Fertility isn’t a topic that tends to come up in the macho, male-dominated world of wildland firefighting — at least not according to Jasper Kehoe, 23, who served as a Colorado wildland firefighter for four summers.
But whenever Kehoe talked about his job in the off-season — working as a student researcher at Colorado State University to assess the impact of wildfire smoke on semen — his colleagues’ ears perked up.
Even more surprising to Kehoe, they wanted to get involved: When he posted about the study in an industry Facebook group, he received more than 150 messages from firefighters who wanted to participate.
“After you get over the stigma of talking about fertility, somewhat of a taboo subject in our community,” Kehoe said, “these firefighters are concerned with the ability to conceive.”
Kehoe helped recruit 144 wildland firefighters to submit pre-, mid- and post-fire season semen samples over the past year. He hopes that his work helps lead to a greater understanding of smoke’s health consequences, as well as more protections for wildland firefighters and others.
When it’s published later this year or the next, the firefighter study will join a new body of research on how wildfire smoke influences human fertility. In comparison to smoke’s effects on pregnancies, it’s a topic that’s been understudied. But with climate change causing more fires, especially in the West, and infertility affecting 1 in 6 people worldwide, interest in the field is growing. And so far, the results hold some warning signs for Westerners who want to have children.
SEVERAL RECENT STUDIES INVOLVE EPISODES OF POOR AIR QUALITY in the Pacific Northwest. Portlanders, for example, suffered from 10 days of severe smoke from nearby wildfires in 2020. At the time, the city’s air quality index, or AQI, almost went off the current scale altogether, with ratings near 500— the highest and most dangerous level, indicating a health serious health hazard.
When researchers at Oregon Health & Science University (OHSU) studied the situation later, comparing before-and-after semen samples from men who’d been undergoing fertility treatment, they discovered that the men’s sperm quantity and motility had dropped in the months following the smoky air’s sudden arrival.
Another study, published earlier this year, analyzed semen samples from 84 men before and after smoke events in Seattle in 2018, 2020 and 2022. Following the exposures, the researchers found that most men’s sperm quality and count declined.
“The changes we found were fairly subtle, and it’s not yet understood whether these impacts on sperm may translate to a change in pregnancy or truly a change in fertility,” said Tristan Nicholson, an assistant urology professor at the University of Washington and senior author of the paper. “But I think this has really motivated, for me and others, an interest in expanding this as (an area of) study.”
The male side of infertility has historically gone underexamined, Nicholson said, and she hopes her research will draw much-needed attention to it.
“Men, my patients, are often the forgotten partner,” she said. “There’s been a lot of focus on infertility being a women’s problem, and I think it really is beneficial to raise awareness that the male partner has an important contribution.”
“There’s been a lot of focus on infertility being a women’s problem, and I think it really is beneficial to raise awareness that the male partner has an important contribution.”
DURING THAT SAME PERIOD OF HAZARDOUS AIR in Portland in 2020, OHSU researchers also examined whether wildfire smoke had any impact on embryos made during in vitro fertilization, or IVF.
As part of that process, female patients undergo about two weeks of drug treatment before their eggs are retrieved and fertilized with sperm. After roughly five days, any resulting embryos are mature enough for transfer into the patient’s uterus.
For this retrospective study, the researchers grouped IVF patients according to when in their cycle the smoke episode occurred: in the weeks prior to their egg retrieval, during their embryos’ development, or after their embryos had already matured.
The patients whose embryos developed during the period of hazardous air were far more likely to find that none of their embryos did well enough to be suitable for transfer. Those whose embryos did mature ended up with a median number of two — 55% fewer than those whose embryos had finished growing prior to the episode.
Even though the lab had several filters, its air still smelled faintly of smoke, said Molly Kornfield, an IVF doctor and the study’s lead author. Her new lab has a “submarine mode” that can prevent outside air from entering at the press of a button.
Kornfield said it’s well-known that long-term exposure to bad air can harm fertility. But she was alarmed to see that “even this acute episode of only 10 days — which, of course, is really severe — can have a negative impact.”
Still, she cautioned that the study had a small sample size. And some of the results were unexpected: Patients who had been exposed to poor air during the weeks before their eggs’ retrieval did not see significant harm to their embryos’ development. Kornfield, who was surprised by that finding, said it underscored the need for more research.
Nicholson agreed. One big question, she said, is whether fertility can bounce back following severe smoke events — and if so, how long it takes. Such information, she said, would help aspiring parents know just how cautious to be.
At the moment, the government’s air-quality recommendations have stricter guidelines for “sensitive groups,” a category that includes children, older adults, pregnant people and those with heart or lung issues.
“What I wonder, and I don’t know yet, is whether people who are trying to conceive, who are trying to start their families, should fall into that category,” Nicholson said. “But I would advise patients to avoid wildfire smoke exposure, given what we know so far.”
Regardless of fertility goals, Westerners should monitor air quality using a reliable data source like airnow.gov. To reduce exposure to unhealthy air, consider limiting outdoor time, keeping windows and doors shut, and wearing an N-95 mask when venturing out.
People who live in smoke-prone areas should consider investing in an indoor air purifier and changing the filters regularly. If that’s not financially possible, some cities have programs that help residents buy or borrow purifiers for fire season. It’s also relatively easy to build an air filter using a box fan and other materials.
After spending years researching the impacts of smoke as an undergraduate student, Kehoe started washing his hands and face as often as he could while on the fire line. He also tried to avoid getting into his sleeping bag when he was dirty. Back home in Kansas City, Missouri, he now has air purifiers running 24/7.
There’s still a lot left to learn. But one thing has come through the smoke: Breathing it in doesn’t seem great for anybody.
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