Summer Ozone: The Pollutant That Peaks in the Afternoon

Most air pollution tracks traffic, so it peaks at rush hour and eases overnight. Ozone doesn't behave that way at all, and the difference matters if you exercise outdoors.

It isn't emitted — it's cooked

Ground-level ozone isn't released by anything. It forms when sunlight drives a reaction between nitrogen oxides and volatile organic compounds, both largely from traffic and industry. Sunlight is a required ingredient, which is why ozone is a summer problem and why it peaks in the afternoon — several hours after the morning emissions that supply it.

That lag also means ozone can be worse downwind in suburbs and countryside than in the city centre that produced the precursors, which surprises people who moved out for the air.

What it does

Ozone is an irritant gas. At elevated concentrations it inflames the airway lining, and the common effects are chest tightness, coughing, a scratchy throat and the sensation that a deep breath won't quite complete. People with asthma are more reactive to it, and it's a recognised trigger for exacerbations.

Unlike particulate, it isn't filtered out by a mask. A respirator stops particles; it does not stop a gas.

The exercise multiplier

This is the part worth internalising. At rest you move maybe six litres of air a minute. During hard exercise that can rise several-fold, and you shift toward mouth breathing, bypassing the nose's filtering and conditioning entirely.

So an hour of hard running on a high-ozone afternoon delivers dramatically more irritant to your airways than the same hour spent sitting outside. The intensity of the session, not just its location, sets the dose.

Reading the index

Air quality indices report a composite, but most will let you see which pollutant is driving the number. In summer it's frequently ozone. Broadly: under 50 is fine, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, and above 150 unhealthy for everyone. Sensitive groups means asthma, COPD, heart disease, older adults, young children and pregnancy.

Indoors is genuinely better here

Ozone is reactive and degrades quickly on contact with surfaces, so indoor concentrations are typically well below outdoor ones as long as you aren't pulling large volumes of outside air in. On a high-ozone afternoon, closed windows help — which is the opposite of the ventilation advice that applies on most other days.