Swan, SH. 2003. Do environmental agents affect semen quality? Epidemiology 14: 261-262. [note]

Epidemiology also invited commentary from an NIH epidemiologist

In 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.

• In the first, 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.
• In 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 United States.

Swan observes that "taken together, these two studies provide good (although not conclusive) evidence that environmental agents ... can alter semen quality."

Coincidentally, 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.

As 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.

Hence 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.

OurStolenFuture.org 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.

At 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.

Teasing 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.

[note] Epidemiology prevents direct links to individual abstracts of published articles on its website. The journal's home page is: www.epidem.com. The abstract and (for subscribers) full text of this article can be found by browsing through published issues of the journal.