air, which may subsequently be inhaled.
The most obvious pathway for exposure to radon in groundwater is by ingestion (drinking). Some studies have indicated a possible link between radon in water and gastric cancer (Mose et al. 1990), although this has yet to be conclusively proven. Swedjemark (1993) has estimated doses derived from radon in drinking water and believes that the dose from the ingestion pathway is most significant in young children (Table 2).
The data given in table 2 are based on Ingestion dose coefficients for adults and children
==> Ingestion dose coefficient for adults based on an annual drinking water consumption of 50l = 1x10-8 Sv/Bq
==>Ingestion dose coefficient for children based on an annual drinking water consumption of 75l = 2x10-8 Sv/Bq
When radon-rich groundwater enters a home, there is also potential for degassing of radon from water to household air, especially as the water heats up and the radon becomes less soluble. In particular, radon is degassed extremely effectively in household appliances such as dish-washing machines and shower units. Figure 3 indicates that, on turning on a shower (which utilises radon-rich groundwater), concentrations of radon in the air increase very rapidly. After the shower has been turned off, radon concentrations in air fall only slowly and persist above recommended maximum levels for a long time.
The Norwegian Radiation Protection Authority has recommended an action level of 500 Bq/l for radon in domestic water, and 200 Bq/m3 in household air (NRPA, 1995).