Ozone and Asthma: How Smog Affects Your Lungs and What to Do on Bad Air Days

Ground-level ozone, often called smog, is a powerful lung irritant. The EPA says it can cause coughing and a scratchy throat, inflame and damage the airways, and increase the frequency of asthma attacks. People with asthma, children, older adults and anyone active outdoors are most at risk.

This guide explains what ozone is, when it’s highest, how to read ozone alerts, and the simple steps that reduce your exposure. It also covers a hidden indoor source: air cleaners that make ozone. For more, see our asthma guides.

Jump to: Quick answer · What ozone is · Health effects · Ozone alert levels · Protect yourself · Ozone indoors · FAQ

Quick answer

• Ozone forms in sunlight when pollutants from vehicles, industry, power plants and products like paints and solvents react. Levels can become unhealthy on hot, sunny days.
• It irritates the lungs: coughing, breathing difficulty, airway inflammation and more asthma attacks.
• Ozone is usually lower in the morning, so the EPA suggests scheduling outdoor activity then on high-ozone days.
• When the Air Quality Index (AQI) hits 101 or more, people with asthma should follow their asthma action plan and keep quick-relief medicine handy.
• Avoid air cleaners that produce ozone, including some ionizers. Choose CARB-certified models.

What is ground-level ozone?

According to the EPA’s Air Quality Guide for Ozone, ozone high in the atmosphere forms the protective ozone layer, but ozone at ground level, where we breathe, harms health. It forms when two types of pollutants, volatile organic compounds and nitrogen oxides, react in sunlight. These come from vehicles, industry, power plants, and products such as solvents and paints.

That’s why ozone is mainly a problem on hot, sunny days. It’s a different trigger from pollen, but both can be high in the same summer weeks. See do seasons affect allergies?

How ozone affects your lungs

The EPA lists these effects of breathing ozone:

• Coughing and a sore or scratchy throat
• Inflamed and damaged airways
• Lungs that are more vulnerable to infection
• Worse lung diseases, including more frequent asthma attacks
• A higher risk of death from lung disease

Children and teens are especially vulnerable because their lungs are still developing and they breathe more air per pound of body weight than adults, the EPA notes.

Large studies back this up. A 2004 study in JAMA of 95 large U.S. urban communities, home to about 40% of the U.S. population, found that a 10-part-per-billion rise in the previous week’s ozone was linked to a 0.52% increase in daily deaths and a 0.64% increase in heart and lung deaths.

What ozone alert levels mean

Source: EPA/AirNow Air Quality Guide for Ozone. For a full explanation of the index and other pollutants like wildfire smoke, see our Air Quality Index guide for asthma.

How to protect yourself on high-ozone days

1. Check the forecast at AirNow.gov or in the free AirNow app.
2. Exercise in the morning, when ozone is usually lower.
3. Choose a less intense activity, shorten it, or take more breaks.
4. Move activities indoors, where ozone levels are usually lower.
5. Keep your asthma under control with an asthma action plan from your doctor.

The EPA adds a heat warning: if you don’t have air conditioning, staying inside with the windows closed can be dangerous in extreme heat. Go somewhere with air conditioning or a local cooling center.

You can also help lower ozone: drive less, don’t let your engine idle, stop refueling when the pump shuts off, and choose low-VOC paints and cleaning products, sealed and stored so they can’t evaporate.

Ozone indoors: air cleaners to avoid

Some air cleaners produce ozone, either as a byproduct or on purpose. The EPA advises avoiding devices that intentionally produce ozone, and notes that some ionizers can raise indoor ozone above public health standards. California’s Air Resources Board (CARB) certifies air cleaners that meet its ozone emission limit of 0.050 parts per million.

For people with asthma, a mechanical filter (HEPA) air cleaner is the safer choice. Read do ionic air purifiers work? and air purifier myths.

This guide is general information, not medical advice. Read our medical disclaimer.

Frequently asked questions

Can ozone trigger an asthma attack?

Yes. The EPA says ozone inflames the airways and increases the frequency of asthma attacks. People with asthma should follow their asthma action plan on high-ozone days.

What time of day is ozone highest?

Ozone forms in sunlight, and levels can become unhealthy on hot, sunny days. The EPA suggests scheduling outdoor activities in the morning, when ozone is lower.

Is ozone the same as smog?

Ground-level ozone is often called smog. It’s different from the protective ozone layer high in the atmosphere.

Is it safe to exercise outside on an ozone alert day?

At AQI 101 to 150, people with asthma and other sensitive groups should make activities shorter and less intense. At higher levels, they should avoid long or intense activity or move indoors.

Do air purifiers produce ozone?

Some do, especially ozone generators and some ionizers. Choose a CARB-certified air cleaner, and avoid any device marketed as producing ozone.

Sources

• U.S. Environmental Protection Agency and AirNow. Air quality guide for ozone.
• U.S. Environmental Protection Agency. Health effects of ozone pollution. Updated February 24, 2026.
• Bell ML, McDermott A, Zeger SL, Samet JM, Dominici F. Ozone and short-term mortality in 95 US urban communities, 1987-2000. JAMA. 2004;292(19):2372-2378.
• U.S. Environmental Protection Agency. Guide to air cleaners in the home.
• California Air Resources Board. Air cleaner information for consumers.

Last reviewed: October 21, 2026.