- Even very low doses of radiation pose a risk of cancer
over a person's lifetime, a National Academy of Sciences panel concluded
Wednesday. It rejected some scientists' arguments that tiny doses are harmless
or may in fact be beneficial.
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- The findings could influence the maximum radiation levels
that are allowed at abandoned reactors and other nuclear sites. The conclusions
also raise warnings about excessive exposure to radiation for medical purposes
such as repeated whole-body CT scans.
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- "It is unlikely that there is a threshold (of radiation
exposure) below which cancers are not induced," scientists said in
the report.
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- While at low doses "the number of radiation-induced
cancers will be small ... as the overall lifetime exposure increases, so
does the risk," the experts said.
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- Scientists for years have debated how extremely low doses
of radiation affect human health.
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- Pro-nuclear advocates, as well as some independent scientists,
have maintained that the current risk models for low-level radiation has
produced more stringent requirements than is necessary to protect public
health.
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- It is an issue in determining decontamination requirements
at abandoned reactors and at federal weapons sites.
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- The academy's panel stood by the "linear, no threshold"
model that generally is the acceptable approach to radiation risk assessment.
This approach assumes that the health risks from radiation exposure decline
as the dose levels drop, but that each unit of radiation - no matter how
small - is assumed to cause cancer.
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- "The scientific research base shows that there is
no threshold of exposure below which low levels of ionized radiation can
be demonstrated to be harmless or beneficial," said Richard R. Monson,
the panel's chairman. He is a professor of epidemiology at Harvard's School
of Public Health.
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- The panel said new and more extensive data developed
over the past 15 years only strengthen the conclusions of the panel's last
report, in 1990, on low-level radiation risks.
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- The scientists estimated that one out of 100 people exposed
to 100 millisievert of radiation over a lifetime probably would develop
solid cancer or leukemia, and that half of those cases would be fatal.
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- It also said that 42 additional cancers can be expected
in the same group from other than low-level radiation sources.
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- A millisievert is a measurement of radiation energy deposited
in a living tissue. People absorb about 3 millisievent of radiation annually
from natural sources and 0.1 millisivert every time they get a chest X-ray.
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- The report noted that exposure from a whole body CT scan
is about 10 millisievert, much higher than a normal X-ray. That raised
concerns about the frequency of such medical diagnostics.
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- The report should not scare people away from nuclear
medicine, said Dr. Henry Royal, a professor of radiology at Washington
University in St. Louis. He said most often the benefits of such tests
and treatments outweigh the risks.
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- But Royal also said that procedures such as CT scans
should be used to deal with a specific medical problems and not part of
annual medical screenings. "You should not be exposed to radiation
for superficial reasons," Royal said in a telephone interview.
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- Some anti-nuclear advocates said the study reaffirms
that stringent regulations are needed when cleaning up abandoned nuclear
sites or considering health risks near nuclear power plants.
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- "The NAS panel puts to rest once and for all claims
that low doses of radiation aren't dangerous ... nuclear advocates have
been making this claim for years" said Daniel Hirsch, president of
Committee to Bridge the Gap, a Los Angeles-based nuclear watchdog group.
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- Mitchell Singer, a spokesman for the Nuclear Energy Institute,
the industry's lobbying arm, said the report "is a positive finding.
It shows there is very little risk of exposure from low levels of radiation."
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- The academy is a private organization chartered by Congress
to advise the government of scientific matters.
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- National Academy of Science: www.nationalacademies.org
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- Copyright © 2005 The Associated Press. All rights
reserved.
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