Male Fertility

Reanalysis of International Data Finds
Sharp Decline in Sperm Density

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Subject: Health – men

National Institute of Environmental Health Sciences

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

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

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.


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See also Male Fertility and Environmental Estrogens