With climate change, we are experiencing more air pollution and more poor air quality events. Specifically, the frequency and severity of wildfires lead to long periods of very poor air quality. Likewise, heat domes and heat waves have led to greater periods of ground-level ozone.
Many athletes are left wondering if training in poor air quality is still worth the health risks. Should they only train during times when pollution is lowest, or visibility is at its clearest? How can outdoor athletes manage these issues?
The push and pull of exercise and air pollution
Endurance athletes typically require extended periods of training outdoors throughout the year and are therefore potentially exposed to significant levels of air pollution. Unfortunately, exercise itself has several unique features which have the potential to worsen the effects of air pollution on the body.
Specifically, when we exercise, our metabolic rate is higher and the amount that we breathe (called minute ventilation) increases to meet our increased oxygen needs, and to get rid of that extra carbon dioxide we produce. With that increase in minute ventilation, the amount of inhaled air pollution is increased and therefore our dose of air pollution is increased.
Furthermore, when we exercise at high intensity, we tend to breathe less through our nose (which filters particles and absorbs ozone) and more through our mouth (which has less of that filtration ability). Thus, when outdoor athletes train, they increase their air pollution dose, and the potential for harmful effects.
Types of air pollution
Firstly, it is important to understand the effects of air pollution on the body. These can be divided into health effects and performance effects. They can also be divided into chronic (long-term) and acute (short-term) effects. Each of these categories of effects are studied differently and have different implications.
Second, it is important to understand the different “recipes” of air pollution. We talk about recipes, because air pollution varies considerably in composition depending on the location, season, time of day, and weather. Pollution is a mixture of gases (such as ozone and nitrogen dioxide) and particles (such as dust, pollen, soot, and heavy metals). Each of these constituents has different effects on the body, which in turn affect the overall response to air pollution.
To simplify a bit (actually, a lot), the most significant pollution exposures for outdoor exercisers are wildfire smoke and ozone. Wildfire smoke is heavy in particles (e.g., soot) that can enter the bloodstream by getting inhaled and depositing in the lungs. For this article we’ll consider wildfire smoke as primarily particulate matter. We can see (and often smell) wildfire pollution as haze and smoke since the particles interfere with the transmission of light.
The other exposure we are seeing more of lately is ground-level ozone, a gas that is created when ultraviolet light reacts with other gases in the environment. Ozone is particularly prevalent on hot, sunny days—especially during heat waves and heat domes. Ozone is invisible and odorless at the levels commonly experienced. So, the only way to tell that the ozone level is high is to check government websites or third-party apps (e.g., Plume Labs or IQAir) for the current and forecasted ozone level.
When wildfires hit, we can be blanketed for days to weeks with very high particle levels. Particles—especially the fine ones—can have significant chronic health effects. Long-term exposure causes lung disease, cancer, cardiovascular disease, dementia, and even diabetes.
However, we do know that active individuals do better than those who are sedentary in the long term when exposed to this kind of pollution. So, although all air pollution is bad for us, staying active in the long term is still critical for our health. The short-term effects of this type of pollution during exercise are much less clear.
Surprisingly, there are no clear performance effects from studies of athletes exposed to particulate-heavy air pollution. Just as surprising is that higher-intensity exercise does not seem to cause more effects when exposed to particulate air pollution. This point is important when we get to recommendations later.
The science around the health and performance effects of ozone seems to be much clearer in the short-term. During and right after exercise, we experience a decreased lung function, increased chest tightness, and decreased performance when exposed to ozone. These performance effects are compounded by heat since ozone levels are normally high on hot, sunny days (when our exercise performance is already affected).
In summary, wildfire pollution probably leads to more chronic health effects, especially from long term exposure, whereas with ozone pollution we notice an effect on our performance and how we feel in a more acute timeframe.
How to mitigate air pollution exposure
The most comprehensive guidance on strategies to reduce performance and health risk during exercise are available from a 2023 position statement developed by a group of international clinicians and researchers in the field. There are four key takeaways.
Use time and distance to separate yourself from air pollution
Air pollution levels vary widely throughout the day due to the effects of weather, traffic, and industrial activity. Typically, levels of all pollutants are lowest in the early morning. Ozone dips again in the evening (whereas particle pollution typically does not).
There can also be larger variations in pollution levels even within a relatively smaller area. Choose your training location based on the current and forecasted pollution levels. For example, train away from local sources of pollution, such as traffic and industrial sites. Geographic features such as coastlines, mountains, and parkland can lead to larger differences in pollution concentration as well.
Always plan your training to minimize your pollution exposure. Use government websites or apps that have an air pollution forecast to determine the best time to train and check the different pollution levels in the areas where you typically train.
Modify your workout
If the air quality is poor even at the best time of day and at your lowest-pollution training location, then consider modifying your workout. It is believed that total inhaled dose is the most significant factor determining the effect of the pollutant, so on bad days, avoid those long, slow base workouts. That six-hour zone 2 ride on a day with poor air quality is going to lead to the greatest pollution effects. Instead, choose a shorter workout, even if it involves higher intensity, such as some hill repeats, a strength session, or even a short race on Zwift.
Consider training indoors
Air quality can be better indoors. However, this is variable, and it depends on the factors specific to your home or gym. Air conditioning, air purifiers, or a filtration system can lead to better air quality indoors if doors and windows are kept closed. This is a great option if you have access to such a facility.
However, do not assume that indoor air quality is always better. Outdoor pollutants (if not filtered) can be present in elevated levels indoors, and there are also indoor sources of pollution, such as candles, stoves, chemicals, climbing chalk, and ski wax.
If you are moving your training indoors, ensure that the air quality is, in fact, better. You can assess this indoor air quality through access to a personal pollution monitor (such as ATMO or PurpleAir).
Wear a mask
A properly fitting N95 mask is designed to reduce particle intake and would reduce the health effects of particle-heavy wildfire smoke (but these will not help with ozone). During a wildfire event, wearing a mask before and after training can be an option. During exercise, there could be a theoretical benefit of wearing the mask, but some athletes might find it uncomfortable or inconvenient.
Always take your long-term health into consideration
Air pollution is bad for our health, and we should reduce our exposure as much as possible before, during, and after training. Remember that poor air quality is not always something you can see, or that can be filtered out by a mask. Plan your outdoor training sessions by factoring in time and distance from polluted areas, and train indoors, if possible, on days with high air pollution. Staying active in the long term is important for health, but during poor air quality events, we should consider these precautions and reduce exposure to pollution as much as possible.