Air pollution appears to alter how sperm genes function, one of the largest fertility studies of its kind has found.
Men exposed to common air pollutants while sperm were developing showed subtle DNA changes that affected whether genes were switched on or off, raising fresh concerns that air pollution may harm male fertility and the health of offspring.
The findings, presented at the annual meeting of the European Society of Human Reproduction and Embryology in London, identified ozone and nitrogen dioxide as the pollutants most strongly linked to these changes.
The study followed more than 2,000 men in Salt Lake City, Utah, between 2013 and 2017. Participants provided semen samples when they enrolled and again after two, four and six months.
Researchers estimated each man’s exposure to outdoor air pollutants, including ozone, nitrogen dioxide, sulfur dioxide and fine particulate matter, during the three months before each sample, the period of sperm production.
Growing evidence that air pollution affects sperm health and may reduce male fertility has been mounting for years, but the biological mechanisms have been unclear.
Most research has focused on pollution’s effects on sperm DNA fragmentation, which refers to damage in the DNA strands inside sperm cells, as well as sperm shape and movement.
The latest work, however, points to a different mechanism – DNA methylation.
Men exposed to common air pollutants while sperm were developing showed subtle DNA changes that affected their genes (stock image)
A major fertility study suggests that air pollution may harm male fertility by altering DNA methylation — chemical tags that act like dimmer switches on genes in sperm cells — offering a potential biological pathway for the first time
DNA methylation is a process where chemical tags attach to DNA and act like dimmer switches, turning genes up or down without changing the underlying genetic code – as one possible explanation.
Most of these tags are wiped clean early in embryo development, but a small number of genes are ‘imprinted,’ meaning they retain these marks and carry them forward, giving them the potential to influence the development of embryos and beyond.
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Scientists analyzed sperm DNA methylation in 1,220 men who provided samples at the six-month follow-up. They identified 39 DNA changes linked to air pollution mixtures, with ozone and nitrogen dioxide appearing to have the strongest influence.
One of the genes identified, GNAS, has previously been linked to poorer semen quality and fetal development.
Studies have found that when the paternal copy of GNAS is altered, it can lead to severe intrauterine growth retardation (IUGR), meaning the baby is born significantly smaller than expected for their gestational age.
IUGR, also referred to as fetal growth restriction, increases the risk of stillbirth, premature birth, brain injury, hypothermia, low blood sugar and the myriad of life-long health complications that can result from these conditions.
‘Because imprinted genes can persist through early embryonic development, this raises important questions about whether fathers’ environmental exposures may influence not only fertility, but pregnancy and offspring health,’ Dr Carrie Nobles, an epidemiologist at the University of Massachusetts Amherst and study leader, said.
She added that air pollution is a complex mixture that varies by season and location.
Approximately 152 million Americans – nearly half of the country – are living in places that receive failing grades for unhealthy levels of ozone or particle pollution, according to the American Lung Association’s 2026 State of the Air report.
Pollution levels shift with the weather. For example, ozone tends to be higher on hot, sunny days because sunlight and heat help form it. In winter, pollutants such as particulate matter from heating may be more common.
A rural farming area has a different pollutant profile than a city center, where nitrogen dioxide from heavy traffic is a constant concern. Cities are particularly prone to high levels of nitrogen dioxide and ozone.
Approximately 152 million Americans – nearly half of the country – are living in places that receive failing grades for unhealthy levels of ozone or particle pollution. Pictured above is wildfire smoke pollution seen in Philadelphia in 2023
Nitrogen dioxide is largely a byproduct of vehicle exhaust and fossil fuel combustion. Ozone, however, is not released directly but forms when other pollutants react in sunlight.
Cities supply plenty of those raw materials, primarily from traffic and natural gas combustion used for heating, cooking and power generation.
A critical next step, Nobles said, is replicating the findings in other studies and researching whether the DNA changes have measurable impacts on fertility and pregnancy outcomes.
Karen Sermon, immediate past chair of the European Society of Human Reproduction and Embryology, said: ‘This is another piece of the puzzle to understand how pollution negatively influences our fertility.
‘We know that couples exposed to air pollution often have difficulties becoming pregnant, and this may be one of the explanations among the myriad ways that pollution affects our reproductive health.’
