Dirty air and running: why you should watch ozone, not the AQI

A city runner opens an app in the morning, sees a yellow air quality index and decides: treadmill today. The logic is clear — while running we push many times more air through our lungs than at rest, and we breathe through the mouth, bypassing the nasal filter. Except a recent meta-analysis suggests we may be looking at the wrong number.

Why exercise amplifies the effect

The mechanism is not in dispute. During intense work, minute ventilation rises many-fold and breathing switches from nasal to oral. The nose is a decent filter: it traps some of the larger particles, humidifies and warms the air. The mouth does none of that. As a result the dose of inhaled pollutants grows disproportionately, and particles penetrate deeper into the airways.

Hence the concern: polluted air raises the risk of respiratory infections and over time may lead to structural changes in the airways, limiting both performance and athletic longevity.

The separate question is: how much of this shows up in performance here and now?

What the meta-analysis showed

A systematic review with meta-analysis in PLOS Climate (2026) gathered 23 studies of short-term associations between air pollution and sporting performance — in real competitive settings, among amateurs and elites. It covered particulates (PM2.5, PM10), gases (ozone, NO₂, SO₂, CO) and the composite AQI.

The result turned out to be unexpectedly narrow. A significant association was found for ozone only:

across all performance types — d = −0.33 (95% CI −0.65 to −0.02);
for endurance specifically — d = −0.54 (95% CI −1.02 to −0.04), a medium-sized effect.

For PM2.5, PM10, NO₂, AQI, CO and SO₂ no reliable associations were found. That runs directly against the everyday habit of treating the “air quality index” as the single number to go by.

An important detail from the authors: the laboratory experiments on which much of the fear is built used concentrations unreachable in ordinary life — ozone up to 750 ppb, whereas real-world measurements rarely exceeded 111 µg/m³. Put simply, it is easy to show an effect in a chamber with a high dose; at an actual race everything is more modest.

Individual studies do still record an influence of particulates. A 2019 Chinese study of marathon runners estimated that a doubling of AQI adds about 4% to finishing time, and an analysis of elite distance runners showed that elevated exposure to PM2.5 and ozone during the preparation period was associated with a roughly 1–1.5% drop in 5 km performance. But in the pooled data from 23 studies these signals did not survive scrutiny — apart from the ozone one.

What a runner should do about it

The practical implications change more than you might expect.

Look at ozone separately, not just at the composite AQI. Most services provide a breakdown by pollutant — ozone is usually labelled O₃.
Account for ozone's daily rhythm. It is a secondary pollutant: it forms under sunlight and peaks on hot, sunny days in the afternoon. A morning session is almost always “cleaner” for ozone than an afternoon one. Conveniently, this also matches heat guidance.
Particulates follow a different logic. PM is tied to traffic and heating, not to sunlight. Here it is not the time of day that matters but the route: moving a couple of blocks away from a main road is more effective than shifting the session by an hour.
Do not extend the conclusion to health. The meta-analysis answered the question “does performance drop”, not “is this safe in the long run”. The absence of a PM2.5 effect on your finishing seconds does not mean an absence of effect on your airways.
Key races and high-volume blocks are worth planning with the ozone season in mind — the elite data hints that exposure across the whole preparation period matters, not just on race day.

Limitations

The authors are honest about the weak points, and they are worth keeping in mind. The design is observational — it shows associations but does not prove causation. Almost everywhere the data came from fixed monitoring stations rather than personal sensors: a runner's actual exposure on a given route may differ substantially from the “city average”. There is little research on sprint and technical disciplines, and very little involving women and amateurs. Finally, the wide confidence intervals even for the ozone effect mean the precise magnitude will still be refined.

Key points

While running, the dose of pollutants rises because of high ventilation and mouth breathing past the nasal filter — the mechanism is not disputed.
In a meta-analysis of 23 studies, only ozone had a significant effect: d = −0.33 across all performance types and d = −0.54 for endurance.
PM2.5, PM10, NO₂, AQI, CO and SO₂ showed no reliable link with performance.
Ozone builds up towards a hot sunny afternoon — run in the morning; particulates are tied to traffic — change your route, not the time.
“No effect on seconds” ≠ “no effect on health”: this analysis did not assess long-term respiratory risk.
The data is observational and exposure was measured at stations rather than on the runner — so the precision of the conclusions is limited.

Sources: “Air pollution and performances in outdoor sports: A systematic review and meta-analysis of short-terms associations in amateur and elite athletes”, PLOS Climate, 2026. https://doi.org/10.1371/journal.pclm.0000988. “Air Pollution and Endurance Exercise: A Systematic Review of the Potential Effects on Cardiopulmonary Health”. https://pmc.ncbi.nlm.nih.gov/articles/PMC12028381/. “Impact of air pollution on running performance”, Scientific Reports, 2023. https://www.nature.com/articles/s41598-023-28802-x