Understanding Ozone: The Good Kind vs. The Harmful Kind

Here’s a piece of environmental messaging that has genuinely confused people for decades: ozone is both a shield and a poison, depending entirely on where it is.

Thirty kilometres above the Earth’s surface, a layer of ozone molecules absorbs ultraviolet radiation from the sun. Without it, skin cancer rates would be catastrophic. The hole in the ozone layer over Antarctica — caused by CFC refrigerants — was a genuine planetary emergency, and the international response through the Montreal Protocol is one of the most successful environmental stories of the 20th century.

That ozone is up there, doing something essential. The ozone in the air you’re breathing right now — street-level, ground-level, the stuff in your city’s afternoon air on a hot summer day — is a different beast entirely. It’s created at ground level by human activity, it’s not protecting anyone from anything, and at the concentrations reached in many cities, it’s actively damaging to human lung tissue.

Ground-level ozone doesn’t come out of a car exhaust directly. It’s manufactured by chemistry — and the recipe requires three ingredients: nitrogen oxides, volatile organic compounds, and sunlight.

The photochemical reaction takes time. Morning rush hour produces the NOx and VOC precursors; by early afternoon, sunlight has had several hours to work on them. This is why ozone levels typically peak between noon and 4pm, hours after the traffic that caused it has dispersed. On hot, sunny, calm days — when the precursors can’t be diluted by wind — ozone builds to its highest concentrations.

Breathing elevated ozone is like bathing your airways in a mild bleach solution. That’s not a metaphor — ozone is a powerful oxidant, and it reacts with the delicate tissue lining the respiratory tract much as it reacts with organic material generally: by oxidising and damaging it.

Immediate effects are reversible once ozone levels drop. But the long-term picture is more serious. A 2009 study in the New England Journal of Medicine following nearly 450,000 adults found that long-term ozone exposure was independently associated with respiratory mortality — even after accounting for PM2.5. People with asthma and COPD bear the heaviest burden, but healthy individuals are not immune. Physical exertion dramatically increases ozone intake, multiplying dose by up to 16× compared to rest.

Ozone’s afternoon peak and weather dependence make it more predictable than many pollutants — which means protecting yourself is largely a matter of timing. Morning is almost always better for outdoor exercise in summer. If you wake up to a hot, still, cloudless summer morning, plan for afternoon ozone to be elevated even if the morning reading looks fine.