Radon and Thoron in the Environment - Aerosols: Measurement, Dosimetry, and Health Effects - page 545

100

80

210 Pb = 500 µBq m -3

60

40

20

0

1 10 100 1,000 10,000

Particle diameter (nm)

FIGURE 21.4 Aerosol particle size distribution measured indoors in a single-family home over a 3 month
interval using 210Pb as a tracer.

UNSCEAR (2006) adopted a bronchial dose conversion coeficient of 9 nSv Bq −3 m −3 h (EEC),
and included a table of 13 published dose conversion coeficients derived from bronchial dose
models.
Harley et al. (2005a,b) developed an integrating particle size sampler that operates for up to
3 months indoors or outdoors. The short-lived radon decay products ultimately form long-lived 210 Pb
and 210 Po (22 years, 138 days) and these radionuclides are used as tracers. Figure 21.4, shows a size
distribution indoors in a typical residence.
A few percent of the 218 Po formed from radon decay associates with water vapor or other
gases and remains as a small cluster of atoms with a diameter of a few nanometers (nm). All
atmospheres contain this nanometer or unattached fraction. Because of their small diameter, the
unattached fraction deposits eficiently in the bronchial airways. A small percentage of the total
activity inhaled accounts for a disproportionate fraction of the bronchial dose with up to about
25% due to the unattached fraction. The unattached fraction is evident in Figure 21.4 in the
1-2 nm size region.
Saccomanno et al. (1996) evaluated the lung cancer histology in 467 Colorado Plateau uranium
miners and 311 nonminers to determine the localization of lung tumors. They showed that 84% of
the lung cancer in uranium miners was located in the bronchial airways and 77% in the bronchial
airways of nonminers who were mostly smokers. The tumors in the pulmonary or gas exchange
region were 16% and 23% in miners and nonminers, respectively. Thus, lung tumors arise mainly
in the bronchial airways whether from 222 Rn exposure or tobacco smoke. Most of the radon decay
products, that is, about 20% of what is inhaled, deposit in the lower lung while only a few percent
deposit in the bronchial region. However, the very large surface area (square meters) for deposition
in the lower or pulmonary lung compared with a much smaller area (a few hundred square centime-
ters) in the bronchial region accounts for the much larger bronchial dose, in spite of the lower actual
radioactivity deposition of the decay products.
Ruzer et al. (1995) measured the deposited radon decay product gamma ray activity in the chest
of metal miners in Tadjikistan by external counting. Measurements on about 100 miners were per-
formed along with iltered air samples to assess the decay product inhalation exposure. Using these
data, actual measured breathing rates were obtained for drillers, assistant drillers, and inspection
personnel. The group average breathing rates were 0.0079, 00067, and 0.0052 m 3 min −1 , respec-
tively, with upper bound values of 0.023 ± 0.004, 0.020 ± 0.004, and 0.015 ± 0.003 m 3 min −1 .