What should you do on a high ozone day?

Short answer. On a high ozone day, do your outdoor exercise in the morning, when ozone is lower, and make it shorter and less intense, as AirNow advises ¹. Watch for coughing or shortness of breath; people with asthma should follow their action plan and keep quick-relief medicine handy ¹. The WHO guideline is 100 µg/m³ as a daily maximum 8-hour mean ⁵.

Is this the same ozone as the ozone layer?

High in the stratosphere, ozone forms the ozone layer that shields the Earth from the sun's ultraviolet rays; at ground level, where we breathe, it harms health ¹². The EPA calls the first "good" and the second "bad" ².

Ground-level ozone is not emitted directly. It forms when nitrogen oxides and volatile organic compounds, from vehicles, industry, power plants, solvents and paints, react in sunlight ¹². The EPA calls it the main ingredient in smog ²; Health Canada calls it a component of smog ⁴.

Why is ozone worst on hot, sunny afternoons?

Because sunlight drives the reaction. AirNow says ozone forms in sunlight, usually on hot summer days, and is worse in the afternoon and early evening ¹. Health Canada says levels are generally higher in spring and summer and, in summer, peak between noon and 6 pm ⁴.

The agencies' time windows differ slightly. Health Canada gives noon to 6 pm ⁴, AirNow the afternoon and early evening ¹, and a Pennsylvania alert from 2025 expected strong formation from late morning through the afternoon ⁸. All three point the same way: AirNow tells people to plan outdoor activities for the morning, when ozone is lower ¹.

Why can the countryside have more ozone than the city?

Ozone travels long distances on the wind, so the EPA says even rural areas can have high levels ². In cities, other pollutants also break ozone down: GOV.UK notes that ozone falls when it reacts with nitric oxide, and that urban pollutants from traffic, industry and other sources can degrade ozone or inhibit its formation ⁷.

The result in the UK: ozone concentrations are typically higher in rural areas than in urban areas ⁷. In 2025, rural background stations averaged 155 hours of "Moderate" or higher ozone pollution per station, against 135 hours at urban background stations ⁷. A drive to the countryside on a hot afternoon is not an escape from ozone.

What does ozone do to the body?

The EPA lists coughing, a sore or scratchy throat, difficulty breathing deeply, and pain when taking a deep breath ³. Ozone can tighten the airway muscles, causing wheezing and shortness of breath, and it increases the frequency of asthma attacks ³. AirNow adds that it can make the lungs more susceptible to infection and aggravate lung diseases ¹.

Even healthy adults can feel these effects ¹. The groups the agencies name as more at risk:

• People with lung disease such as asthma ¹³; Health Canada adds COPD, cardiovascular disease and diabetes ⁴.
• Children and teenagers, whose lungs are still developing and who breathe more air per pound of body weight than adults ¹.
• Older adults ¹³⁴.
• People who are active outdoors, including outdoor workers ¹⁴.

If you have a heart or lung condition, Health Canada advises asking your health care professional about extra ways to protect yourself when pollution is high ⁴. This guide is not individual medical advice.

When should you exercise on an ozone day?

In the morning. At the "Unhealthy for Sensitive Groups" and "Unhealthy" levels, AirNow's ozone guide tells sensitive groups to plan outdoor activities in the morning, when ozone is lower, and it gives the same advice to everyone else up to "Very Unhealthy" ¹. Exercise itself is not the problem; the guide says you can reduce exposure and still get exercise ¹.

The simple steps AirNow gives when the index is unhealthy ¹:

• Choose a less intense activity.
• Shorten it, or take more breaks.
• Reschedule to the morning or to another day.
• Move the activity inside.

Health Canada's version is to avoid or reduce strenuous outdoor activity and exercise during smog episodes ⁴. Austria's environment agency, at the EU alert threshold, advises avoiding unusual and strenuous physical activity particularly at midday and in the afternoon ⁶.

Does staying indoors help?

It can help, but the ozone guidance puts the emphasis elsewhere. AirNow says to move activities inside "where ozone levels are usually lower" ¹, and Austria's agency suggests that at-risk people, such as children with bronchial hyper-reactivity, consider staying indoors in smoke-free areas when the alert threshold is passed ⁶. The main lever in the ozone guidance is less exertion at the wrong hour, not sealing the house ¹⁴.

There is an important limit. Ozone days are often hot, and AirNow warns that without air conditioning, staying inside with the windows closed may be dangerous in extreme heat: go somewhere with air conditioning or a cooling centre ¹. Heat safety is covered in the heatwave guide. For particles and smoke, where a sealed, filtered room does matter, see what to do during a pollution episode.

What are the official ozone thresholds?

Thresholds use different averaging times, so compare like with like.

The EU target is above the WHO guideline, and many stations exceed it: in 2025, 22.4% of reporting stations in Europe were above the current EU target value threshold ⁵. In the United States and Canada, ozone is reported through each country's index, the AQI ¹ and the Air Quality Health Index ⁴; the air quality index guide explains how to read them.

