Radon and Thoron in the Environment - Aerosols: Measurement, Dosimetry, and Health Effects - page 547

the mixture. UNSCEAR (2006) adopts 40% as an equilibrium factor for indoor exposure and 60%
for outdoor exposure. Thus, the radon gas concentration times 0.4 or 0.6 is accepted as the numeri-
cal value of the EEC for indoor and outdoor environments.
Figures 21.5 and 21.6 show that as the inhaled particle size changes with the particular environ-
ment, the dose could change by factors of 2-3. The median particle size of inhaled particulates
present in various indoor and outdoor environments can change signiicantly, but usually over short
time periods such as during cooking when the particle size decreases. The unattached fraction can
change with the total aerosol particle loading, decreasing with higher aerosol concentration.
21.10 DOSE TO OTHER ORGANS
Radon is quite soluble in body tissues and there is always some dose to organs other than the lung. The
annual bronchial dose from 222 Rn and that to other organs such as soft tissues, female breast, and skin
(also from atmospheric plate out of decay products) is small compared with the bronchial dose. For
an average radon exposure of 40 Bq m −3 and decay product equilibrium of 40%, the dose factor from
Figure 21.5 for a breathing rate of 1.2 m 3 h −1 and a median particle size of 0.2 μm, the dose is 10 nGy
Bq −1 m −3 h. Assuming this exposure full-time each year, the calculated dose is 40 × 0.4 × 10 × 8760 h,
or about 1.4 mGy year −1 . The dose to female breast and soft tissues would be 0.003 and 0.001 mGy
year −1 , respectively (Harley and Robbins, 1992), or less than 1% of the bronchial dose. UNSCEAR
(2006) adopted an annual per capita effective bronchial dose of 1.15 mSv for the global population.
21.10.1 d ose to tHe F etus FroM r adon in d rinking w ater
Robbins and Harley (2002) calculated the dose to the developing fetus from ingested water by the
mother. The model indicates an increase from 9 weeks to about 14 weeks and then a decrease. This
is due to the assumed changing blood low rates. The dose at 1 week is zero. This is a consequence
of the very small 222 Rn concentration in maternal blood in which the embryo loats.
The equivalent dose values can be compared with the average dose to any developing fetus from
the natural external gamma ray and cosmic ray radiation of approximately 1 mSv during pregnancy
(NCRP, 1988). The maximum equivalent dose to the fetus is about 0.4% of the fetal-life external
gamma-ray and cosmic-ray dose for each 100 Bq ingested in water by the mother.
If we assume an average consumption per day of 0.6 L of raw tap water at a concentration of
100 Bq L −1 , the calculated total dose to the fetus over the term of the pregnancy is 0.25 mSv or 25%
of the normal background radiation dose.
21.10.2 r adon and c HildHood l eukeMia
An epidemiologic study of childhood leukemia in Denmark (2400 cases, 6697 controls) from 1968
to 1994, suggested a weak, but statistically signiicant, association of residential radon exposure
and acute lymphoblastic leukemia (ALL). The Danish study estimated a relative risk (RR) = 1.56
(95% CI, 1.05-2.30) for a cumulative exposure of 1000 Bq m −3 years. For an exposure duration of 10
years, their RR corresponds to a radon concentration of 100 Bq m −3 . There are two dose pathways
of interest for alpha particles to damage potential stem cells for ALL. One is the alpha dose to bone
marrow, and two, the dose to bronchial mucosa where an abundance of circulating lymphocytes
is found. Compared with an exposure of about 1 mSv year −1 from natural external background,
radon and decay products contribute an additional 10%-60% to the bone marrow equivalent dose.
The other pathway for exposure of T (or B) lymphocytes is within the tracheobronchial epithe-
lium. Inhaled radon decay products deposit on the relatively small area of airway surfaces and
deliver a signiicant dose to the nearby basal or mucous cells implicated in human lung cancer.
Lymphocytes are collocated with basal cells and are half as abundant. Using a 10 year exposure to
100 Bq m −3 , dose estimates suggest that the equivalent dose to these lymphocytes could approach