Exposure to commonly used insecticides has been associated with lower sperm concentration in men, according to a study published in the journal Frontiers in Endocrinology.

The researchers analysed semen and urine samples from 179 men attending an assisted reproductive technology clinic in Jinan, China.

They found that men with the highest levels of neonicotinoid insecticides in their semen had a 27% lower sperm concentration than those with the lowest levels.

Neonicotinoids are widely used in agriculture to control insect pests. They are often applied to seeds before planting, and residues can remain in food and water.

Neonicotinoids are a family of chemicals used to kill insect pests that damage crops. They work by disrupting the insects' nervous systems.

The chemicals are used on crops and can be applied directly to plants or as coatings on seeds before planting. Plants can absorb them as they grow, and residues may remain in food and water.

The study examined four neonicotinoid insecticides which include imidacloprid, acetamiprid, clothianidin and thiamethoxam as well as substances produced when the chemicals break down.

The researchers investigated the chemicals together because people can be exposed to mixtures rather than to a single insecticide at a time.

The researchers found that desnitro-imidacloprid, a breakdown product of imidacloprid, accounted for more than 40% of the estimated decline in sperm concentration associated with the mixture.

The findings add to research investigating whether exposure to pesticides and other environmental chemicals contributes to declining sperm counts.

Questions about exposure monitoring

The study found that some pesticide breakdown products were present at higher concentrations in semen than in urine.

Urine tests are commonly used to assess exposure to chemicals because substances and their breakdown products can be excreted through urine. However, the researchers said measurements from urine might not fully capture the levels of these substances in semen.

This matters because the two types of samples can reflect different periods of exposure. Urine measurements generally provide information about more recent exposure, while semen quality can be affected by exposures over several weeks or months.

A professor of environmental health sciences at the University of Michigan School of Public Health, John Meeker, said the study added human evidence to concerns about the possible reproductive effects of neonicotinoids. However, he identified the timing of sample collection as a limitation.

The researchers collected urine and semen samples at the same time. Meeker said collecting several urine samples before obtaining a semen sample could help researchers assess the relationship between exposure and sperm quality more accurately, particularly when chemical concentrations are very low.

The study's authors called for longer-term research to establish the relationship between neonicotinoid exposure and male reproductive health and to help inform regulatory limits.

Implications for fertility

Sperm concentration is one measure of semen quality and can contribute to assessments of male fertility. Researchers have also investigated whether poor semen quality is associated with other health conditions.

A 2016 study cited in the research found that men diagnosed with male-factor infertility had a higher risk of certain adverse health outcomes, including cardiovascular and metabolic conditions, in the years following their fertility assessment.

Meeker said the 27% difference observed in the neonicotinoid study might not affect the fertility of every man.

However, he warned that a similar reduction across a large population could have consequences for men whose semen quality was already low or close to the lower range associated with impaired fertility.

Such men could move into a subfertile or infertile range, particularly if they were also exposed to other substances that affect reproductive health.

A professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai, Epidemiologist Shanna Swan, described the reported reduction in sperm concentration as substantial.

Swan published a major study in 2017 reporting a 52% decline in sperm concentration among men in Western countries over approximately four decades. That research examined possible links between declining sperm counts and environmental and lifestyle factors, including exposure to certain synthetic chemicals and pesticides.

She said the latest study raised further questions about the possible reproductive effects of neonicotinoids, given the extent of human and animal exposure to the chemicals.

Swan added that more research was needed into their effects on female reproductive health and the combined effects of exposure on couples experiencing infertility.

The researchers said further studies would be needed to establish the nature of the relationship between neonicotinoid exposure and semen quality, determine the effects of long-term exposure and assess the implications for reproductive health.