It’s Looking Smoky Outside—When Is it Safe to Exercise?

san francisco
Depending on weather, wildfire smoke can linger in places for a long period of time after the burn.Photo: Om Malik

Fire season is in full swing once again. Fourteen states are currently battling large wildfires as rivers of smoke flow across the western half of the United States. Among those states, California, notorious for its hot, arid and windy climate, resumes its yearly position as facing the largest, most devastating fires. Over two million acres have burned, incinerating over 4,000 structures, and, according to a recent statement issued by Governor Gavin Newsom, putting more than a quarter of the state’s population in high fire-risk areas.

Many of us are becoming increasingly more intimate with the drier and warmer seasons caused by climate change, and subsequently, more scattered and active fire seasons. As the western U.S. experiences more expansive fires each year, it’s becoming clear that the health implications of these wildfires extend far beyond the burn.

Satellite images show wind bring large plumes of wildfire smoke across state borders. Photo: NASA


Smoke, the most obvious and most visible offender which knows no borders, can reach communities thousands of miles away from the site of the fires. NASA images (above) of the active fires in California have shown winds bringing smoke into Nevada, Utah, Idaho and more, causing air quality in some regions to dip into harmful levels of toxicity. Areas like San Francisco, stuck in between three of California’s largest fires of the summer thus far, the LNU, SCU, and CZU Lightning Complexes, saw the worst air quality in the world on August 19.

bonny doon
Flames from the CZU Lighting Complex engulf Bonny Doon in Santa Cruz in August. Photo: Taylor West”


Coronavirus provides us with yet another challenge. For many, the pandemic has brought forth the importance of exercise and time spent outdoors as crucial to mental and physical wellbeing. Physical activity is known to help keep our immune systems strong, relieve stress and help many cope with the self-isolating nature of the pandemic. And after a tightly locked-down spring, rangers have reported seeing an abrupt emergence of people flocking to the trails in search of respite from their homes, and perhaps to reap the physiological benefits that nature has to offer.

However, as we head further into fire season with COVID-19 still poised as an insidious threat, the reality of contracting the respiratory disease after exposure to toxic wildfire smoke now carries a different kind of weight.  For the millions of people who find themselves in the plumes of smoke, how do we know if it’s safe to exercise outside? Additionally, what effects can the inhalation of wildfire smoke have on the body, and how might this contribute to one’s vulnerability to coronavirus?

Where to find an accurate reading of the air quality in your location, as well as fire containment in the U.S.

The length of time that smoke particulates remain in the air is heavily contingent on the weather and wind patterns, making it especially important to check the air quality daily.

AirNow is a website that provides an hourly assessment of the air quality from over 500 sites across the U.S. These high-grade, state-regulated sensors are created by partnering federal agencies including the EPA, National Oceanic and Atmospheric Administration (NOAA), National Park Service, NASA, and the CDC with local and tribal air quality agencies. In addition to providing information about the smoke plumes, their predicted trajectory and effect on air quality, the site also provides a look at current fire conditions and their containment.

AirNow has also begun incorporating readings from Purple Air, an online system which provides real-time air quality measurements fed by crowdsourced data. For a more localized assessment of air quality, many people have purchased low-cost sensors that are manufactured by Purple Air to place just outside their homes. These readings, which feed an online network that collects and shares data on a public map, can also be found on AirNow.

On this map (seen below), the squares represent readings from Purple Air sensors; the circles are readings from the government monitored AirNow sensors; and in some states (including California), the triangles represent readings from AIRSIS, a federally run smoke monitoring system.

Image taken from Central California, displays air quality readings from three types of sensors. Image: AirNow


What makes wildfire smoke so toxic? 

Smoke from wildland fires largely consists of fine particulate matter known as PM2.5. The size of this particulate matter, about 2.5 microns (or a twentieth of the width of a strand of hair), is known to be of greater concern due to its tendency to get lodged deep within the lungs, causing a variety of health issues. It’s most commonly known to irritate the eyes and respiratory tract, exacerbating underlying health conditions like asthma and heart disease, and there is evidence that wildfire smoke can lead to respiratory infections such as pneumonia and bronchitis.

Arcata in Humboldt County (Northern California) faces a dark, smoky morning after Labor Day weekend. Photo: Katie Rodriguez


Exercising in smoky weather:

When we exercise, we’re often inhaling more through our mouth than our nose, which helps the polluted air bypass our nose filtration system and go directly into our body. “The issue about exercise is that you increase your effective dose to the wildfire smoke,” says Dr. James Balmes, a professor at UC Berkeley whose research focuses on the respiratory, cardiovascular and metabolic health effects of various air pollutants.

“Walking is exercise that people can do when the air is worse. But when you’re jogging or cycling or playing sports like basketball outdoors, you start breathing through your mouth because you need to increase your minute ventilation (how much air you’re breathing in per minute). Because your muscles are using more oxygen, there is greater blood flow to those muscles, and you breathe faster and deeper and through your mouth.”

In other words, the type of exercise we should do depends on how poor the air quality is, what sort of underlying health conditions one has, and how contained the fire is. Activities like backpacking during fire season should be heavily scrutinized, by choosing a destination that is far from existing fires and during a time that is not forecasted to be too hot. However, according to Dr. Balmes, due to coronavirus we should be assessing the air quality through a more acute lens.

aqi scale
Courtesy Image


Assessing air quality in the times of COVID-19:

We know that sustained exposure to air pollutants is known to increase susceptibility to respiratory issues, making it likely that too much smoke inhalation can exacerbate symptoms of coronavirus and potentially, increase mortality from COVID-19. “We don’t know a ton about SARS-CoV-2 (the novel coronavirus) itself because we haven’t had enough time during this wildfire season,” shares Dr. Balmes, “But based on what we know about other forms of air pollution which have been looked at in China, Netherlands, Italy and (one study ) in the US, we do worry that wildfire smoke will increase the risk of moving from asymptomatic SARS-CoV-2 infection to a more symptomatic infection of COVID-19.”

In assessing air quality readings, even the healthiest of people are suggested to consider less strenuous forms of exercise, even on days where the AQI reads, “Unhealthy for Sensitive Groups”. For those safe from fire danger but facing lingering smoke trails, individuals can check air quality forecasts provided by AirNow and Purple Air for an indication of when conditions might improve.

How to filter out harmful pollutants at home:

For access to exclusive gear videos, celebrity interviews, and more, subscribe on YouTube!