What Is Ground-Level Ozone and Why Does It Matter?

Wait — Isn't Ozone Good?

This is where people get confused. There are two kinds of ozone, and they couldn't be more different.

Stratospheric ozone — the "ozone layer" — sits miles above the Earth's surface. It blocks ultraviolet radiation from the sun. Without it, you'd get a sunburn in minutes. This ozone is protective and essential. When people talk about the "hole in the ozone layer," they're talking about this one.

Ground-level ozone is the opposite story. It forms near the surface, in the air you actually breathe. At this level, ozone is a pollutant. It doesn't protect you. It hurts you.

How Does It Form?

Ground-level ozone isn't released directly from a tailpipe or smokestack. It's created by a chemical reaction in the atmosphere. Here's the recipe:

Take nitrogen oxides (NOx) — which come from car engines, power plants, and industrial boilers — and volatile organic compounds (VOCs) — which come from gasoline vapors, solvents, paints, and chemical plants. Put them together in sunlight and heat. The result is ozone.

That's why ozone is worst on hot, sunny days in summer, and worse in cities with heavy traffic and industry. But wind can carry ozone and its ingredients hundreds of miles, so rural areas aren't safe either. You can live far from a highway and still breathe unhealthy ozone on a bad day.

What Does It Do to You?

Ozone attacks lung tissue. Scientists describe the effect like a sunburn on the inside of your lungs. In the short term, breathing elevated ozone can cause coughing, throat irritation, chest pain, and shortness of breath. These effects happen even in healthy people.

For people with asthma, the effects are worse. Ozone can trigger asthma attacks, increase the need for medication, and send people to the emergency room. Children are especially vulnerable because their lungs are still developing and they tend to spend more time outdoors in summer — exactly when ozone is highest.

Long-term exposure is more serious. Repeated exposure to ozone is linked to the development of chronic obstructive pulmonary disease (COPD), a progressive condition that makes it permanently harder to breathe. Studies across the U.S. and globally have found that higher ozone levels are associated with increased risk of premature death, even at levels below the current federal standard.

What's the Standard?

Under the Clean Air Act, EPA sets a legal limit for ozone called the National Ambient Air Quality Standard (NAAQS). The current standard, set in 2015, is 0.070 parts per million, measured as the fourth-highest daily maximum 8-hour average concentration, averaged over three years.

That's a mouthful. Here's what it means in practice: EPA doesn't expect the air to be perfect every single day. An occasional spike — from a wildfire, an unusual heat wave — is tolerated. But the pattern has to be below the limit. If an area's monitors consistently show readings above 0.070 ppm, the area is designated "nonattainment," and the state has to create a plan to bring it into compliance.

How Do You Know If Your Air Is Bad?

EPA and state agencies operate ozone monitors across the country. You can check real-time air quality at AirNow.gov, which translates the raw data into the Air Quality Index (AQI) — a simple 0-500 scale.

Most weather apps and local news stations include the AQI in their forecasts. If you have asthma or a lung condition, check it before spending extended time outdoors in summer.

What's Being Done About It?

Ozone has been declining in much of the U.S. for decades, thanks to the Clean Air Act. The biggest drivers of improvement have been cleaner vehicle emission standards (the Tier 2 and Tier 3 programs), cleaner power plants (through programs like the Cross-State Air Pollution Rule), and tighter controls on industrial VOC emissions.

When an area fails the ozone standard, the state is required to submit a State Implementation Plan (SIP) explaining how it will reduce emissions enough to meet the standard. These plans can include stricter rules on factories, vehicle inspection programs, requirements for cleaner fuels, and controls on consumer products like paints and solvents that release VOCs.

The system works — but slowly. It can take years or even decades for an area to move from nonattainment to attainment. Cleveland just did it after eight years of reductions from cleaner cars, a coal plant closure, and tighter state rules. It's possible — but it requires sustained effort at every level of government.

It Harms Plants and Crops, Too

Ozone doesn't just affect people. It damages vegetation by interfering with plants' ability to produce and store food through photosynthesis. Over time, this weakens plants and makes them more vulnerable to disease, pests, and drought. Agricultural research suggests ozone at current U.S. levels can reduce soybean yields by 15-20% and affects other major crops as well. Forests, parks, and wilderness areas are also harmed — especially during the growing season when ozone levels tend to be highest.

Sources: EPA, Ground-Level Ozone Basics; EPA, Health Effects of Ozone Pollution; American Lung Association, Ozone; State of Global Air, Health Impacts of Ozone (2019 data); 80 FR 65292 (October 26, 2015), 2015 Ozone NAAQS. Have a correction? Contact us.