Uranium mine workers inhale a considerable amount of radioactive dust
containing radon gas. The decay products of radon settle in the lungs, and
radiations from them can cause lung cancer.
Dose–Response Relationship
A meaningful dose–response relationship for carcinogenesis should be
based on data with both low and high radiation exposures. However, the
low-dose data (below 10 rad or 10 cGy) that have been accumulated thus
far are mostly inconclusive, because of the small sample size, lack of appro-
priate control, incomplete dosimetry, and other related factors. So risks at
low doses are primarily estimated by extrapolation of the data from high-
dose experiments. Several authoritative committees (international and
national) are responsible for establishing the dose-response relationship,
and they are the United Nations Scientific Committee of the Effects of
Atomic Radiations (UNSCEAR), the Committee on the Biological Effects
of lonizing Radiations (BEIR), the International Commission on Radio-
logical Protection (ICRP), and the National Council on Radiation Protec-
tion and Measurements (NCRP) in the United Sates. These committees
base their analysis on the data of the Japanese survivors of the A-bomb
attacks on Hiroshima and Nagasaki, data on human exposures mentioned
above (see Epidemiologic Evidence of Carcinogenesis), and data from in
vitro cell culture and animal studies.
The risk versus dose relationship has been controversial, particularly
about the minimum level of radiation dose that induces cancer (Murphy,
1991). Some experts propose that the dose–response relationship is linear,
without a threshold dose, and that a very minimal dose can cause cancer
(Fig. 15.15). A threshold dose is a dose below which no radiation damage
occurs in an individual. The LNT dose–response relationship has been
based on the extrapolation of high-dose data to low-dose data (below 10
rad or cGy) and has drawn a considerable debate as to its validity. In one
argument, the opponents of the theory question the validity of such extra-
polation, because the mechanism of radiation damage may be quite differ-
ent at doses that differ by orders of magnitude. Also, this group points out
that the people living in high natural background radiation areas (e.g.,
Rocky Mountains) do not show to have any more apparent adverse health
effects than those in low-dose areas (e.g., sea level). On the other hand, the
proponents of the LNT theory suggest that a single hit by a radiation can
cause the mutation of a cell, and consequently result in carcinogenesis in a
later period, thus supporting the theory. The recent BEIR VII report
strongly supports the LNT theory, suggesting that even the smallest dose
can cause a small risk of cancer in humans.
All intermediate and high-energy data primarily obtained from the
Japanese survivors of the A-bomb attacks are fitted by a linear quadratic
curve (Fig. 15.15). The curve is linear at lower doses and becomes propor-
Long-Term Effects of Radiation 251