New research this week suggests that humanity’s war against insectkind has had some unintended consequences: declining sperm counts. The study, a review of the existing data, found a clear association between increased exposure to insecticides and lower sperm concentrations in adult men. The authors say that the evidence is strong enough to warrant new regulations that would reduce people’s exposure to these chemicals.
Several studies indicated that men’s average sperm count has steadily declined over the past half-century, particularly since the early 2000s. Scientists speculated on many possible reasons for this worldwide drop, such as increased rates of obesity or greater exposure to environmental toxins, insecticides included. Researchers at George Washington University, George Mason University, and Italy’s Ramazzini Institute wanted to get a better sense of the data linking insecticides to sperm quantity so they decided to perform a systematic review of relevant studies around the world.
They analyzed 25 studies that were conducted over the past 25-plus years that looked at men’s occupational and environmental exposures to two widely used classes of insecticides: organophosphates and N-methyl carbamates. These studies also measured men’s sperm concentrations (sperm concentration can be used to calculate total sperm count). The team saw a clear pattern, even after accounting for other possible factors.
“What we found is that there was a consistent robust finding across those 25 studies—that increased exposure to these insecticides was associated with decreases in sperm concentration,” senior study author Melissa Perry, dean of the George Mason University College of Public Health, told Gizmodo over the phone.
The authors say their paper, published Wednesday in the journal Environmental Health Perspectives, is the most comprehensive systematic review on this link to date. But like all research, the study does have its caveats.
Importantly, it can only show a correlation between insecticide exposure and sperm counts, not demonstrate a clear cause-and-effect relationship. Many of the studies were cross-sectional as well, meaning that they only studied people at a single point in time. There is also still some debate over whether sperm counts have truly declined over time, as well as whether any such decline has actually impacted male fertility in general. Lastly, the study can’t tell us exactly how insecticides might be damaging sperm.
Other research has supported a causative connection between insecticides and sperm. Just last month a separate meta-analysis concluded that organophosphate exposure was associated with reduced sperm counts and other markers. Studies in animals have indicated that these chemicals can directly interfere with hormone receptors key to male fertility.
The authors say more research should be funded and conducted to better understand the exact role that insecticides might be playing in declining sperm counts, among other important questions. These studies ideally would proactively track insecticide exposure and sperm quality in men over a long time, in what’s known as a prospective cohort study. But they also argue that people and governments should already be taking steps to limit our collective exposure to these chemicals, given what we know.
“At this point in time, I do believe that this is compelling, convergent evidence that men should avoid being exposed to insecticides, particularly if they’re planning on having a family or wanting to father children,” Perry said.