What ozone pollution is and how to avoid it

The short answer

Ground-level ozone is a gas that forms when pollution from cars, power plants and industry reacts in sunlight. It's worst on hot, sunny afternoons and early evenings. On high-ozone days, move hard exercise to the morning or indoors. Air purifier ratings cover particles only, and ozone generators add ozone, so never use one.

Ozone near the ground is a pollutant

Ozone is a gas made of 3 oxygen atoms. Where it sits decides whether it helps or hurts. High up in the stratosphere, it forms the layer that shields the earth from the sun's ultraviolet rays. At ground level, where you breathe, the EPA calls it a harmful air pollutant and the main ingredient in smog. The EPA's short version is "good up high, bad nearby."

Sunlight makes it

Ground-level ozone isn't emitted directly. It forms when 2 kinds of pollution, nitrogen oxides and volatile organic compounds (VOCs), react in sunlight. The EPA names cars, power plants, industrial boilers, refineries and chemical plants as sources. AirNow adds products such as solvents and paints.

The EPA says ozone is most likely to reach unhealthy levels on hot, sunny days in cities. It can still run high in colder months. Wind can carry it long distances, so rural areas get high ozone too.

Afternoons are the worst time

AirNow says ozone is worse in the afternoon and early evening, so plan outdoor activities for the morning.

The daily ozone AQI uses the day's highest 8-hour average. AirNow's live reading uses an ozone NowCast that rises and falls with the hourly levels. Our AQI guide explains how to read both.

The EPA requires ozone monitoring during the months when weather favors ozone. Some warm states monitor all year. Colder states may monitor for as little as 5 months in summer.

Here's what ozone does to your lungs

According to the EPA, depending on how much you breathe, ozone can:

Cause coughing and a sore or scratchy throat
Make it harder to breathe deeply, and make deep breaths hurt
Inflame and damage the airways
Make the lungs more likely to get infections
Worsen asthma, emphysema and chronic bronchitis
Bring on asthma attacks more often

The EPA says some of these effects show up even in healthy people. They can be more serious with asthma, and they lead to more missed school, medication use, ER visits and hospital stays. Over the long term, the EPA links ozone to worse asthma and says it's likely one of many causes of asthma developing.

Some people need to take more care

The EPA lists the people most at risk:

People with asthma or other lung disease
Children and teenagers. The EPA says children are at greatest risk because their lungs are still developing, they breathe more air per pound of body weight than adults, and they're more likely to be active outdoors when ozone is high.
Older adults
People who are active outdoors, especially outdoor workers

The EPA also notes people with certain genetic traits, and people who get less vitamin C and E in their diet, are at greater risk.

Change when and how hard you exercise

Exercise makes you breathe more ozone. On high-ozone days, AirNow's advice is to:

Pick a less intense activity
Go shorter, or take more breaks
Move it to the morning or to another day
Move it inside, where ozone levels are usually lower

AirNow's advice by AQI level, condensed:

AQI	Sensitive groups	Everyone else
Orange (101 to 150)	Go shorter and less intense, take more breaks, plan for morning	Consider going shorter and less intense
Red (151 to 200)	No long or intense outdoor activity, schedule for morning, consider moving indoors	Cut back on long or intense activity, take more breaks
Purple (201 to 300)	Avoid all physical activity outdoors	Avoid long or intense exertion, consider moving indoors
Maroon (301 and up)	Avoid all physical activity outdoors	Avoid all physical activity outdoors

People with asthma should follow their asthma action plan and keep quick-relief medicine handy.

One caution: AirNow says that without air conditioning, staying inside with the windows shut can be dangerous in extreme heat. If you're too hot, go somewhere cool.

You can get alerts by email through EPA's free EnviroFlash service, or check the forecast on AirNow.

Air purifiers sized for smoke aren't rated for ozone

Ozone is a gas. The EPA says most air cleaner filters are designed for either particles or gases, and the CADR rating on a purifier's box covers particles only. A HEPA purifier sized by its CADR tells you nothing about ozone.

To remove gases, the EPA says you need an activated carbon filter or another gas filter, and carbon works best when there's a lot of it. There's no widely used rating for how well a purifier removes gases, so you can't compare models on ozone.

Ozone generators add ozone to your home

Some machines sold as air purifiers make ozone on purpose. The EPA says no federal agency has approved them for occupied spaces. At levels within health standards, ozone does little to clean indoor air, and EPA-cited studies found some units pushed indoor ozone well past health limits even when used as directed. The California Air Resources Board strongly advises against using them at home.

Sellers use names like "energized oxygen," "activated oxygen" or "super-oxygenated air." CARB-certified air cleaners have been tested to emit no more than 0.050 parts per million of ozone. Our air purifier guide covers what to buy.

You can help cut ozone

AirNow's suggestions: drive less, don't idle, stop fueling when the pump shuts off, tighten your gas cap, and use low-VOC paints and cleaners. Some areas call Ozone Action Days when levels are expected to be high.

What helps

Exercise early

Ozone peaks on hot afternoons.

Check the forecast

AirNow posts it the day before.

Skip ozone generators

They add ozone to your home.

Questions people ask

When is ozone highest?

On hot, sunny days, in the afternoon and early evening, according to AirNow. Mornings are usually lower, which is why the EPA's advice is to exercise early.

Can I smell ozone?

Sometimes, but don't count on it. The EPA says people's ability to smell ozone varies a lot and fades quickly when you're around it. No smell doesn't mean low levels.

Does a HEPA air purifier remove ozone?

A HEPA filter is built to catch particles. The EPA says removing gases takes a gas filter such as activated carbon, and there's no common rating to show how well any purifier removes ozone.

What ozone level is considered unhealthy?

The national 8-hour ozone standard is 0.070 parts per million. On the AQI, the top of the yellow range is 0.070 ppm, and anything above it is orange or worse.