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Monday, November 24, 1997, 12:00 a.m. EDT
Reanalysis of International Data Finds Sharp Decline in Sperm Density
After an extensive review of data from 61 published studies, three California researchers have concluded that a decline in average sperm density reported in the U.S. and other Western countries may be even greater than previously estimated.
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).
The study was conducted by epidemiologists Shanna Swan, Eric Elkin and Laura Fenster of the California Department of Health Services. It appears in the November issue of Environmental Health Perspectives, the monthly scientific journal published by the National Institute of Environmental Health Sciences.
Since the early 1930's, there has been considerable interest in declining semen quality as a key predictor of male reproductive dysfunction. The vast majority of studies designed to answer this question have focused on sperm density - the number of sperm contained in one milliliter of sample. (One milliliter is approximately 1/30th of an ounce.)
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 (Evidence for decreasing quality of semen during past 50 years, E. Carlsen, A. Giwercman, and N. Skakkebaek, British Medical Journal, vol. 305, page 609) 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. Swan said, "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.
Since no mathematical model can ever fit the data perfectly, there is always a certain discrepancy between what the model predicts and what actually happens - this is often referred to as variability. "A perfect model would explain 100 percent of this variability," says Swan. "Our model accounts for 80 percent - that is the best fit of any model that has been proposed."
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.
Researcher Shanna H. Swan, Ph.D., can be reached at (510) 450-3818.
CDC and NIH Join in Testing Exposure of Americans to Environmental Estrogens and Other Chemicals
NIH's National Institute of Environmental Health Sciences and the Centers for Disease Control and Prevention's National Center for Environmental Health have launched a study of blood and urine samples to determine the amount of exposure that Americans have to environmental estrogens.
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 study underway by NIH and CDC will address these questions.
Richard J. Jackson, M.D., director of CDC's National Center for Environmental Health, said, "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 longstanding expertise in measuring toxic substances in people's blood and urine and is a valuable public health collaboration with NIEHS."
Kenneth Olden, Ph.D.,director of both NIEHS and the National Toxicology Program, which is headquartered at NIEHS, said, "The study will help us develop priorities for studying the potential adverse health effects of exposure to environmental estrogens. We hope this kind of collaboration will be expanded in the future to address many other toxic substances that we know or suspect cause cancer, reproductive, and other health effects."
The NIEHS and NTP are providing $2.1 million to CDC to measure approximately 50 environmental estrogens in 200 persons to determine levels of exposure to the population. CDC and NIEHS will jointly agree on the final list of environmental estrogens to be measured in people. Among the more familiar chemicals that will be tested for are: insecticides such as arsenic, dieldrin, mirex, lindane, parathion and DDT and its metabolites; herbicides such as 2,4-D, alachlor and atrazine; nematocides such as aldicarb; fungicides, plant and fungal estrogens, and industrial chemicals such as cadmium, lead, mercury, PCBs and dioxins. CDC will use existing analytical methods for blood and urine to measure most of the chemicals and develop new analytical methods to measure 10 to 20 of the environmental estrogens.
The coordinator for this research for NIEHS and NTP, George Lucier, Ph.D., said, "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."
By measuring chemicals in people's blood and urine, scientists can determine what chemicals Americans are being exposed to, how much exposure is occurring to each chemical, what population groups are at high risk of excessive exposure, and whether interventions aimed at reducing exposure to a chemical have actually been effective and reduced the chemical level in people.
For example, blood lead measurements obtained as part of the National Health and Nutrition Examination Surveys conducted by CDC's National Center for Health Statistics have documented a more than 78% reduction in lead in the U.S. population, since 99.8% of lead has been removed from gasoline and lead is no longer used in food and drink cans in the U.S. Similar assessments could be made for other toxic substances to determine whether the U.S. populations' exposure is increasing or decreasing. This exposure information helps prioritize public health efforts in environmental health and direct toxicologic research towards exposures of most health concern.