[CPEO-MEF] NAS press release on perchlorate report

[Lenny Siegel <lsiegel@cpeo.org>]

THE NATIONAL ACADEMIES
FOR IMMEDIATE RELEASE
January 10, 2005

Report Assesses Health Implications of Perchlorate Exposure

WASHINGTON -- A new report by the National Academies' National Research
Council on the health effects of perchlorate, a chemical that in high
doses can decrease thyroid function in humans and that is present in
many public drinking-water supplies, says daily ingestion of up to
0.0007 milligrams per kilogram of body weight can occur without
adversely affecting the health of even the most sensitive populations.
That amount is more than 20 times the "reference dose" proposed by the
U.S. Environmental Protection Agency in a recent draft risk assessment.

Environmental releases of perchlorate -- a component of rocket fuel and
fireworks -- have been discovered in 35 states, and more than 11 million
people have perchlorate in their drinking water at concentrations of 4
parts per billion or higher. As it considers a first-ever national
standard for acceptable levels of perchlorate in drinking water, EPA has
issued a series of draft risk assessments, each containing a reference
dose upon which a standard could be based. Controversies over the
scientific conclusions reached in the risk assessments, however, led the
federal government to request that the National Research Council review
the issue. 

The most recent EPA risk assessment, published in 2002, proposes a daily
reference dose of 0.00003 milligrams per kilogram (mg/kg) of body
weight, which the agency said would correspond to a drinking-water
concentration of 1 part per billion based on certain assumptions about
body weight and daily water consumption. The committee that wrote the
Research Council report did not include a corresponding drinking-water
concentration with its reference dose because the assumptions that are
used to derive drinking-water standards involve public-policy choices
that were beyond the committee's charge.

Perchlorate inhibits the thyroid's uptake of iodide, which is essential
for the production of thyroid hormones. One potential consequence of
that effect is low thyroid hormone production, or hypothyroidism. EPA
has predicted that an ultimate consequence of that effect is the
development of thyroid tumors -- a conclusion the agency based on the
occurrence of a few thyroid tumors in rats exposed to perchlorate. The
committee disagrees with EPA's conclusion and thinks that perchlorate
exposure is unlikely to lead to thyroid tumors in humans. Humans are
much less susceptible to disruption of thyroid function or formation of
thyroid tumors than rats, and therefore the way rats responded to
perchlorate exposure is not a good indicator of how humans would react.

In the past, high doses of perchlorate were used to treat patients with
hyperthyroidism, or excessive thyroid hormone production, but a few
patients had serious adverse reactions, and the use of perchlorate in
this manner was largely abandoned. More recently, patients with
hyperthyroidism have been treated effectively and safely with moderate
doses of perchlorate for up to two years. Perchlorate has been
administered to healthy subjects in doses ranging from 0.007 mg/kg to
9.2 mg/kg per day with no changes in thyroid hormone production to
suggest any adverse effect on thyroid function. On the basis of these
and other studies, the committee concluded that a perchlorate dose of
more than 0.4 mg/kg per day would be required to adversely affect
thyroid hormone production and cause hypothyroidism. However, the dose
required to cause hypothyroidism in pregnant women, infants, children,
and people with low iodide intake or pre-existing thyroid dysfunction
might be lower. 

There have been studies on the health effects of human populations
exposed to perchlorate, but they were studies in which data were
available for geographic areas, not for individuals. Relationships
observed at the geographic level may not apply at the individual level,
and therefore such studies cannot provide direct evidence of causation.
They can support a possible association between two events, however,
which allowed the committee to reach some conclusions based on those
studies. In particular, the committee found that the available evidence
is not consistent with an association between exposure to perchlorate in
the drinking water at concentrations up to 120 parts per billion during
pregnancy and changes in thyroid hormone production in normal-birth
weight, full-term newborn infants. The evidence is insufficient to
determine whether or not there is an association between perchlorate
exposure and adverse neurodevelopmental outcomes in children.

Because of the weaknesses in the studies of the health effects in human
populations exposed to perchlorate in the environment, the committee
recommended against using them to determine a reference dose. Rather, it
recommended using a 2002 clinical study in which groups of healthy men
and women were administered perchlorate in daily doses ranging from
0.007 mg/kg to 0.5 mg/kg for 14 days. The study found no statistically
significant inhibition of iodide uptake by the thyroid at the 0.007
mg/kg daily dose. The findings in this study are supported by the
results in four other studies of healthy subjects, including a six-month
study. The committee recommended that an uncertainty factor of 10 be
applied to the 0.007 mg/kg per day level to protect the fetuses of
pregnant women who might have hypothyroidism or iodide deficiency. This
results in the 0.0007 mg/kg per day reference dose recommended in the
report. 

One committee member, concerned over the adequacy of the data, dissented
and thought that an additional uncertainty factor of three should be
applied. The rest of the 15-member committee responded by pointing out
that the key study examined the effects of four dose levels in a total
of 37 subjects, and that four other studies had remarkably similar
results. The committee was unanimous in all other findings and recommendations.

The committee emphasized that the reference dose should be based on
inhibition of iodide uptake by the thyroid in humans, which is not an
adverse effect but the key biochemical event that precedes any health
effects caused by perchlorate exposure. The committee called this a
"conservative, health-protective approach to perchlorate risk
assessment." It also suggested studies that have the potential to more
precisely define "safe" perchlorate exposures. Future findings could
result in the need to adjust the reference dose recommended in the
report, the committee acknowledged.

The study was sponsored by the U.S. Environmental Protection Agency, the
U.S. Department of Defense, the U.S. Department of Energy, and NASA. The
National Research Council is the principal operating arm of the National
Academy of Sciences and the National Academy of Engineering. It is a
private, nonprofit institution that provides science and technology
advice under a congressional charter. A committee roster follows.

Copies of Health Implications of Perchlorate Ingestion will be available
later this winter from the National Academies Press; tel. 202-334-3313
or 1-800-624-6242 or on the Internet at http://www.nap.edu. Reporters
may obtain a pre-publication copy from the Office of News and Public
Information (contacts listed above).

For the original news release, list of Committee members, and the link
to the full report, go to
http://www4.nationalacademies.org/news.nsf/isbn/0309095689?OpenDocument

-- 


Lenny Siegel
Director, Center for Public Environmental Oversight
c/o PSC, 278-A Hope St., Mountain View, CA 94041
Voice: 650/961-8918 or 650/969-1545
Fax: 650/961-8918
<lsiegel@cpeo.org>
http://www.cpeo.org
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