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.

The confusion between the two has sometimes been weaponised — by those who want to downplay air pollution regulations, the talking point that "we need ozone" obscures the fact that ground-level ozone serves no protective function and exists entirely as a byproduct of combustion pollution. Understanding the difference is not a minor technical point. It's the foundation for understanding why ozone alerts exist, and why they matter.

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.

Nitrogen oxides (NOx) are emitted by car engines, power plants, and industrial processes whenever fossil fuels burn at high temperatures. Volatile organic compounds (VOCs) come from fuel evaporation, industrial solvents, paints, and even trees (though biogenic VOCs are a smaller contributor to urban ozone than industrial and transport sources). When these two precursors mix in the presence of sunlight, they react to form ozone.

Why it peaks in the afternoon 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.

Geographic patterns Cities with intense sunlight and high traffic are the worst ozone environments: Los Angeles, Houston, Denver, and parts of the Northeast. But ozone can travel — it's a regional pollutant that forms downwind of emission sources, which means suburban and rural areas can record high ozone levels from urban emissions carried on the wind. The Appalachian Mountains, ironically, record some of the highest ozone readings in the eastern US due to prevailing wind patterns.

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 Even moderate ozone exposure causes measurable drops in lung function in healthy adults. Symptoms include coughing, throat irritation, chest tightness, and shortness of breath — particularly during exercise, when breathing rate is elevated. These effects are typically reversible once ozone levels drop.

Long-term effects The evidence on chronic ozone exposure tells a more serious story. 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, COPD, and other respiratory conditions bear the heaviest burden, but healthy individuals are not immune.

Why exercise amplifies the risk Physical exertion increases breathing rate and volume, meaning you inhale far more ozone per minute during a run than at rest. Exercising outdoors during peak ozone hours on a summer day can produce a respiratory dose comparable to sitting in a moderately polluted environment for much longer. This is the specific scenario the WHO and EPA warnings target when they recommend limiting outdoor exercise on high-ozone days.

Ozone's afternoon peak and weather dependence make it more predictable than many pollutants — which means protecting yourself is largely a matter of timing.

Check before heading out Ozone forecasts are included in the standard AQI you see in weather apps and on PollutionProfile. The key numbers: • Under 100: Most healthy people can exercise outside without concern • 101–150 (Orange): Sensitive groups — asthma, COPD, elderly, children — should reduce prolonged outdoor exertion • Above 150 (Red+): Limit outdoor exercise for everyone, particularly in the afternoon

Shift your workout window Morning is almost always better for outdoor exercise in summer. Ozone hasn't had time to build, temperatures are lower, and traffic-related precursors from the previous day have largely dispersed overnight. By 7am, you're typically breathing the cleanest outdoor air of the day.

Hot, sunny, calm days are the highest-risk combination 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. Wind, clouds, and temperature drops all reduce ozone formation. When the forecast is for heat and high pressure sitting over the city, check the afternoon ozone reading before scheduling outdoor time.

Indoors isn't entirely safe Ozone infiltrates buildings — though at lower concentrations than outdoors. Tightly sealed homes with HVAC recirculation reduce ozone infiltration. Opening windows in the afternoon during a high-ozone day brings outdoor ozone inside.