Sperm Decline Worse Than Believed Earlier

Their analysis of data collected from 1938 to 1990 indicates that sperm densities in the United States have exhibited an average annual decrease of 1.5 million sperm per milliliter of collected sample, or about 1.5 percent per year, while those in European countries have declined at about twice that rate (3.1 percent per year).

Despite enormous differences in data collection methods, study population and time period, most studies have come to the same conclusion -- that sperm density has declined. In fact, a 1992 review of 61 such studies revealed a steady decline, from 113 million sperm per milliliter in 1938 to 66 million in 1990, or about 1 million sperm per milliliter per year.

However, 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, each of which can influence the observed trend. One of the researchers, epidemiologist Shanna Swan of the California Department of Health Services, commented, "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."

Using a statistical model that corrects for individual differences in these key variables as well as geographic area, Swan and her colleagues reanalyzed the data from 56 of the studies cited in the 1992 paper. The investigators excluded three non-English language studies and two others that included men who had conceived only after an infertility workup.

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.

For non-Western countries, a group that included Brazil, India, Israel, Hong Kong, Kuwait, Nigeria and Thailand, the trend was slightly positive. However, because these data were taken from only 13 studies, all of which were published after 1978, this trend was not statistically significant.

Although the authors do not address the specific causes of this phenomenon, some recent studies have focused on the relationship between environmental exposures and declining sperm quality. In one such 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.

While there is no evidence that this apparent decline in sperm density has led to reduced fertility, the authors say sperm count may be a surrogate indicator of effects on the male reproductive system. They say, for example, 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.

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