Race-day nitrogen dioxide slowed marathon runners by about a minute and a half for each extra microgram per cubic meter of air. The effect comes from 2,756,553 net finishing times at six World Marathon Majors raced from 2010 to 2024, according to the University of Leicester in its September 2026 announcement.
Nitrogen dioxide is a traffic-related gas pollutant measured at ground level on race day. The races studied were Berlin, Boston, Chicago, London, New York City and Tokyo. The analysis was published 29 Aug 2026 as a cross-sectional study in Sports Medicine–Open, according to Springer Nature in the journal article.
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Table of Contents
- How much slower were runners?
- Who loses the most time?
- What did the study not prove?
- What can runners and races do?
How much slower were runners?
Each 1 µg/m³ rise in nitrogen dioxide was linked to finish times 1.43 minutes slower for men and 1.56 minutes slower for women. Those averages cover millions of results across 15 years. Small shifts in race-day air can therefore move a personal goal by minutes.
A two-unit difference would equal roughly three minutes for most runners. That matters for time cutoffs, pacing plans and Boston qualifying attempts. It also helps explain why similar fitness can produce different results on different days.
Who loses the most time?
Recreational runners paid the largest penalty. Their slowdown was about six times larger than for elite runners, at roughly 2.12 versus 0.34 minutes per µg/m³, according to Running Lookout reporting on the paper in its September 2026 report.
Runners aged 18–29 showed larger nitrogen dioxide effects than older runners. Longer time on course means longer exposure to traffic air. Less even pacing and more mouth breathing can also raise inhaled dose.
What did the study not prove?
The design documents association, not causation. As a cross-sectional study of ambient race-day pollution, it compares different races and days rather than following one runner through clean and dirty air.
Exposure was based on ambient race-day levels, not air quality along every mile of each 26.2-mile route. Fine particulate matter effects were not statistically significant in this analysis. Temperature, humidity and course traffic still shape results.
What can runners and races do?
The authors recommend that planners and runners weigh air quality and event-traffic policies alongside heat and humidity, especially for recreational participants. Start-time choice, traffic closures and route selection all change roadside exposure. Runners can take practical steps on smoky or high-traffic days: The Leicester team is developing the P-STEP app using real-time, location-specific air-quality data to help people plan outdoor exercise with lower exposure.
- Check the local air-quality index the night before and race morning
- Shift hard workouts away from rush-hour roads when pollution peaks
- Wear no mask for racing unless already tested in training
- Seek medical advice promptly for chest tightness, wheezing or unusual fatigue