How air pollution can affect athletes

Air pollution peaks, such as those that have recently occurred in several regions of the world, regularly make headlines. The World Health Organization (WHO) defines air pollution as

“contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere”.

It is now widely recognized that this phenomenon has an impact on the health of the overall population. But what about athletes? Are there any specific risks associated with their activities?

To gain a better understanding of this broad topic ahead of the Paris Olympics, we need to start with the basics: what are the most dangerous particles, and what are the main sources of pollution?

The main air pollutants

The WHO has classified the different pollutants and taken a close look at their physiological effects. The main ones are:

• Suspended particulate matter (SPM) of various sizes and chemical compositions. Particles with a diameter of 20 μm (PM20) fall quickly, therefore few of them remain in the air except in emission zones. Suspended particles most frequently found in the atmosphere include PM10 (with a diameter less than or equal to 10 µm), PM2.5 and ultrafine particles (PM<0.1 µm).

  The smaller they are, the greater their impact on our organs and their likelihood of causing or aggravating respiratory, cardiovascular or other pathologies. Exposure to PM2.5 (even at levels below current standards) can increase the risk of stroke, cognitive impairment, dementia, Alzheimer’s and Parkinson’s disease.

• Carbon monoxide (CO) is a gas formed during the incomplete combustion of elements containing carbon. Motor vehicles, heating systems, incinerators, refineries and many other industries are major producers of carbon monoxide. Because our red blood cells have a much stronger affinity for carbon monoxide than for oxygen (O₂), the gas causes a rapid drop in blood oxygen levels, sometimes leading to death.

• Sulfur dioxide (SO₂) is a sulfurous gas of volcanic and industrial origin. While emissions have decreased considerably in developed countries in recent years, the same is not true everywhere, especially in places where fuel oil and high-sulfur diesel are still widely used. Exposure to suflur dioxide is linked to increased hospital admissions and death from cardiovascular and respiratory causes.

• Nitrogen oxides (NOx) are gases derived from nitrogen-rich fuels, mainly caused by road traffic and electric generators. Nitrogen dioxide (NO2) is a key contributor to a series of secondary pollutants of photochemical origin, such as ozone and organic nitrate and sulfate particles measured as PM10 or PM2.5.

• Volatile organic compounds (VOCs) can be released by leaks in pressurized systems (natural gas, methane, etc.) or exhaust systems, or originate from fuel evaporation (benzene, etc.), cigarette smoke or household products. Their effects range from a mere disturbing smell to carcinogenic potential. More disturbingly, as they break down in the atmosphere due to solar radiation and heat, they form other harmful compounds such as ozone.

• Ozone (O3) is one of the most widespread atmospheric pollutants. It is formed from a chemical reaction between nitrogen oxides, carbon monoxide, sunlight and hydrocarbons. Pushed by the wind, it accumulates over large towns and surrounding hills, especially on sunny days.

  Ozone aggressively irritates airways and leads to increased hospital admissions and deaths, especially among people with breathing pathologies. It also contributes to cognitive decline, dementia and Alzheimer’s disease.

Pollutant levels

Research into the link between pollution and health has led WHO to set thresholds for pollutants, that should not be exceeded. These limits were used to establish an air quality index and practical guidelines that also apply to physical activities.

The different air quality levels are colour-coded from green (good) to dark purple (poor air quality). An explanation of the color codes can be found on the websites of official national or regional organizations. Other international agencies give an overview for each country .

When should athletes be careful?

In some circumstances, athletes should be cautious when exercising, whether it be outdoors or indoors.

When close to heavy car traffic

Combustion of gasoline or diesel fuel produces exhaust gases that contain a wide range of potentially harmful pollutants: carbon monoxide, nitrogen oxides, volatile organic compounds and particulate matter. At high temperatures, large quantities of ozone can also form.

Brake and tire wear and friction with the road surface also need to be considered. Particles from the road surface are released into the air by the sheer forces or turbulence generated between tires and the ground.

Stadiums, often built close to major roads (for easy access) and to parking lots, can be polluted areas during games. The New Delhi marathon is regularly run in health-threatening conditions.

During agricultural work

The risk of exposure to pesticide concentrations is difficult to assess. Few studies have been carried out with respect to sports activities, but a systematic review identified an increase in the frequency of cancers and Parkinson’s disease in certain locations among outdoor workers exposed to these substances.

A study of a cohort of 682 golf course managers in the United States also showed excess mortality due to cancer, and more specifically cancer of the prostate and large intestine, non-Hodgkin’s lymphoma and tumors of the brain or nervous system.

During megafires

In its sixth report, the Intergovernmental Panel on Climate Change (IPCC) predicted that climate change will lead to a roughly 30% increase in the occurrence of wildfires, which cause a wide range of health problems.

