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9) How do scientists determine whether an environmental agent, such as a static EM field causes or contributes to the development of cancer?




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This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder jmoulder@its.mcw.edu and the Medical College of Wisconsin with numerous contributions by others.

9) How do scientists determine whether an environmental agent, such as a static EM field causes or contributes to the development of cancer?

There are certain widely accepted criteria [11,63,64], often called the
"Hill criteria" [11], that are weighed when assessing epidemiological
and laboratory studies of agents that may cause human cancer. Under
these criteria one examines the strength, consistency, and specificity
of the association between exposure and the incidence of cancer, the
evidence for a dose-response relationship, the laboratory evidence, the
biological plausibility of the association, and the coherence of the
proposed association with what is known about the agent and about
cancer.

- The first criterion is whether there a clear increase in cancer
incidence associated with exposure. The excess cancer found in
epidemiological studies is usually quantified in a number called the
relative risk (RR). This is the incidence of cancer in an "exposed"
population divided by the incidence of cancer of an "unexposed"
population. Since no one is unexposed to static fields, the comparison
is actually "high exposure" versus "low exposure". A RR of 1.0 means no
effect, a RR of less than 1.0 means a decreased incidence of cancer in
the exposed group, and a RR of greater than 1.0 means an increased
incidence of cancer in the exposed group. A strong association is one
with a relative risk (RR) of 5 or more. Tobacco smoking, for example,
shows a RR for lung cancer 10-30 times that of non-smokers.

- The second criterion is whether most studies show about the same
increased incidence of the same type of cancer. Using the smoking
example, essentially all studies of smoking and cancer have shown an
increased incidence of lung and head-and-neck cancers.

- The third criterion is whether cancer incidence increase when the
exposure increases? Again, the more a person smokes, the higher the
increased incidence of lung cancer.

- The fourth criterion is whether there is laboratory evidence
suggesting that the cancer is associated with exposure. Epidemiological
associations are greatly strengthened when there is laboratory evidence
to support such an association.

- The fifth criterion is whether there are plausible biological
mechanisms that suggest that there should be an association between the
agent and cancer. When it is understood how something causes disease,
it is much easier to interpret ambiguous epidemiology. For smoking,
while the direct laboratory evidence connecting smoking and cancer was
weak at the time of the Surgeon GeneralĀ¹s report, the association was
highly plausible because there were known cancer-causing agents in
tobacco smoke.

- The sixth criterion is whether the association between exposure to an
agent and cancer is coherent (consistent) with other things that we know
about the biophysics of the agent and the biology of cancer.

These criteria must be applied with caution [11,63,64]. First, it is
necessary to examine the entire published literature; it is not
acceptable to pick out only those reports that support the existence of
a health hazard. Second, it is necessary to directly review the
important source documents; it is not acceptable to base judgments
solely on academic or regulatory reviews. Third, satisfying the
individual criteria is not a yes-no matter; support for a criterion can
be strong, moderate, weak, or non-existent. Lastly, these criteria must
be viewed as a whole; no individual criterion is either necessary or
sufficient for concluding that there is a causal relationship between
exposure to an agent and a disease.

 

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