The World Health Organization (WHO) states that 99% of the world’s population lives in places where air pollution exceeds recommended limits. Thus, we should keep the questions of how, when and if to engage in outdoor physical activity top of mind.
Interestingly, among the organs in the human body, the lungs are unique because they are directly exposed to the environment and, therefore, hazardous substances in the air (e.g., aeroallergens, chemicals, gases, particulate matter). Increases in ventilation, which occur during physical activity, will further intensify the airways’ exposure to noxious substances. But perhaps surprisingly, the respiratory system doesn’t accrue any positive adaptations in response to habitual physical activity. This contrasts with the myriad beneficial adaptations seen in the cardiovascular, neuromuscular and central nervous systems in response to regular physical activity. Thus, for the respiratory system, there is no tradeoff between the positive effects of exercise and the negative effects of inhaling polluted air.
Airborne Pollution and Health
Pollution refers to the introduction of substances into the natural environment at levels that might be harmful for animals and plants. Airborne pollution can be derived from natural substances (e.g., dirt and sediment) or human-made substances (e.g., excess CO2 and other gases from internal combustion engines, plastic, pesticides). According to the WHO, the incidence of stroke, heart disease, obstructive lung disease, lung cancer and pneumonia are clearly linked with air pollution. The WHO estimates that ambient air pollution caused ~4.2 million deaths globally in 2019. Specifically, air pollution accounts for an estimated 43% of deaths and disease from chronic obstructive lung disease, 17% of lower respiratory tract infections and 29% of lung cancer cases. Clearly, air pollution is an important contributor to disease.
Particulate matter (PM) is a particularly important type of pollution that consists of combinations of inhalable particles ranging from dust to nitrates to black carbon. These particles come in a distribution of sizes, but the smaller ones are more damaging because they can travel deeper into the respiratory system, even entering the blood by crossing the air-blood barrier in the lungs. Hence the link between PM and cardiovascular disease! Unfortunately, human technology is a significant source of small PM. The finer particles are derived from fuel combustion in, among other things, power generation facilities and motor vehicles. PM is also generated by wildfires. This is critically important, given our warming climate has led to more frequent wildfires globally, including the devastating fires in the western United States. Meanwhile, smog — “smoke fog” — is a combination of gases and PM that is hazardous to respiratory system health. Coal fire and traffic emissions are principal sources of smog, explaining why this is generally confined to urban areas. These realities provide clear examples of the interactions among human activities, environmental conditions and health.
Global climate change due to increased atmospheric greenhouse gases (CO2 being the primary culprit) will amplify the deleterious effects of air pollution on health. The causes and consequences of the bidirectional relationship between climate change and air pollution are manifold and complex. Yet, levels of PM, ozone and aeroallergens, among others, will increase with a warmer, more humid and more volatile climate.
What Can Be Done?
Unfortunately, the negative health effects of poor air quality mean that outdoor exercise is not always beneficial. The simplest advice is to avoid exercise in outdoor environments when air pollution is high.
How high is too high? The air quality index (AQI) is an overall measure of air quality that ranges from 0 to 301 and higher. The AQI is partitioned into six color-coded categories of increasing values from “Good” (AQI 0-50) to “Hazardous” (AQI ≥301). Importantly, the potentially negative health effects of a given AQI are not the same for all persons. While the benefits of exercise outweigh the risks of breathing polluted air until AQI levels reach about 150, people in vulnerable groups (e.g., the elderly, children, those with lung disease) may be at risk when the AQI is as low as 50. Further, the AQI often fluctuates regularly with time of day; planning exercise when air quality is better is a good strategy to adopt. Face masks can also provide some protection against poor air quality.
In the longer term, and with the view that our individual choices make a cumulative difference, reducing reliance on fossil fuels and taking other actions to minimize human-made pollution (e.g., using public transport, walking or cycling) will be essential in reducing the negative health effects of exercise in outdoor environments.
Acknowledgements
Thank you to my colleague Jill Wagner and to Dr. Michael Koehle for their helpful critique of this article.
Hans C. Haverkamp, PhD, is an associate professor of exercise physiology in the Department of Nutrition and Exercise Physiology at the Washington State University College of Medicine. He has served on the ACSM Annual Meeting Program Committee since 2021 and in several leadership roles in the New England chapter of ACSM from 2013 to 2017. Dr. Haverkamp is an internationally recognized expert on the respiratory system responses to exercise, with a particular focus on airway mechanical responses to exercise in health and disease.