Wildfires and megafires release high concentrations of carbon dioxide and other atmospheric pollutants. Smoke can also travel thousands of kilometers and spread this pollution.

In June 2017, during the wildfires in Portugal and Spain, pollutant levels reached the upper limit of measurement instruments. In Portugal, for 7 to 14 days, daily PM concentrations locally exceeded European and national thresholds.

During Australia’s “black summer” (2019-2020), wildfires lasted five months in the east and south of the country. Their smoke was identified as the cause for the breathing problems of many tennis players at the Melbourne Tennis Open. The megafires in Canada and the impressive image of New York’s orange skyline at the beginning of June are current examples of “unbreathable” air that led to restrictions on outdoor sports activities.

Smoke disrupted the Australian Open in 2020.

During certain seasons

In Europe, outdoor workout can lead to exposure to particulate matter (including pollen in the spring) when the climate is temperate (autumn-winter-spring), and to ozone when it is hot (spring-summer). With global warming, pollens are also increasingly allergenic and widespread.

On synthetic turf

A few years ago, concerns were raised about the composition of recreational fields and artificial lawns made of recycled tires containing polycyclic aromatic hydrocarbons, vulcanizers, plasticizers, antioxidants and heavy metals. Inhalation, ingestion and contact with these harmful residues presents a risk.

These particles pose a greater risk by inducing genetic mutations rather than by affecting immediate respiratory health. Their quantity measured on sports fields were found to exceed the legal or recommended safety thresholds for each compound, some of which were carcinogens.

What’s more, there are still no guidelines to protect people from the possible harmful effects of the many chemical substances in tire rubber.

Indoors

Indoor sports activities expose athletes to specific risks. Both the indoor air itself and the air coming from outside are polluted by small particles and ozone. Outside air often reacts with indoor surfaces.

Recent but still insufficient studies have measured PM, VOC and CO₂ pollution in gyms, fitness centers and indoor sports facilities. Their concentrations are essentially a function of the number of participants, the type of activity and ventilation. Yoga, for example, does not tend to resuspend particles, and therefore has less impact than other activities.

High concentrations of VOCs can come from alcohol-based hand sanitizers used in fitness centers or cleaning products, air fresheners or diffusers, but also from new equipment (mats, small fitness equipment, etc.).

Effects on performance

It should be noted that some athletes are more sensitive than others. The effects of air pollutants on performance have been studied in two ways: by compiling the times achieved by athletes in a certain type of race over different seasons, and by tracking the best results in international marathons. These results were then matched with local environmental and climate measurements.

Particles. An increase in PM2.5 and PM10 tends to increase marathon and 5km running times (of women notably). There are two possible explanations: a drop in VO₂ max (the maximum amount of oxygen the body can use during exercise) and an increase in the perception of effort.

This correlation has been found even when WHO thresholds were not exceeded, but when that was the case, variations in performance could not necessarily be perceived on an individual scale or in short-term studies.

Each PM10 increase of 10 µg/m³ lengthened running times by 1.4% among female marathon runners. And the more polluted the air, the more visible the effects. It probably took an average marathoner around 12 minutes longer to cross the finish line at the 2014 Beijing Marathon, when the air was highly polluted, than on a day when the air was moderately polluted.

Ozone seems to have the greatest impact on performance. In addition to reducing achievements, it increases the number of dropouts sometimes by as much as 50% in the case of high concentrations.

At low concentrations, the effects are barely noticeable by individuals, but one author has estimated that performance dropped 0.39% with each ozone increase of around 20 µg/m.

Most often, ozone and particles combine their effects. Data from five Ironman events held annually in the United States over a seven-year period showed that ozone affected swimming performance, while PM2.5 had a greater impact on cycling and running. Each 20 µg/m³ increase in ozone added 1% to the final mean time, and each 1 mg/m³ increase in PM2.5 added 0.12%. Obviously, the most highly trained marathon runners, who finish faster, are less affected by pollutants.

Comparable data were obtained with team sports. Among professional Bundesliga players and teenage footballers, poor air quality was associated with a drop in the distance covered, the high-intensity effort and the number of passes. In the US National Football League, it was demonstrated that the defense team was less effective when PM₂⋅₅ concentrations were high.

Professional referees are also affected. The number of arbitration errors has been reported to increase by 11% with each 1ppm rise in carbon dioxide (three-hour average) and by 2.6% with PM2.5 (12-hour average).

The effects of pollution on performance are often overshadowed by the more significant effects of heat. But their impact is real, and while sometimes small, it can be enough to change a podium or a score that hangs on just a few seconds.

In an increasingly hot and polluted environment, will athletes who are less sensitive to pollutants be selected in the future to optimize their team’s performance? And more importantly, will this research have an impact on public health policy?