Sperm Counts Fell by 62.3% in Past 50 Years — Exposure to Two Pesticide Classes Commonly Used on Food May Be Partly to Blame
Organophosphate and N-methyl carbamate pesticides used at residences and on food crops were associated with lower sperm counts, according to a study in Environmental Health Perspectives — and the higher the exposure, the greater the effect.
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Two common classes of pesticides may be responsible for the 50-year-long drop in global sperm counts.
According to a paper in Environmental Health Perspectives, men with the highest exposure to organophosphate and N-methyl carbamate pesticides had sperm counts almost one-third lower than men in the lowest-exposure group.
During the last 50 years, sperm concentrations in human semen have dropped by 51.6%, and total sperm counts fell 62.3%. Low sperm counts reduce a man’s ability to father children.
‘Strength of evidence warrants reducing exposure’
Led by Melissa Perry, Sc.D., an epidemiologist at George Mason University, investigators examined relevant epidemiologic studies published before Aug. 11, 2022, in PubMed, Scopus and Web of Science article archives plus the Centers for Disease Control and Prevention’s NIOSHTIC-2 Publications Search and the Science.gov gateway to federal science activity.
From 20 studies on 21 populations of 1,774 adult men and 42 exposure levels, the difference in sperm concentration between more- and less-exposed men was 30%.
Researchers determined their data to be “of moderate quality” but with sufficient evidence to associate the highest exposure levels to drops in sperm count.
“The strength of evidence warrants reducing exposure to OP [organophosphate pesticides] and NMC [N-methyl carbamate] insecticides now to prevent continued male reproductive harm,” they concluded.
As a precaution against selection bias, which is common in literature-based reviews or meta-analyses, the scientists ran their choices through the Navigation Guide systematic review methodology, a method for reducing bias and providing transparency to environmental health studies.
This process injects the data analysis with “what-if” scenarios with the intent to reduce the observed effect. However, every attempt led to the same strong association between pesticide exposure and reduced sperm counts.
“We were surprised to see such robust findings,” Perry said in a video interview on the George Mason University website. “We weren’t really sure, if we looked at all of the studies combined, would we find an aggregate effect, and if we applied such advanced calculations, would we still see this association show up.”
Organophosphates interfere with nerve impulse transmission
Organophosphate pesticides, which are widely used on food crops, include products that contain diazinon, ethoprop, tribufos and phosmet, which are applied in both agricultural and non-agricultural settings.
Diazinon and phosmet control insects, ethoprop kills worms and other soil pests while tribufos defoliates cotton right before harvest.
Organophosphates interfere with nerve impulse transmission by blocking acetylcholinesterase, an enzyme that breaks down the neurotransmitter acetylcholine, which plays a role in memory, learning, attention, arousal and involuntary muscle movement.
Once the acetylcholinesterase enzyme stops working, acetylcholine remains and builds up on the nerve cell, causing it to fire repeatedly.
Animals and humans exposed to organophosphates exhibit symptoms that include hyperactivity, uncoordinated movements, tremors, convulsions and paralysis.
The U.S. Environmental Protection Agency (EPA) originally planned to complete the quindecennial safety review for organophosphate pesticides between 2024 and 2025. But after recognizing the health risks posed by this class of pesticides, the agency announced on March 15, 2023, that it was moving its reevaluation up by two years.
“The science is clear,” said Michal Freedhoff, assistant administrator of the EPA’s Office of Chemical Safety and Pollution Prevention, in an EPA press release. Freedhoff added:
“Some uses of these four pesticides pose a serious health risk to the people that are exposed to them. … That’s why we’re taking early action now. While we know there’s still a lot of work to finish our review of these pesticides, today’s announcement helps deliver on our promise to protect farmworkers and uphold our commitment to environmental justice.”
N-methyl carbamates in high doses can paralyze respiratory system
N-methyl carbamates, used in homes, gardens and agriculture, are a diverse family of chemical ingredients. Two carbamates, aldicarb and fenoxycarb, are used mainly in residential settings while aldicarb and methomyl are mostly used in agriculture.
Since they are acetylcholinesterase blockers, they work similarly to the organophosphates.
Human exposure to N-methyl carbamates causes weakness, blurred vision, headache, nausea, tearing, sweating and tremors in humans. Very high doses kill by paralyzing the respiratory system.
Pesticides are comprised mainly of solvents that dissolve, stabilize and disperse the active chemical ingredients. The risks of exposure to the solvent are an open question because the pesticide formulations are often proprietary.
Although solvents are generally not as toxic as carbamates or organophosphates, the alcohols, glycols, petroleum distillates and chlorinated hydrocarbons they contain are sources of additional toxic exposure.
Pesticide production proceeds unabated and these products continue to be widely applied despite the increasingly apparent dangers, and the fact that they are dangerous even if used as directed.
“Pesticides are specifically formulated to be biologically active, to kill things,” Perry said.
Pesticides not the only cause of declining sperm counts
Many risk factors have been associated with low sperm counts, including:
- Varicoceles, reversible swelling of veins around the testicles, which also reduce sperm quality.
- Infections of sex organs or tissues, including sexually transmitted diseases.
- Ejaculation problems resulting from diabetes, spinal injuries or surgery.
- Medications, particularly alpha-blocker blood pressure medications. Some of these problems can be reversed but some are permanent.
- Benign or malignant tumors and their associated treatments.
- Drug, alcohol and tobacco use.
- Emotional stress and depression.
- Hormone imbalances.
- Sperm blockages.
- Chromosome defects.
- Celiac disease.
- Age, nutrition and lifestyle.
This long list of risk factors reduces the possibility that any one case of low sperm count is due to pesticide exposure alone. A deeper understanding of the pesticide-sperm link, and the relative contributions of these risk factors, will only arise through new experimental designs, larger study sample sizes and more studies.
“At the same time, given the weight of evidence in front of us, as our meta-analysis shows, there is plenty of evidence to advocate for reducing exposure to insecticides, especially among men who are intending to plan their family and father children,” Perry said.
“Taken together, this should be adequate evidence for policymakers to make some important decisions about how to reduce individual insecticide exposure, and to recognize it as a public health issue.”
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