SH, RL Kruse, L Fan, DB Barr, EZ Drobnis, JB Redmon, C Wang, C Brazil
and JW Overstreet and the Study for the Future of Families Research
Group. 2003. Semen quality in relation to biomarkers of
pesticide exposure. Environmental
this paper, Swan et al. report that the risk of low
sperm quality in Missouri men is strongly associated with pesticide
exposures, especially alachlor, diazinon and atrazine.
While the sample size is relatively small, the odds ratios revealed
by their research are extraordinarily high and very unlikely to
be the result of chance. The scientists conclude that exposures
to these agricultural chemicals is most likely to be through contaminated
is the first time scientists have shown a link between levels of
current, widely used pesticides and semen quality. Its importance
is magnified even more because it is also the first to show an effect
in the general population, not just in farmers or in pesticide workers.
strongest association was with alachlor. Men with the highest alachlor
levels were 30x more likely to have low sperm quality. This result
was highly significant statistically, with the probability of it
occuring by chance calculated to be less than 1 in 1000 (p = 0.0007).
did they do? Swan's team obtained semen and urine samples
from 426 men living in rural Missouri (208) and urban Minneapolis
(218), each of whom was a partner of a pregnant woman. Thus by definition
the men were fertile. The first stage of this analysis, describing
the subjects and documenting geographic differences, was published
earlier in 2003. Each of the semen samples was analyzed for
sperm count (sperm per milliliter), sperm morphology (% normal sperm)
and sperm motility (% motile sperm).
current study carries that analysis farther. From the initial sample,
Swan et al. selected a subset of the men with the highest
(52 men, of them 25 from Missouri) and the lowest sperm counts (34
men, 25 from Missouri) and analyzed their urine for traces of 15
pesticides many in widespread use today.
then used a variety of analyses to examine associations between
pesticide level and sperm quality.
did they find? Their
first analysis showed that men in Missouri were exposed to a different
suite of pesticides than those in Minneapolis. This is not surprising,
as most of the Missouri men were from rural areas in the midst of
agriculture, where most Minneapolis men were from an urban setting.
And because of crop and climatological differences between Missouri
and Minneapolis, the mix of pesticides applied agriculturally would
be expected to differ between the samples also.
pesticides, including the herbicides alachlor and atrazine and the
insecticide diazinon, were much more likely to be encountered at
higher levels in Missouri than in Minneapolis.
then turned their attention to links between pesticides and semen
quality. The results were striking: Men with higher levels of alachlor,
atrazine and diazinon were significantly more likely to have lower
sperm counts than men with lower levels of pesticides in their urine.
The risk of poor semen quality was elevated 30-fold with higher
alachlor levels; 17-fold with IMPY (the diazinon metabolite), and
12-fold with higher atrazine levels. These odds ratios were highly
statistically significant (p= 0.0007, p= 0.0004 and p= 0.01, respectively).
results suggested reduced semen quality was linked to two other
herbicides, metolachlor and 2,4-D, also, but the pattern was of
only borderline statistical significance. Further research with
a larger sample would be necessary to determine whether these associations
were real. For one pesticide, acetochlor, the data suggested that
greater exposure was linked to higher sperm count, but this too
was of borderline significance.
saw no link between exposure and sperm quality for DEET or malathion.
upon interview data, few of the men could have been exposed to the
agricultural pesticides through occupation. Swan et al.
conclude that the likely route of exposure is via drinking water.
This is consistent with the fact that these pesticides are known
to be common in drinking water sources in the mid-West, and that
they are not removed by usual water treatment methods.
does it mean? As
noted above, this study is important because it links, for the first
time, reduction in sperm quality to exposure to several pesticides
in widespread use today, and it does so in a study of men selected
from the general population instead of men likely to be exposed
occupationally (e.g., farmers or pesticide workers). The exposures,
moreover, are most likely through drinking water.
associations are very strong for alachlor, diazinon and atrazine.
Odds ratios of this magnitude are rarely seen in epidemiological
studies of the general population, and are on par with
the elevation in risk linking lung cancer to cigarette smoking.
That association is now firmly established by many studies of many
different types, a depth of proof that is still lacking for this
new work. The magnitude of the odds ratios reported by Swan et
al. should force public health authorities to take immediate
note of these results, invigorate research into the issue, and stimulate
evaluations of public health measures to be taken as scientific
does this mean for couples attempting to conceive? This can't be
answered definitively from the current study. The way that Swan
et al. constructed their case and control groups was to
draw them from partners of pregnant women, i.e., men who were confirmed
to be fertile. By definition, the reductions in sperm quality in
this men was insufficient to cause infertility--although whether
it took longer for the partners of men with low sperm quality to
become pregnant was not assessed.
determine whether or not alachlor, atrazine and diazinon can reduce
sperm count sufficiently to impair fertility will require repeating
the study with a sample of men from couples seeking infertility
treatment. In these couples, some cases of infertility will be due
to male problems, others to female. Then when an analysis like Swan
et al.'s is performed, sperm quality should span a range
from high to low, including sufficiently low to impair fertility.
The key question will be whether the cases of male infertility are
associated with high exposures, perhaps exposures beyond the levels
seen in this study. It is entirely plausible, for example, that
by restricting their sampling to partners of pregnant women, Swan
et al. inadvertently excluded men with even higher levels of pesticide
exposure, if those caused complete fertility impairment.
et al. conclude with an understatement: "If further
study confirms these findings, the implications for public health
and agricultural practice could be considerable." The findings
are so striking that even without scientific certainty of causation,
they pose an important challenge to public health authorities.