Health Risks of Ionizing Radiation: - Clark University

104 Uranium Miners
enrichment facilities. The exposures of millers have
not been studied as intensively as the exposures
of miners or radiation workers generally. Some of
the papers that we discuss in this section include
the exposures of millers, and some of the papers in
section 6 discuss exposure at uranium processing
and enrichment facilities. The bulk of this section,
however, is focused on the exposures of miners.
The mine environment is dangerous is many
ways, and lung cancer risk factors include silica,
arsenic, chromium, and nickel. Exposure to radiation
in mining typically occurs through inhalation of
air and dust containing radon (Rn-222); the term
“radon” is usually used in reference to an air
mixture of alpha-emitters including radon and its
radioactive decay products, or daughters. Radon
daughters can be retained in the lungs for long
periods, so exposure continues long after the initial
inhalation. It should be noted that although Rn-222
is a decay product of uranium, exposure to radon
is not at all limited to uranium mining--miners of
other ores and residents of many homes are exposed
to the naturally occurring decay products of both
uranium and thorium. The exposures of uranium
miners are thus comparable to exposures in some
other mines and to residential radon exposures,
although residential exposures are usually lower in
magnitude. Comparisons with residential exposures
are discussed at the end of this section, but based
on the availability of exposure information and the
relatively high exposures, epidemiological data on
miners provides some of the best information that
we have about alpha radiation exposure, especially
lung exposure due to inhalation.
Studies of miners and other radon-exposed
cohorts are hard to compare to the rest of the
radiation epidemiology field because exposure is
not measured in conventional units. Exposure to
radiation in mines is measured in “working levels”
which correspond to 200 pCi (picocuries) of radon
daughters per liter of air. Exposure to the equivalent
of one working level for one month of work is called
one Working Level Month (WLM). The conversion
of WLM to sieverts is not straightforward because it
depends on breathing rate and the size of the radon
daughter aerosols. UNSCEAR (2000) suggests that
the average value is roughly 5.7 mSv per WLM 3 .
Although there has been an awareness of the
hazards of mining for over a centrury, it was not
until 1959 that the US passed a radon safety standard
for workers’ health at 1 WL (or 12 WLM per year).
This standard was adjusted in 1971 to an annual
allowable exposure of 4 WLM (Caufield 1989).
Still, by 1971 many miners in the US and elsewhere
had already been exposed to much greater amounts
of radiation than standards allowed, and the earlier
epidemiological reports easily showed correlations
between exposure to radiation in uranium mining
activities and adverse health effects.
8.2 US studies
Early US studies looked at large groups of uranium
miners from various mines in the western part of
the country (see Figure 8-2). Wagoner et al. (1964)
showed a significant excess of lung cancer mortality
among uranium miners and millers (15 observed
vs. 6.9 expected, p