Study links common insecticides to lower sperm concentration in men

Higher exposure to commonly used insecticides was associated with lower sperm quality in men, according to a new study that adds to a growing body of evidence about the reproductive health risks of pesticides. 

The study, published in Frontiers in Endocrinology, analyzed the semen and urine samples of 179 men attending an assisted reproductive technology clinic in Jinan, China, and found that higher neonicotinoid levels in semen were linked to a 27% drop in sperm concentration.

The study comes at a time when global fertility rates are down by more than half from where they were 50 years ago and global sperm counts have decreased by 62% over the last five decades. Researchers say there are likely a number of factors for declining sperm counts, but increasingly point to chemical exposures as key drivers.

Neonicotinoids are the world’s most widely used class of insecticides, and are often used in mixtures with one another to target multiple pests. They are commonly applied as coatings to seeds planted by farmers, and residues of the chemicals can persist in food and water.

While safety regulators often evaluate pesticides individually, the study looked at the specific mixtures of multiple insecticides inside human semen. Researchers evaluated a mixture of primary neonicotinoid insecticides (including imidacloprid, acetamiprid, clothianidin, and thiamethoxam) and their breakdown products.

Comparing men in the highest exposure quantile (the top 25%) to those in the lowest, researchers found a 27% drop in sperm concentration. Desnitro-imidacloprid, a specific breakdown product of imidacloprid, proved to be the most damaging ingredient in the mix, driving over 40% of that decline.

While standard safety tests generally rely on urine, researchers found that urine samples don’t provide the full picture, as pesticide breakdown products were found at higher levels in semen than in urine.

The study’s authors concluded that modern-day measurement methods are falling short, and that standard urine monitoring doesn’t fully capture the extent of exposure, calling for long-term research to inform regulatory thresholds for neonicotinoids. 

“This study adds human data to a growing body of scientific evidence that neonicotinoid insecticides may adversely impact male reproductive health,” said John Meeker, a professor of environmental health sciences at the University of Michigan School of Public Health. Additional studies should be conducted to fill some of the limitations from this study, he said.

One of the limitations, according to Meeker, was that the urine and semen samples were collected at the same time, but represent different exposure windows. On one hand, urine reflects recent exposure, while changes in semen quality may reflect cumulative exposure over weeks or months. 

“A study that is able to collect multiple urine samples prior to the semen sample collection may be better equipped to detect associations due to the challenges with measuring very low concentrations in semen,” Meeker said.

Declining sperm count is of concern to researchers because along with impacting fertility, it can also be a key indicator of overall health. A 2016 study found that men diagnosed with male factor infertility had a significantly higher risk of adverse health outcomes, including cardiovascular and metabolic conditions, in the years after an infertility evaluation. 

Meeker said that while a 27% drop in sperm count might not affect a healthy individual’s fertility, widespread neonicotinoid exposure that lowers sperm counts across a large population “would likely result in many people with low or borderline semen quality to cross into the subfertile or possibly even infertile range, especially when considering what we know about other reproductive toxicants that people are also commonly exposed to simultaneously.”

Prior studies have linked chemical exposures to a drop in sperm counts. 

“This is a substantial drop,” epidemiologist Shanna Swan said of the 27% drop in sperm concentration. Swan is adjunct professor in obstetrics and gynecology at the University of California San Francisco.

In 2017 she published a landmark study which found a 52% decline in sperm concentration among men in Western countries over four decades. Her research pointed to synthetic chemicals like phthalates, PFAS (also known as “forever chemicals”), and agricultural pesticides as primary culprits.

Swan said that while the current study looked at semen quality rather than fertility as an endpoint, she believes it raises important questions, given “the extensive exposure of humans (and animals) to these pesticides,” adding that the effects on female reproduction and exposure to couples must be further examined when considering infertility.

Featured image by Alexandre Daoust on Unsplash.