Environmental Engineering Reference
In-Depth Information
15.8 RADIOACTIVE AEROSOLS AND LUNG IRRADIATION
The incidence of elevated rates of lung cancer among miners exposed to radon decay products has
led to programs that control worker exposure. Furthermore, such evidence is the basis for concern
about lung cancer risk among the general public due to radon in homes, which can lead to a substan-
tial number of putative lung cancer deaths.
In conducting epidemiological studies or in controlling worker or general-public risks, human
exposures to the decay products must represent or estimate, in some way, time-integrated airborne
concentrations of the relevant radionuclides (“exposure”) or calculated energy deposition in the lung
tissues affected (“absorbed dose”). Intermediate quantities that may also be used are the amount
of activity deposited in the whole lung or relevant part of the lung (“deposited activity”) or intake,
which was oficially included together with concentration in the Norms of Radiation safety. The
use of any of these quantities in either epidemiological studies or control programs involves, either
explicitly or implicitly, presumptions about their relationships with one another. However, although
it is the dose that is the cause of changes in tissue that lead to cancer, it is airborne concentration
that is measured and corresponding estimates of exposure that are used as correlates or predictors
of risk.
The absorbed dose or the activity (intake) in lung tissue is calculated to understand the actual
insult leading to lung cancer, or to permit the quantitative comparison of doses arising from atmo-
spheres having different characteristics such as in mines and homes. If exposure is used as an index
of dose (activity), for example, in epidemiological studies, presumptions are made implicitly about
a comparability or constancy of breathing rates and of the retention of decay products in the lung.
Since both dosimetric and epidemiological data for miners are the main source of a risk cancer
mortality assessment for miners and the general population, it is important to understand to what
degree correlation between the measured radon decay products concentration in the air and dose to
the lung takes place in the real mining environment.
In this connection, it should be noted that, especially in mining environments, inhalation rates
depend on the groups involved and on working conditions, speciically the physical load, which can
vary substantially. Retention of the decay products depends on the properties of the aerosols and
also on the load of work and is typically not known accurately.
It should be pointed out that measurements of the airborne concentrations are not complete
enough to provide directly the value of exposure, because the concentrations to which individuals
are exposed vary substantially in time and space. For example, variation of the ventilation rate even
for a short period of time can lead to a substantial change in concentration. Moreover, the concentra-
tions of radon decay products and other nuclides in the breathing zone may differ substantially from
the value measured by the actually implied standard instrument (Domanski et al., 1989). Finally,
the very concept of “workplace,” associated concentration, and load of work are indeinite, since
miners are typically at a number of places during their working day, with variable concentrations (as
well as nature of work). Since the measurements of concentrations in mines may be performed only
once or twice a month (not to mention estimated retrospectively), we cannot expect reliable corre-
spondence between exposures estimated from measurement results and actual personal exposures,
that is, time-integrated concentrations. Thus, the use of measurements of airborne concentration as
a basis for estimating or comparing dose can lead to substantial errors, both because of the lack of
correspondence to concentration measurements and personal exposures and because of uncertainty
and variability in breathing rate and radionuclide retention. These errors, in aggregate, may be as
much as an order of magnitude and therefore make dosimetric and consequently risk assessment
data unreliable (Ruzer, 2001).
It should also be mentioned that the dose to the lung from radon itself is negligible in compari-
son with that from its decay products. However, until now assessment of the risk in many studies is
based on the measurement of radon with an assumption on the degree of equilibrium between radon
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