As wildfires become more common across the U.S. and Canada, their effects may reach beyond breathing problems and heart health—new research suggests they may also affect male fertility.
A study from UW Medicine found that wildfire smoke can reduce sperm quality in men undergoing fertility treatments.
The study was published in the journal Fertility and Sterility. Researchers analyzed semen samples from 84 men who provided sperm for intrauterine insemination (IUI) procedures between 2018 and 2022 in Seattle.
Major wildfire smoke events hit the area in 2018, 2020, and 2022, offering a unique chance to compare sperm quality before and during smoke exposure.
By looking at samples taken before and during these wildfire seasons, researchers noticed consistent drops in sperm concentration, total sperm count, and the number of moving (motile) sperm. One measure—the percentage of sperm that moved well—showed a small increase, but it wasn’t enough to make up for the overall decline in quality.
Each man in the study served as his own control. That means his sperm samples taken before wildfire season were compared to his samples during wildfire exposure. The results were similar across all three years, showing that the changes were not random.
“These results reinforce growing evidence that environmental exposures—specifically wildfire smoke—can affect reproductive health,” said senior author Dr. Tristan Nicholson, a reproductive urologist at UW Medicine’s Men’s Health Center.
The findings match earlier studies from Oregon Health & Science University that also linked air pollution and wildfire smoke to problems with reproductive health.
Tiny particles in wildfire smoke are known to harm the lungs, heart, and brain. But their impact on male fertility has not been studied much until now. This study highlights the need for more research into how wildfire smoke affects men and women who are trying to conceive.
Although this study didn’t focus on pregnancy outcomes, the researchers reported that 11% of the women whose partners were in the study became pregnant, and 9% had live births. These rates are in line with previous reports.
One important question the study did not answer: Can sperm recover after wildfire smoke exposure? Dr. Nicholson says her team is now running a new study to track how and when sperm quality improves after being exposed to wildfire smoke.
As wildfires become more frequent and intense, understanding how they affect reproductive health may help protect individuals and couples planning to start a family.
The study is published in Fertility and Sterility.
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