Health Risks of Ionizing Radiation: - Clark University

13
DISCUSSION
Our intention in creating this review was to provide
accessible information regarding exposures to low
doses of ionizing radiation. We ended up including
studies of people exposed to a wider range of
doses, including unknown doses, because all of
these studies have generated important and relevant
information. In this concluding section we return to
our original mission, focusing specifically on low
doses of ionizing radiation. Given the substantial
uncertainty and complexity 1 associated with low
doses of radiation we do not attempt to quantify
risks but instead review the challenge of interpreting
these data and making decisions in a ‘gray’ area.
We begin by returning to one approach to low-dose
radiation modeling, the threshold.
The idea of a threshold dose. People who
are exposed to low doses of radiation might find
themselves talking with a health expert who tells
them that they were exposed to a harmless dose.
This message can be conveyed in many ways but
it rests on an important assumption, the assumption
that there is a dose of radiation below which no
health effects will occur. This dose is often referred
to as a ‘threshold’ dose.
Based on evidence from epidemiology, animal
studies, cellular studies, and the basic physics of the
interaction of radiation with matter, most scientists
assume that there is no threshold dose. They
assume that any dose of radiation, no matter how
small and including radiation that we are exposed
to naturally, carries a risk. This risk is assumed to
be proportional to dose so that a very small dose
is associated with a very small risk. This model of
the health effects of radiation is often named the
linear no-threshold hypothesis; linear refers to the
proportional relationship between dose and risk. All
major agencies and committees support this model
(BEIR 1990, ICRP 1991, EPA 1999, UNSCEAR
2000, Upton 2003 2 ).
The linear no-threshold model is not universally
accepted, however, and some experts still present
threshold doses as a way of demonstrating that a
particular exposure was harmless. The Agency for
Toxic Substances and Disease Registry (ATSDR),
for example, occasionally uses a threshold model
of risk in its Public Health Assessments. One such
document claimed that there is a human cancer
threshold of 0.1 Sv, and that doses below 0.1 Sv
would not lead to cancer 3 . The Health Physics Society
has stated that “below 5-10 rem (0.05-0.1 Sv), risks
of health effects are either too small to be observed
or are nonexistent” and they recommend that risks
below this level not be estimated (HPS 2004). This
is a minority perspective but it maintains credibility
among some people, including those who have an
interest in relaxed exposure standards.
Positive results at low doses. Despite claims
to the contrary there have been many statistically
1 Risks have been shown in this review to depend on age at exposure, time since exposure, dose rate, type of radiation,
background cancer risk factors, gender, and the endpoint of concern.
2 The National Council on Radiation Protection and Measurements (NCRP) recently formed a Scientific Committee
to reassess the model and they determined that it was the most plausible perspective on dose response in Report No.
136 (Upton 2003).
3 The document stated that “studies of children who received x-rays in utero indicate that there is a threshold dose for
radiogenic leukemia that lies in the range of 10 to 50 rad (0.1 to 0.5 Gy)” (ATSDR 2002).
167