Fertility, a subject often relegated to private conversations, is now being scrutinized within the robust and traditionally masculine domain of wildland firefighting, revealing unexpected anxieties and a growing body of scientific inquiry. Jasper Kehoe, a 23-year-old who spent four summers as a wildland firefighter in Colorado, discovered this firsthand. While discussing his off-season work as a student researcher at Colorado State University—investigating the effects of wildfire smoke on semen—he found his colleagues’ attention was not only piqued but they actively sought to participate. This engagement, even surprising Kehoe, highlighted a shared concern about reproductive health that transcended the rugged image of their profession. He received over 150 messages from firefighters eager to contribute to the study after posting about it in an industry Facebook group.

"After you get over the stigma of talking about fertility, somewhat of a taboo subject in our community," Kehoe remarked, "these firefighters are concerned with the ability to conceive." His research endeavor involved recruiting 144 wildland firefighters to provide semen samples at three distinct points: before, during, and after the fire season over the past year. Kehoe anticipates his work will contribute significantly to a broader understanding of smoke’s health consequences, ultimately advocating for enhanced protections for wildland firefighters and other populations exposed to this environmental hazard.

The forthcoming publication of the firefighter study is poised to join an emerging collection of research exploring the intricate relationship between wildfire smoke and human fertility. While the impact of smoke on pregnancies has garnered more attention, the specific effects on fertility, particularly male fertility, remain comparatively understudied. However, with the escalating frequency and intensity of wildfires globally, exacerbated by climate change, and with one in six individuals worldwide facing infertility, scientific and public interest in this area is rapidly expanding. Early findings from these investigations are already presenting cautionary signals for individuals in regions susceptible to wildfire smoke who aspire to start families.

Recent scientific investigations have focused on periods of significant air quality degradation in the Pacific Northwest. For instance, residents of Portland, Oregon, endured ten days of severe wildfire smoke in 2020, during which the city’s Air Quality Index (AQI) approached unprecedented levels, nearing 500—the highest and most dangerous threshold, signaling a critical health hazard. In the wake of this event, researchers at Oregon Health & Science University (OHSU) analyzed semen samples from men undergoing fertility treatments. Their findings revealed a notable decrease in sperm quantity and motility in the months following the intense smoke exposure.

A separate study, published earlier this year, examined semen samples from 84 men in Seattle, collecting data before and after smoke events in 2018, 2020, and 2022. The analysis indicated a decline in sperm quality and count across most participants subsequent to these exposures. Tristan Nicholson, an assistant professor of urology at the University of Washington and senior author of this paper, noted that while the observed changes were subtle, the long-term implications for pregnancy and fertility remain a subject requiring further investigation. "But I think this has really motivated, for me and others, an interest in expanding this as (an area of) study," she stated, emphasizing the historical underrepresentation of the male factor in infertility research. Nicholson expressed hope that her work would bring much-needed attention to this often-overlooked aspect of reproductive health, where men are frequently considered the "forgotten partner" in discussions about infertility.

During the same period of hazardous air quality in Portland in 2020, OHSU researchers also embarked on an inquiry into the potential impact of wildfire smoke on embryos created through in vitro fertilization (IVF). The IVF process involves approximately two weeks of hormonal treatment for female patients prior to egg retrieval, followed by fertilization with sperm. Mature embryos are typically ready for uterine transfer around five days post-fertilization. For their retrospective study, the researchers categorized IVF patients based on the timing of the smoke episode relative to their treatment cycle: in the weeks preceding egg retrieval, during embryo development, or after embryo maturation.

The findings indicated a significantly higher likelihood that patients whose embryos developed during the period of hazardous air quality would have no viable embryos suitable for transfer. For those whose embryos had already matured, the median number of usable embryos dropped by 55%, resulting in a median of two embryos compared to unaffected cycles. Molly Kornfield, an IVF physician and the study’s lead author, shared that even though the laboratory was equipped with multiple air filters, a faint smell of smoke persisted. Her current laboratory features an advanced "submarine mode" designed to seal out external air at the push of a button. Kornfield acknowledged the well-established risks of long-term exposure to poor air quality on fertility but expressed alarm at the demonstrable negative impact of even a concentrated, ten-day period of severe smoke exposure.

However, Kornfield also urged caution due to the study’s limited sample size. Some findings were unexpected; for instance, patients exposed to poor air in the weeks before egg retrieval did not exhibit significant harm to their embryo development. This surprising result, Kornfield suggested, underscored the critical need for more extensive research. Nicholson concurred, highlighting a significant unanswered question: whether fertility can recover after severe smoke events, and if so, the duration of this recovery period. Such information, she explained, would be invaluable for prospective parents seeking to understand the necessary precautions.

Currently, government guidelines for air quality recommendations categorize certain groups as "sensitive," including children, older adults, pregnant individuals, and those with pre-existing heart or lung conditions. Nicholson questioned whether individuals actively trying to conceive should be included in this "sensitive" category, adding, "But I would advise patients to avoid wildfire smoke exposure, given what we know so far."

Regardless of fertility aspirations, individuals residing in areas prone to wildfire smoke are strongly encouraged to monitor air quality through reliable sources such as AirNow.gov. To mitigate exposure to unhealthy air, practical measures include limiting outdoor activities, keeping windows and doors closed, and utilizing N-95 masks when venturing outdoors. For those in smoke-affected regions, investing in an indoor air purifier and ensuring regular filter replacement is advisable. Some municipalities offer programs to assist residents in acquiring or borrowing purifiers during fire season. Furthermore, accessible DIY solutions, such as constructing an air filter using a box fan and other materials, are also available.

Kehoe, after years of studying smoke’s effects as an undergraduate, adopted stricter personal hygiene practices while on the fire line, frequently washing his hands and face and avoiding sleeping in soiled gear. Now back in Kansas City, Missouri, he maintains air purifiers running continuously in his home. While much remains to be discovered about the full spectrum of wildfire smoke’s health impacts, the evidence accumulating suggests a pervasive negative influence on well-being, underscoring the urgent need for continued research and public health awareness.