Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers


The science of sperm count declines


No aspect of the extensive scientific evidence considered in Our Stolen Future commanded more media attention than the controversial reports of declining sperm counts, launched by the 1992 paper by Carlsen et al. reporting 40% decline in sperm count over the second half of the 20th century.

Animal studies clearly demonstrated that in utero exposure to low levels of endocrine disruptors at critical times in development cause low sperm counts and other sperm maladies. Human studies document exposure to these compounds. Data presented by Carlsen et al. suggested that major declines were occurring in people.

All of a sudden, endocrine disruption seemed less theoretical and more concrete in its impacts--not just concrete... potentially catastrophic if the decline was real and if it was continuing. Industry scientists set out to undermine the validity of the Carlsen report, seizing upon many virtually unavoidable difficulties in retrospective studies of something as difficult to measure in an unbiased way as human sperm density. At times the New York Times seemed in collusion with this effort, via the biased reporting of the reporter on that newspaper's staff who handled the story.

Shortly after Our Stolen Future went to press, a series of new studies began to emerge revealing, it would appear, significant geographic variation in sperm count and in sperm count trends. The bottom line for the sperm story is that:


Sperm counts have decreased significantly in some areas and held steady in others. There are no reports of significant increases in sperm count. Mathematically this means there has been an overall average decline. But more practically, it means that there may be clues as to the causes of decline in the geographic patterns.




Sperm count varies geographically. The variability among areas is as large as the largest temporal declines. A large number of studies are now underway, documenting these geographic changes and looking for their causes. Research published in 2003, for example, finds large differences in the United States comparing men in the mid-West to men on the coasts.




There is no agreed-upon explanation of why declines have taken place, if they have, nor why there is geographic variation.




Endocrine disruption is one of the plausible explanations for the declines, and for the geographic variation. This is particularly the case because controlled experiments with laboratory animals reveal endocrine disrupters decrease sperm count when the male fetus is exposed at critical times of development in the womb. Results show ways in which in utero exposure to endocrine disruptors can both increase and decrease adult sperm count.


Humans are exposed quite commonly to substances, such as phthalates and dioxin, that are known to cause sperm count declines in experimental animals. Data from the CDC released in September 2000 indicate that within the US one of the subpopulations with highest exposures may be women of child-bearing age. The mixture of exposures varies significantly among areas and demographic groups.



Serious epidemiological work on phthalates is now underway. Studies published in 2003 link phthalate levels to DNA damage in sperm and to sub-optimal sperm characteristics (sperm count, mobility and deformity). Other links to low sperm count are also being found, including exposures to certain PCB congeners and to maternal smoking during pregnancy.


  It is not likely that there is a single cause of low sperm count. The process of sperm formation can be disrupted at multiple points, some in the developmental formation of the glands and organs essential for sperm production, some in altering hormonal sensitivity, and some in the ongoing process in adulthood of spermatogenesis. Different agents are likely to attack different parts of the chain of events that leads to the production of healthy sperm. This reality will confound all but the most sophisticated of epidemiological studies. Taking cues from animal experiments will be essential.





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