SH. 2003. Do environmental agents affect semen quality?
Epidemiology 14: 261-262. [note]
also invited commentary from an
this invited commentary for the scientific journal Epidemiology,
Univ. Missouri reproductive epidemiologist Dr. Shanna Swan places
two new scientific reports in the context of the ongoing
debate about environmental impacts on semen quality.
Danish researchers establish a clear link between maternal smoking
and a son's risk of lower sperm counts. The sons of women who
smoke are more than 4 times as likely as sons of nonsmokers to
have sperm counts beneath 20 million sperm per milliliter, a level
judged by the World Health Organization to indicate impaired fertility.
the second, Harvard and CDC scientists report that phthalate
exposure is associated with poorer sperm quality, at phthalate
levels well within the range commonly experienced by men in the
observes that "taken together, these two studies provide good
(although not conclusive) evidence that environmental agents ...
can alter semen quality."
on the same day these studies (and commentary) appeared in Epidemiology,
a companion paper to the Harvard
study (above) was published on line by the journal Environmental
Health Perspectives, showing a link between PCBs and semen quality.
Swan notes, the effort to establish (or refute) long-term trends
in sperm count globally have proven elusive. Data are consistent
with a decline over the past half-century, but because of differences
in study design, changes in measurement, and spotty geographic coverage,
they don't allow firm conclusions to be reached.
scientists have turned their attention to studying geographic variation
in semen quality using standardized approaches and gathering extensive
companion information, with the intent of elucidating factors driving
that geographic variability. Striking geographic differences are
emerging in this research, in the
US and in Europe.
commentary: These new studies, as
well as others in progress, provide insights into exposures that
may be involved. Lab studies with animals show conclusively that
sperm quality can be affected by environmental agents. Body burden
measurements clearly demonstrate that human exposure to these agents
is widespread. Now human epidemiology is finding patterns
in human semen quality associated with the environmental agents
known to act adversely in animals.
this stage in the research, it is important not to lose sight of
the fact that simultaneous exposures to multiple agents is a certainty.
Strong evidence that one agent is involved doesn't constitute proof
that another is innocent. Multiple factors can be contributing simultaneously,
with the relative contributions changing from one place to another
or over time.
apart their separate and combined contributions will be a significant
challenge. But waiting for scientific certainty about causation
before instituting measures that will reduce exposures is not appropriate.
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