Declining Semen Quality
been reported that human sperm counts have been falling around the world
and environmental chemicals are suspected as a cause.
Background: In 1992 a Danish team of investigators published a report
in which they examined all the scientific papers published between 1950
and 1992 containing data on semen quality. A decrease in the concentration
of sperm both the number of sperm in a man's ejaculate and the volume
of his ejaculate were decreased. This study has received considerable
attention and criticism by the scientific community but if nothing else
has lead to a flurry of studies from around the world, designed to examine
for changes in human semen quality.
Trends: The Danish team reported a global decline in semen quality
of approximately 2% per year over the preceding 50 years. Similar trends
have been reported in the UK and Paris as well as elsewhere.
Consistency of the data: A re-analysis of the data from the Danish
report has confirmed the original findings. However, in recent years
numerous papers have appeared in the scientific press varying results.
Some authors have reported a decrease in semen quality over time whereas
others have found no change or an increase in semen quality. A number
of these studies have shown that there are regional differences in semen
quality. For example, the semen quality in men residing in the Thames
Water Shed was lower compared to men living outside of the Thames Water
Shed. Regional differences in semen quality have also been described
in Canada and the United States. Interestingly, in some regions semen
quality has been found to be widely divergent from other geographic
areas although environmental contamination is similar. A further complication
in linking environmental contaminants with effects on semen quality
is the disconnection between lower sperm counts and decreased fertility.
For example, the time taken to achieve a pregnancy does not appear to
be increased in regions for which lower semen quality has been reported.
Experimental evidence: Animal studies using rodents have shown that
various industrial chemicals possess the capacity to alter semen quality.
Biological plausibility: Exposure to various chemicals has been
shown to result in lower semen quality in men in certain occupational
settings where exposures are high. Indeed, exposure to the pesticide
Kepone and the nemotocide dibromochloropropane (DBCP) resulted in lower
sperm counts and in some men complete absence of sperm. In the DBCP
example the mechanism of altered sperm counts was found to be due to
contaminant induced destruction of the precursor germ cells. While these
data demonstrate that semen quality can be reduced by contaminant exposure,
and an endocrine target has been adversely affected, an endocrine mechanism
has not been shown. Reports of decreased semen quality have not measured
exposure and thus it is difficult to infer from these studies that the
outcome measured is in any way related to chemical contaminants. Furthermore,
while environmental chemicals may induce changes in semen quality the
mechanism has yet to be demonstrated.
Experimental animal studies have shown that chemicals such as methoxychlor,
PCBs among others reduce epididymal sperm counts and daily sperm concentration
in dosed animals. However, again an endocrine mechanism has not been
demonstrated. Two reports have been published and provide an overview
of the biological plausibility of an endocrine mechanism that requires
Numerous other factors in addition to potential effects of environmental
contaminant are known to affect semen quality. For example, an association
between lower semen quality and prescription medications, cigarette
smoking, age, heat, and solvent exposure has been documented.
Conclusion: The weight of the evidence supports a conclusion that
man-made chemicals can induce changes in human semen quality. However,
it has not been shown that such is occurring in the general population
or that an endocrine mechanism is involved.