A runner braves the dark-orange smoke at Crissy Field in San Francisco as multiple wildfires burn across California and Oregon in 2020.

In summer 2020, the California sky became a warning light. As fires raged across northern parts of the state, smoke particles blanketed the atmosphere, coloring the skies red-orange and shades of yellow. That year, the state set a grim record: its worst five days of air pollution to date.

Jasmine Sanchez, a Morgan Hill-based runner, still remembers the smell of burning wood from the nearby Santa Clara Unit Lightning Complex fires.

“I don’t think I will ever forget that week in 2020,” Sanchez said. “My mom and I made it out to a local trail one day, and I still ran, but I was a little concerned, like, ‘Oh, I don’t even know if this is safe or healthy.’ After that, the fire conditions got even worse, so I just decided I had to stay in.”

Sanchez, founder of hydration company Vessel Athletics, hasn’t had to abandon her running plans since August 2020. But as tinder-dry California enters another nerve-wracking fire season and as more research emerges about the health dangers of inhaling smoke particles, staying in could be the hard choice more runners and outdoor enthusiasts are asked to make this summer — and for summers to come.

For several days throughout August, September and October of 2020 — the worst wildfire season on record — the Air Quality Index (AQI) in much of the Bay Area was above 150 — a level considered unhealthy. Because of smoke from nearby wildfires, there were high levels of fine particulate matter (PM2.5), a dangerous type of pollutant, in the air.

The region has thus far avoided reaching that threshold again, though the East Bay and Santa Clara Valley saw a number of days that were deemed unhealthy for sensitive groups last August and the Bay Area Air Quality Management District has issued two air quality advisories since May warning residents to avoid smoke from two since-contained fires — the Owens Fire in Mendocino County and the Old Fire in Napa County.

PM2.5 is nearly 1/40th the size of fine beach sand, making it easily inhalable.

“Once the particulate gets in the bloodstream, it can go anywhere, basically, and cause problems,” said Mary Prunicki, director of air pollution and health research at the Sean N. Parker Center for Allergy and Asthma Research at Stanford University.

Research shows running strengthens the heart, burns calories, releases endorphins, boosts the immune system and more. However, on days when the air quality is poor, the consequences of being outdoors may outweigh the benefits of running.

Runners and other outdoor athletes are at an increased risk of breathing in particle pollution. A 2019 study published in the journal Scientific Research and Essays found people inhale up to 20 times the normal volume of air per minute during exercise, exposing them to more air pollutants. People also often breathe through their mouths when exercising. While nasal breathing filters out some of the inhaled pollutants, mouth breathing allows them to easily enter the lungs.

“From air pollution studies, we know without a doubt that the more you’re exposed to PM2.5, the shorter your life expectancy,” Prunicki said. “And then we know from wildfire smoke studies that when a population is exposed to wildfire smoke, there’s going to be increases in respiratory problems, cardiac problems and neurologic” problems.

Mimi Kellogg, a Palo Alto-based runner and the co-host of the podcast “Runners of the Bay,” said the first time wildfire smoke affected her was in fall 2020 as a vast wildfire raged in upper Northern California.

Jasmine Sanchez, of Morgan Hill, approaches the finish line during the Stars and Strides Run on July 2 in San Jose. Sanchez remembers cancelling her running plans in 2018 and 2020 due to wildfire smoke and now feels a tinge of nervousness each time summer rolls around.

That September, Kellogg was preparing for a virtual Boston Marathon where remote participants would complete a 26.2-mile run anywhere in one continuous attempt and upload their finishing times within a few days. But during her training she started to notice the hazy skies. Smoke from the August Complex fires had drifted across Northern California, affecting the air quality in the Bay Area.

“My friends and I would wake up, and we would check the air quality and say, ‘OK, what’s the threshold that we would consider running this marathon in?’” she said. Kellogg remembered one day where she got a few miles into a run before deciding to go home due to the air quality.

While the U.S. Environmental Protection Agency advises people to “avoid prolonged or heavy exertion” outdoors when the AQI is 201 or higher, Prunicki, the Stanford health expert, said to take this advice “with a grain of salt.”

“Listen to your body instead of the number that’s being given,” she said, emphasizing that different people have different reactions to the air quality, and some may experience effects even when the AQI is below 201.

Vic Thasiah, a Ventura-based trail runner, is used to conducting this kind of personal calculus.

Hailing from the San Joaquin Valley, one of the most polluted regions in the state, Thasiah has long been cognizant of environmental issues. In 2019, he founded Runners for Public Lands, a nonprofit organization and community of runners dedicated to environmental advocacy and conservation.

Back in the San Joaquin Valley, “some days vigorous exercise outside is off the table, and running outside, by definition, is vigorous exercise,” Thasiah said. “So, if you happen to be a runner, you’re used to your training getting disrupted, and sometimes even feeling like you don’t want to race on particular days because the exposure and the impacts and consequences are not desirable.”

It is important that people in areas affected by wildfires adapt personal strategies for dealing with air pollution, said Bryant Baker, a board member of Runners for Public Lands and the conservation director for Los Padres ForestWatch.

Baker said air pollution “may mean that people will have to adjust where and when they run.” Some days, runners may have to opt for the treadmill instead of the trail.