What is an Ozone Action Day?

A U.S. air quality alert. In Pennsylvania, the state Department of Environmental Protection declares an Air Quality Action Day when the AQI is forecast to be Code Orange or higher ⁸. Children, older adults and people with respiratory conditions are told to avoid heavy outdoor activity, and residents and businesses are asked to carpool, use transit, limit idling and refuel after dusk ⁸. AirNow lists "watch for Ozone Action Days in your area" among its tips ¹.

What official agencies recommend

• United States (EPA, AirNow). Check the AQI; at unhealthy levels, shorten and lighten outdoor activity and move it to the morning; people with asthma keep quick-relief medicine handy ¹.
• Canada (Health Canada). Check the Air Quality Health Index for the best time to be active; avoid or reduce strenuous exercise during smog; summer peak noon to 6 pm ⁴.
• European Union (EEA, national agencies). Information at 180 µg/m³ and alert at 240 µg/m³ over one hour ⁶; target value 120 µg/m³ over 8 hours ⁵. In Austria, above the information threshold very sensitive people may notice minor symptoms during strenuous activity; other precautions are needed only above the alert threshold ⁶.
• United Kingdom (GOV.UK). The Daily Air Quality Index moves to "Moderate" above 100 µg/m³ over 8 hours ⁷; ozone is typically higher in rural areas than in urban ones ⁷.
• WHO. 100 µg/m³ over 8 hours, 60 µg/m³ for the peak season ⁵.

Where they diverge: the WHO's 100 µg/m³ is stricter than the EU's 120 µg/m³ target ⁵, and Austria says usual outdoor activities such as walking remain safe even for vulnerable people above the information threshold ⁶, while AirNow already asks unusually sensitive people to ease off in its "Moderate" band ¹.

Frequently asked questions

Can I still go for a run on an ozone day?

Usually yes, earlier. AirNow advises scheduling outdoor activity in the morning when ozone is lower, and making it shorter and less intense as the index rises ¹. At "Very Unhealthy," sensitive groups should avoid all physical activity outdoors ¹.

Does a mask protect against ozone?

None of the agencies cited here recommend a mask for ozone; their advice is timing, intensity and moving indoors ¹⁴⁶. Masks for smoke particles are a separate topic, covered in the wildfire smoke guide.

Why is the ozone level high when there is no traffic around me?

Ozone can be carried long distances by the wind ², and city pollutants that break it down are scarcer in the countryside ⁷. In the UK, ozone is typically higher in rural areas than in urban ones ⁷.

What should someone with asthma do?

Follow their asthma action plan and keep quick-relief medicine handy, as AirNow advises at every unhealthy level ¹. Austria's agency tells at-risk people to contact their doctor for personal advice ⁶.

Is ozone only a summer problem?

Mostly. Health Canada says levels are higher in spring and summer and lower in winter ⁴, although the EPA notes high readings can occur in colder months too ².

Know your local risk

Ozone and other air risks differ from one region to the next: see air quality country by country.

Your Phaenyx plan's "Water and air" section lists the water and air risks for your postal code. The Phaenyx Safe app keeps the reflex sheets, including this one, available offline. Build your Phaenyx plan.

Sources

1. AirNow (U.S. Environmental Protection Agency) — Air Quality Guide for Ozone — https://document.airnow.gov/air-quality-guide-for-ozone.pdf (accessed 2026-10-27)
2. U.S. Environmental Protection Agency (EPA) — Ground-level Ozone Basics — https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics (accessed 2026-10-27)
3. U.S. Environmental Protection Agency (EPA) — Health Effects of Ozone Pollution — https://www.epa.gov/ground-level-ozone-pollution/health-effects-ozone-pollution (accessed 2026-10-27)
4. Health Canada — Air pollution: what is ozone? — https://www.canada.ca/content/dam/eccc/documents/pdf/aqhi/20200830-1e.pdf (accessed 2026-10-27)
5. European Environment Agency (EEA) — Ozone - O3 | Air quality status in Europe 2026 — https://www.eea.europa.eu/en/analysis/publications/air-quality-status-report-2026/ozone-o3 (accessed 2026-10-27)
6. Umweltbundesamt (Environment Agency Austria) — Air information – current ozone levels — https://www.umweltbundesamt.at/en/services/laboratory-services/services-measurements/current-ozone-levels (accessed 2026-10-27)
7. GOV.UK (Department for Environment, Food & Rural Affairs) — Ozone (O3) – Air quality statistics — https://www.gov.uk/government/statistics/air-quality-statistics/concentrations-of-ozone (accessed 2026-10-27)
8. Pennsylvania Department of Environmental Protection — DEP Declares Code Orange Air Quality Action Day for Ozone for Thursday, September 18, 2025, for Southwest Pennsylvania — https://www.pa.gov/agencies/dep/newsroom/2025-09-17-dep-declares-code-orange-air-quality-action-day-for-ozone-for-september-18-2025-for-southwest-pa (accessed 2026-10-27)