How does EPA estimate the risk of
health effects from low-level exposures?
Since we cannot measure the
stochastic effects of very low-level exposure, health
physicists must extrapolate the risks from what they know
about high-level exposure. Most radiation protection organizations
estimate the risk of health effects from low-level exposures,
all the way to zero exposure, as proportional to those
of high-level exposures.
This method of estimating risk is called the 'linear no-threshold
model.' It assumes is no "threshold" exposure level
below which the risk is zero.It also assumes that the risk
increases proportionally to the exposure. If the exposure
doubles, the risk also doubles. Some scientists strongly
dispute the no-threshold assumption.
Scientists use several methods to estimate the risks from
exposure to a particular substance. Using one method, they
compare the number of people who experience a particular health
effect in two groups. The groups are the same except that
one group has been exposed to the substance and one group,
known as the "control group," has not.
A way of refining the risk estimate is to identify two groups
of people who have been exposed to the substance. One group
is made up of people who are experiencing a particular health
effect and the other group consists of people who are not.
This method allows scientist to identify other risk factors
(such as a family history of the disease) that may make one
group more likely to experience the health effect.
For example, estimating the risk of cancer in smokers,
requires comparing the occurrence of cancer in a group of
smokers to a group of non-smokers. Each group would be similar
in age, education, occupation, income, etc.
Identifying other risk factors, such as a family history
of the cancer, level of exercise, and alcohol use, can be
done by comparing a group of smokers who have cancer with
a group of smokers who do not.
We use similar methods to estimate the risks from exposure
to ionizing radiation. However, the estimates include uncertainty,
because of major challenges in making the estimates:
Developing an exposure history can be extremely difficult.
Separating the effects from exposure levels that are tens
or hundreds of times smaller than exposures due to background
is extremely difficult.
Determining whether radiation exposure is the cause of
a particular occurrence of a health effect, such as
cancer. Many chemicals are also carcinogens.