“I do think COVID has offered a glimpse into our ability as a society to deal with these types of issues,” Baker said. “Most people adopted masks very quickly. … We’ve done more work remotely indoors. … I don’t think people quite have made the connection yet, but those are climate change adaptations in a lot of ways.”

Sales of home fitness equipment saw a 25% lockdown bump in 2020, with the global market valued between $10.18 billion and $11.6 billion, depending on the market research firm. The appetite for personal workout gear is projected to climb through the decade as exercise companies incorporate more smart technology that allows users to run or pedal in place while interactive screens attempt to virtually transport them outside. But the expensiveness of the equipment could make such options available only to those who can afford them.

Marginalized communities are already disproportionately affected by pollution and face barriers to outdoor access.

A 2022 study published in the journal Nature found racial and ethnic minorities and residents of low-income areas are exposed to higher average levels of particle pollution than others. In 2020, the Center for American Progress released an extensive report on “the nature gap,” documenting economic and racial disparities in access to the outdoors, such as the lack of green space in low-income communities.

Baker talked about the need to reconsider outdoor labor practices and offer personal protection equipment and resources to those in need. Air pollution adaptations need to be accessible not just for runners, he stressed, but also for low-income and communities of color, outdoor workers and future generations that will continue to grapple with a combustible climate.

Baker also noted that naturally occurring fires can have ecological benefits and are not the largest source of air pollution.

“In our focus on wildfire-related particulate matter and air pollution, we tend to forget, to the benefit of polluting industries, like the oil industry (and) the automobile industry,” Baker said, adding that those industries contribute the vast majority of air pollution in the Bay Area. These same industries contribute significantly to global CO2 emissions, fueling the very climate crisis that is increasing the frequency and intensity of wildfires.

Even though the onus is on the government to regulate polluting industries, individuals can help reduce air pollution and combat climate change, including driving less; avoiding gas-powered landscaping equipment, such as gas lawn mowers; and burning less wood. (During the winter months, residential wood burning is the No. 1 cause of air pollution in the Bay Area.)

While people can effect change through personal choices, Thasiah emphasized the power of collective action and alluded to the long history of sports activism in the U.S. Runners for Public Lands was inspired by a number of environmental actions, such as the time that a group of indigenous prayer runners trekked 800 miles to Bears Ears National Monument in Utah to raise awareness of the dangers posed to indigenous and public lands by environmental deregulation.

Runners for Public Lands is working to promote environmentally responsible and sustainable practices within the running community, protect public lands and improve equitable access to the outdoors through actions, such as cleanups and rallies.

There are “60 million-plus runners in the country,” Thasiah said. “I think that as they get more and more concerned and activated around these kinds of social (and) environmental issues that make running possible and enjoyable, that’s a huge force.”

Tennis was Kellogg’s sport of choice growing up. She ran but mostly as a way to stay deft and energetic on the court. Running also offered some respite from daily stressors like school. But when she moved to the Bay Area after college, running took on a new meaning: It became a way to build community in a new city.

“I didn’t know anyone, so I was like, ‘Well, maybe running is a way that I can make friends,’” Kellogg said. “And so it was really only then that I started learning about races and working with a coach.”

Now an avid runner, Kellogg zips through the rolling hills of Portola Valley; through the redwood forests of Wunderlich Park, beneath the shade of 100-year-old trees; through the marshlands of Baylands Nature Preserve, home to wild roses and rare birds; and through the streets of downtown Palo Alto.

“There’s a connection that (runners) have with the Earth and the ground that I don’t think people realize,” Kellogg said. “A street could look completely flat, and you’re like, ‘No, I know this is a slight uphill grade, and so I know that running out, it’s going to be harder, and running back, I’m going to fly.’”

Running in Northern California, where the trails are postcard-perfect and summer never seems to end, is often idyllic. But Kellogg recognizes that the pristine trails and favorable weather conditions aren’t guaranteed.

The “wake-up call” for her was 2020. For Sanchez, it was 2018, when the Monterey Bay Half Marathon she had signed up to compete in was canceled due to poor air quality.

Sanchez says she often thinks about climate change and feels a tinge of nervousness when summer rolls around.

“Climate change is and will continue to impact my habits as a runner,” she wrote in a follow-up text. “As someone who enjoys outdoor recreation for both the physical and mental benefits, I have to continue to educate myself on how I can be a good outdoor steward.”

Baker said adaptation will be necessary moving forward as the issues of air pollution and climate change persist. But he doesn’t think of it as a negative.

“I don’t even look at it as pessimism. It’s actually more optimism about our ability to change and adapt,” he said. “We’re a really resilient species, and we can learn from nature.”

Chasity Hale is a San Francisco Chronicle staff writer. Email: chasity.hale@sfchronicle.com Twitter: @chas_hale

Chasity Hale is a journalism master's student at Stanford University, where she also received her bachelor's degree in communication and creative writing. She has interned at NPR Music, worked at the Stanford Daily student-run newsroom, and written for various publications, including the San Franciscan magazine and Peninsula Press. Raised in Miami, Hale is intimately aware of the dangers that climate change poses to coastal cities and is passionate about reporting on the environment through the lenses of equity and justice. She is excited to be joining the Race & Equity team this summer.