Environmental Health Perspectives
November 15, 2005


By Sarah Todd Davidson

Any exposure to radiation may cause cell damage that could lead to
cancer, according to a June 2005 report from the National Research
Council. The risk noted by the report, though small, is a third higher
than the risk of 8.46 cancers per 10,000 people exposed to 1 rem (or
10 millisieverts [mSv]) currently used by U.S. regulators. The report
contradicts critics who believe there is a threshold below which
radiation is harmless; it also fails to support those who say low
doses of radiation cause greater health damage per unit dose than high

The seventh Biological Effects of Ionizing Radiation (BEIR) report,
sponsored by several federal agencies, assessed and updated the health
risks from low linear energy transfer (low-LET) radiation, which
deposits little energy in a cell and thus tends to cause little
damage.The last BEIR report that addressed these health risks was
published in 1990.

Richard Monson, a professor of epidemiology at the Harvard School of
Public Health and chair of the group that conducted the study, says,
"We judged that the most reasonable shape is a line through the
origin." Simply put, this means any low-LET ionizing radiation may
increase the risk of a cell becoming cancerous--there is no threshold
below which there is no risk--and as exposure increases, so does the
health risk. Researchers refer to this straight line as the linear-no-
threshold model.

Less than 20% of people's low-level radiation exposure comes from
anthropogenic sources. The Earth and cosmic sources emit the
remainder. Nearly 80% of human-induced exposure comes from medical
procedures, about 15% from products like tobacco and building
materials, and around 5% from exposure at work.

For the purpose of the BEIR VII report, the authoring committee
defined low-LET radiation as levels up to about 100 mSv. For
comparison, a chest X ray averages around 0.1 mSv. The committee
concluded it's likely that about 1 out of 100 people would develop a
tumor or leukemia from exposure to 100 mSv above background. Of that
same 100 people, experts would expect 42 to develop cancers for other
reasons, but at the press conference marking the release of the
report, the committee said it did not fully exclude the possibility of
some radiation exposure being a factor in those cases.

The BEIR VII report employed statistical data to draw its conclusions
and reviewed studies of people exposed at work and in medical
settings. It also relied heavily on data from the Japanese atomic bomb

As these survivors age, more is revealed about the relationship
between radiation exposure and eventual health outcomes. Investigators
have also improved their estimate of the levels of exposure this
population received. But critics question the heavy reliance on the
Japanese survivors because of the "healthy survivor" effect--those who
survived the bombing might have been hardier than those who died early
on, potentially skewing the results.

Many researchers say the latest report helps reaffirm the general
accuracy of federal standards in place for limiting health risks from
low-level radiation. "We believe the data are more convincing than
fifteen years ago and show that the radiation protection standards we
use are reasonable," says Monson.

Mike Boyd, a health physicist who works on setting and updating those
standards for the Environmental Protection Agency, concurs. "I don't
think we'll be changing any federal standards," he says. "I'm not
willing to say there will be no impact. This report will go into our
estimation of risk and could lead to refinements, but generally
standards should stay the same."

Although most scientists agree the report incorporated the majority of
pertinent data up through 2003, information about low-LET radiation
continues to emerge. One hypothesis under investigation, says
biologist Andrew Wyrobek of Lawrence Livermore National Laboratory, is
the possible adaptive response cells developed over eons of natural
exposure. Other hypotheses include genetic instability (the idea that
some cells already have genetic mutations and are thus more prone to
becoming cancerous, given the incentive) and the "bystander effect"
(in which cells respond adversely to nearby irradiation although they
themselves weren't hit directly). These concepts were among those
reviewed for the BEIR VII report but were not incorporated into the
risk estimates.

Most experts agree that the BEIR VII report won't be the last in the
series. "Right now there is just a lot we don't know about how cells
react to very low doses of radiation," says Wyrobek. "But with
multiple exposures from more and more people undergoing medical
diagnostics in the low-dose range, and increased amounts of
radioactive waste, it's important to understand these ranges better."
Says Boyd, "I will be excited to see some future academy report after
we find out more about how radiation affects cells at very low doses."