Jasper Kehoe, 23, who spent four summers as a wildland firefighter in Colorado, says fertility is not often talked about in the macho, male-dominated world of wildland firefighting.
But whenever Kehoe talked about his offseason work as a student researcher at Colorado State University evaluating the effects of wildfire smoke on semen, his colleagues’ ears perked up.
Even more surprising to Kehoe was that they wanted to participate. He posted about the study in an industry Facebook group and received more than 150 messages from firefighters interested in participating.
“After overcoming the stigma of talking about fertility, which is taboo in our community, firefighters are concerned about their ability to conceive,” Kehoe said.
Over the past year, Kehoe has recruited 144 wildland firefighters and helped them submit semen samples before, during and after fire season. He hopes his research will lead to a better understanding of the health effects of smoke, leading to greater protections for wildland firefighters and others.
If the study is published later this year or next, the firefighter’s study will join a growing body of research into how wildfire smoke affects human fertility. Compared to the effects of smoking on pregnancy, this is an understudied topic. But interest in the field is growing as climate change causes more fires, especially in the West, and as infertility affects one in six people worldwide. And so far, the results contain some red flags for Westerners looking to have children.
credit: Marissa Garcia / High Country News
Some recent studies include episodes of poor air quality Located in the Pacific Northwest. For example, residents of Portland suffered from 10 days of heavy smoke from nearby wildfires in 2020. At the time, the city’s air quality index (AQI) was almost completely off its current scale, with ratings near 500. This is the highest and most dangerous level and represents a serious health hazard.
Researchers at Oregon Health and Science University (OHSU) later looked into the situation, comparing before and after semen samples from men undergoing fertility treatment and found that men’s sperm volume and motility decreased in the months following the sudden arrival of the smoky air.
Another study published earlier this year analyzed semen samples from 84 men before and after smoking incidents in Seattle in 2018, 2020, and 2022. After exposure, researchers found that most men’s sperm quality and number decreased.
“The changes we found are quite subtle, and we don’t yet know whether these effects on sperm translate to changes in pregnancy or truly changes in fertility,” said Tristan Nicholson, assistant professor of urology at the University of Washington and lead author of the paper. “But I think this has really sparked interest for me and others to expand this as an (area of) research.”
Nicholson said the male side of infertility has historically been understudied, and said she hopes her research will bring much-needed attention to the issue.
“My male patients are often the forgotten partners,” she says. “There’s a lot of attention to how infertility is a women’s issue, but I think it’s really helpful to raise awareness that male partners play an important role.”
“There’s a lot of attention to how infertility is a women’s issue, but I think it’s really helpful to raise awareness that male partners play an important role.”
dangerous air at the same time OHSU researchers in Portland in 2020 also investigated whether wildfire smoke affects in vitro fertilization or embryos created during IVF.
As part of the process, female patients undergo approximately two weeks of drug treatment until their eggs are retrieved and fertilized by sperm. After approximately five days, the resulting embryo is mature enough to be implanted into the patient’s uterus.
For this retrospective study, researchers grouped IVF patients according to when their smoke episodes occurred within their cycles. This may occur several weeks before egg retrieval, during embryo development, or after the embryo has already matured.
Patients whose embryos developed during periods of hazardous atmosphere were much more likely to find that none of their embryos developed well enough to be suitable for implantation. The median number of patients whose embryos reached maturity was 2, 55% fewer than those whose embryos had completed development before the episode.
Even though the lab had several filters, there was still a faint smell of smoke in the air, said Molly Cornfield, an IVF doctor and lead author of the study. Her new lab has a “submarine mode” that allows her to block out outside air with the push of a button.
Kornfield said it is well known that long-term exposure to bad air can negatively impact fertility. But she was alarmed to learn that “even this acute episode for just 10 days, which is, of course, really serious, can have negative effects.”
Still, she cautioned that the study’s sample size was small. And some of the results were unexpected. Patients exposed to poor air quality in the weeks leading up to egg retrieval did not cause any significant harm to the developing embryos. Kornfield said the findings surprised him and highlighted the need for further research.
Nicholson agreed. One of the big questions, she says, is whether fertility can be restored after a serious smoking accident, and if so, how long. She says such information could help aspiring parents know how careful they should be.
Currently, government air quality recommendations set out stricter guidelines for “sensitive groups” including children, the elderly, pregnant women and people with heart or lung problems.
Firefighters are barely visible as smoke from the wind-driven Bane Fire fills the air with an orange glow at the Western Riverside Animal Shelter on May 19, 2026 in Jurupa Valley, California. credit: Gina Ferrazzi/Los Angeles Times via Getty Images
“What I’m wondering about is whether people who are trying to conceive, who are trying to start a family, should fall into that category,” Nicholson said. “However, given what we know so far, I would advise patients to avoid exposure to wildfire smoke.”
Regardless of fertility goals, Westerners should monitor air quality using reliable data sources like airnow.gov. To reduce exposure to unhealthy air, limit your time outdoors, close windows and doors, and consider wearing an N-95 mask when you go outside.
People who live in smoke-prone areas should consider investing in an indoor air purifier and replacing the filters regularly. If that isn’t financially possible, some cities have programs to help residents buy or rent purification equipment for fire season. It’s also relatively easy to build an air filter using box fans and other materials.
After years of researching the effects of smoke as an undergraduate, Kehoe learned to wash his hands and face as often as possible while on the fire line. He also tried not to get into his sleeping bag when he was dirty. Back home in Kansas City, Missouri, he now runs his air purifier 24/7.
There is still much left to learn. But what we learned through smoke is that inhaling smoke is not good for anyone.
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