Marathon performance depends on more than training, endurance, and pacing. Environmental conditions, including temperature, humidity, wind, and air quality, can also influence how runners perform.

Recent research suggests that nitrogen dioxide, a pollutant commonly associated with traffic emissions, may be linked to slower marathon finishing times.

A 2026 study published in Sports Medicine–Open examined more than 2.7 million marathon performances across six major international races. The findings indicated that higher nitrogen dioxide concentrations were associated with slower results, particularly among recreational runners. Although the research cannot establish direct causation, it highlights the potential importance of air quality during endurance events.

What the Study Revealed

Researchers from the University of Leicester analyzed 2,756,553 finishing times from the Berlin, Boston, Chicago, London, New York City, and Tokyo marathons between 2010 and 2024. They compared race results with estimated concentrations of nitrogen dioxide (NO₂) and fine particulate matter (PM₂.₅), while considering factors such as heat index, wind speed, and changes over time.

Nitrogen dioxide showed the clearest association with performance. Each additional microgram per cubic meter was associated with finishing times approximately 1.43 minutes slower for men and 1.56 minutes slower for women.

These figures describe statistical associations across the study population, not guaranteed delays for individual runners. Nevertheless, the results suggest that environmental conditions may influence performance over the demanding 42.195-kilometer distance.

PM₂.₅, which consists of tiny airborne particles, did not show a statistically significant association with finishing times after adjustments. This difference highlights why individual pollutants should be examined separately rather than treating all air pollution as having identical effects.

Why Nitrogen Dioxide Matters During Exercise

Nitrogen dioxide is produced mainly through combustion, including emissions from road traffic and other fuel-burning activities. Its concentration varies according to traffic levels, weather, and local environmental conditions.

During a marathon, runners breathe faster and more deeply than they do at rest. This increases the amount of air entering the respiratory system and may increase exposure when pollutant concentrations are elevated.

Nitrogen dioxide can irritate the airways and contribute to respiratory inflammation. Air pollution may also affect cardiovascular function, potentially making sustained exercise more demanding. For endurance athletes, even a modest additional burden could make maintaining a consistent pace more difficult.

However, the researchers did not directly measure each runner's inhaled pollution dose or establish the precise biological mechanisms behind the observed results. Further research is needed to determine how much nitrogen dioxide itself contributes to slower performance.

Why Recreational Runners May Be More Affected

One notable finding was the difference between recreational and elite athletes. The association between nitrogen dioxide and finishing time was approximately six times greater among recreational participants. For recreational runners, each 1 µg/m³ increase was associated with a delay of approximately 2.12 minutes. The corresponding estimate for elite runners was about 0.34 minutes.

Several factors might explain this difference. Elite athletes generally have extensive training experience, refined pacing strategies, and highly developed endurance capabilities. Recreational participants may spend longer on the course, potentially increasing their exposure duration.

Differences in fitness, race strategy, and individual susceptibility could also contribute. The study found a stronger association among runners aged 18–29 than among those aged 60 and older, although this does not prove that younger athletes are universally more vulnerable.

Expert Insight

Professor Bryan Williams, chief scientific and medical officer at the British Heart Foundation, commented on the findings in the University of Leicester's research announcement. Williams stated: “This large study suggests air pollution, specifically nitrogen dioxide, has a negative impact on how well people can exercise.”

Practical Steps for Marathon Preparation

Runners can include air quality checks in their training routines. Local forecasts can help identify periods when pollution levels are elevated. When conditions are unfavorable, exercising indoors or choosing routes away from heavy traffic may reduce exposure. Race organizers can also monitor environmental conditions, consider traffic management measures, and communicate air quality information to participants.

The analysis of more than 2.7 million marathon performances suggests that higher nitrogen dioxide concentrations are associated with slower finishing times across six major international races. Recreational runners showed a stronger association than elite athletes, while PM₂.₅ did not demonstrate a statistically significant relationship after adjustments.

Although further research is necessary to clarify the causes, the findings highlight air quality as a useful consideration in endurance training and event planning. Understanding environmental conditions can help runners prepare more effectively while preserving the substantial benefits of regular physical activity.