Sperm Decline/Environmental Estrogen Link?

Researchers at the California Department of Health Services conducted an extensive review of data from 61 published studies, representing sperm density information gathered from 1938 to 1990. This review shows that sperm densities in the United States have declined every year during that period, with an average annual decrease of 1.5 million sperm per milliliter of collected sample, or about 1.5 percent per year. Sperm densities in European countries have declined at about twice that rate (3.1 percent per year).
 
A review of  the same 61 studies published in 1992 (E. Carlsen, et al.; British Medical Journal, vol. 305, page 609) also showed a 50 year trend in declining sperm counts, but not quite as dramatic. The California researchers noticed that these studies did not take into account such factors as the age of the subjects, the length of abstinence prior to sample collection, and method of sample collection. Any of these factors could have influenced the observed trend.

"Most of the critics have suggested ways in which the data analysis might be skewed, but no one has ever looked at the data from these earlier studies to see whether these hypothetical biases are actually present," noted epidemiologist Shanna Swan.

Swan and colleagues utilized statistical modeling to correct for individual differences in these key variables as well as geographic area, and then reanalyzed the data from 56 of the studies cited in the 1992 paper. Three non-English language studies and two others that included men who had conceived only after an infertility work-up were excluded from the new analysis.

While the results of their analyses also showed a significant decline in sperm density, it was the rate of the decline, particularly in Western countries, that was most surprising. "We observed a decrease of about 1.5 million sperm per milliliter per year in the United States, and a corresponding decrease of about 3 million sperm per year in Europe,"
reports Swan.

There is still no evidence that this apparent decline in sperm density has led to reduced fertility. However, sperm count may be a surrogate indicator of effects on the male reproductive system. The researchers note that in countries such as Denmark, England and the United States, where sperm counts have fallen, the incidence of testicular cancer has increased dramatically over the last 25 years, while in Finland, where sperm counts are still relatively high, testicular cancer rates have remained low.

There has been a flurry of recent research looking of the potential relationship between exposure to environmental pollutants and declining sperm quality. In one study, researchers reported a significant correlation between lowered sperm densities and increased levels of organochlorine compounds in the subjects' seminal fluid. In another, investigators found that a general decline in sperm concentration during the years 1949 to 1981 was statistically linked to an overall increase in several environmental exposures, including environmental estrogen.
 

NEW FEDERAL STUDY

In sufficient amounts, these chemicals can act like the female hormone estrogen. Although the effects of any exposure are unknown, some scientists have suggested that environmental estrogens might be reducing sperm counts in men and causing breast cancer, fibroids and other reproductive diseases in women. At present, scientists know little about which of the environmental estrogens people are exposed to and how much exposure they have. The new study will address these questions.

"This kind of assessment of exposure to environmental estrogens is absolutely critical to the scientifically credible assessment of  potential health risk from these compounds. The study builds on CDC's long-standing expertise in measuring toxic substances in people's blood and urine and is a valuable public health collaboration with NIEHS." Richard J. Jackson, M.D., director of CDC's National Center for Environmental Health, said.
 
The study will measure approximately 50 environmental estrogens in 200 persons to determine the level of exposure to the population. Among the more familiar chemicals that will be tested are: insecticides such as arsenic, dieldrin, mirex, lindane, parathion, and DDT and its metabolics; herbicides such as 2,4-D, alachlor and atrazine; nematocides such as aldricarb; fungicides, plant and fungal estrogens, and industrial chemicals such as cadmium, lead, mercury, PCBs, and dioxins.

"This project will give us an idea of human exposure to each of the chemicals and help us set priorities for the studies done in the National Toxicology Program. Comparing the levels with other health and toxicity data, we should be able to determine if some of the higher exposures we find are linked to increased incidences of disease." said George Lucier, Ph.D., coordinator for the research project.

The research appears in the November 1997 issue of Environmental Health Perspectives, the monthly scientific journal published by the National Institute of Environmental Health Sciences.