Are Pesticides Killing Your Sperm?
Why you probably don't have to worry about your little swimmers just yet
There’s nothing we love to hate more than man-made substances. As a society, we have decided that things which we collectively make, regardless of how useful they may be, are secretly killing us in a myriad of ways. Every week there’s a new story about how some common household exposure is going to destroy humanity, whether it be mobile phones, screen time, or how much coffee everyone drinks these days.
The latest in the trend is pesticides. According to the headlines, common pesticides in our food are killing men’s sperm across the globe. As NBC put it, “Global decline in male fertility linked to common pesticides”, which is, if true, really quite terrifying.
The subtext to these headlines is genuinely scary. If the results are correct, then we could be looking at a huge reduction in the human population very soon, and all because we used pesticides to help with growing crops.
Fortunately, the headlines are extremely misleading. While there may be some association between pesticides and sperm, it’s very vague and only really applies to people who work with the substances on a day to day basis. For the average person eating meals at home, there’s no evidence that you need to worry about these pesticides murdering your little swimmers.
The Science
The new study that has caused all of this ruckus is a systematic review and meta-analysis of epidemiological studies looking at organophosphate and carbamate pesticide exposure and how that relates to semen production in adults. This is a type of study where the researchers use a relicable methodology to search through all of the literature on a subject, and combine all of the studies on a topic into one big meta-estimate.
After searching through a range of databases, this particular review aggregated together a total of 21 studies which represented 42 different estimates of how pesticide exposure was associated with sperm concentration. These figures were pooled together in the main analysis. This showed that higher exposure to organophosphates and carbamates was associated with a reduction in sperm concentration of 0.3 standard deviations. To put that in context, I had a look at the average standard deviation across a few studies, and an 0.3 SD difference would mean a reduction of about 25% in sperm concentration measured as millions per ml of fluid.
The systematic review itself was also really good. Honestly, this paper is one of the best reviews I’ve seen in years. Methodologically, it’s pretty much perfect - I have some quibbles with how the authors rated their studies for bias, but aside from that it’s genuinely an excellent review.
And yet, I’m still unconvinced that there’s actually a connection between these pesticides and sperm. Why?
Problem One - Samples
The first reason that this result is dicey is due to the samples that the authors were able to identify. There were 21 total studies, sure, but of those 15 looked explicitly at farm workers and had no non-farm control group, while only 6 looked at more diverse populations. Even the studies of men in the community who weren’t farm workers were highly selected, like this paper run on men attending an infertility clinic, or this one that included mostly farm workers but also a tiny group of people from a nearby town as well.
It’s hard to know what these studies could tell us about regular daily exposures to pesticides in food. Most of the men included in the meta-estimate worked every day on farms spraying these chemicals - that may have no connection to the experience of your average non-farm worker who has drastically lower exposure to the pesticides.
This is actually demonstrated by the study’s results. When the authors of the review stratified the estimates into occupational vs environmental exposures, there was no impact on pesticide exposure whatsoever for men who didn’t work directly with the chemicals. In other words, the association between pesticides and less sperm was only seen in farm workers, and not in your average person anywhere else.
Problem Two - Biased Studies
The second issue was that much of the research that the authors of this review identified just…wasn’t very good. The median sample size across all the papers was 86. Most studies didn’t control for potential confounding factors, and the ones that did often failed to include important things like BMI, income, and how long people had been exposed. There was only one study in the entire review that looked at what could reasonably be called a population sample - all the other papers looked at highly confounded groups like men who attended fertility clinics. Virtually all of the 21 papers were cross-sectional, which means that they didn’t even track pesticides and sperm over time.
All of this means that it’s very hard to draw causal conclusions between pesticides and low sperm from this research. Indeed, as the authors themselves note, the low quality of the included literature “makes causal inference challenging at this time”. The study shows that pesticides may be associated with lower sperm counts, which is decidedly less certain a finding than the headlines suggest.
Problem Three - Sensitivity Analyses
In addition to problems one and two, there’s also the fact that the authors did a bunch of sensitivity analyses which seem to rather undercut their main results. For example, there was no association between pesticides and sperm count when they only looked at studies with adequate assessment of exposure. Similarly, papers that took into account how long men had been abstinent before giving their, uh, samples, did not show an association either.
All of this weakens the perceived association between pesticides and low sperm. If the association is only present in certain groups, and doesn’t appear to apply when studies control for fairly basic measures of sperm quality, then it’s hard to see how it could be a real connection.
Bottom Line
As with all environmental exposures, there is a plausible connection between your little swimmers and pesticides. However, this particular study mostly just shows how weak the evidence around this topic currently is. While there is some potential for organophosphates and carbamates to be impacting people’s sperm concentration, the evidence shown here - which, remember, is all of the evidence we have so far on the topic in humans - is decidedly unconvincing. In my opinion, the biggest take-home from this paper is that we need better research on the topic, which is decidedly more boring than the headlines but also a bit closer to the truth.
Perhaps even more importantly, even if you take these results entirely at face value they only really apply to farm workers. Yes, for people who are exposed to large quantities of these pesticides every day there was a bit of an association, but for the general population of people who produce semen the results show no real impact.
As someone who works directly in public health, I’m generally pretty fond of regulating harmful substances, but I just don’t see any strong evidence here. We might need to improve our legislation to protect farm workers, but otherwise there’s no reason to worry about pesticides in your food killing off your sperm.