U.S. Environmental Protection AgencyOffice of Air Quality, Planning and Standards
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Clean and Dirty Air Clean Air and Dirty Air
On a clear breezy day, the air smells fresh and clean. Clean air is air that has no pollutants (dirt and chemicals) in it. Clean air is good for people to breathe.
On a hot day with no wind, the air can feel heavy and have a bad smell. Once in a while, the air can even make your chest feel tight, or make you cough. Dirt and chemicals that get into the air make the air dirty or polluted. Dirty air is not good for people to breathe.

Dirty Air Can Make You Sick

When the air has some dust, soot or chemicals floating in it, healthy people who are inside probably won't notice it. People who are outside might notice it.

People with asthma, a disease that can make it hard to breathe, and children who play outside a lot might feel a little funny. When you are active outdoors, for example, when you run and jump a lot, you breathe faster and take in more air. Any pollutants in the air go into your lungs.

When the air is very dirty, almost everyone will notice it. It would be good if we could stop breathing on those days, but of course we can't!

How Can I Tell if the Air is Clean or Dirty?

Have you ever been stopped behind a truck or a bus at a traffic light? When they start up, sometimes a puff of dark smoke comes out of their exhaust pipe. Image of car

At times like that you can see dirty air - it looks hazy and brownish. If your window is open, you might be able to smell the pollution. But sometimes the air can be dirty and you can't see it or smell it. So you need another way to tell if the air is dirty.

The Environmental Protection Agency

The environment is everything around you - the air, the land, and the rivers and oceans. The Environmental Protection Agency (EPA) is a government office that works to keep the air, the land, and the water clean. Clean air, land, and water helps keep us healthy. The EPA takes samples of the air at more than 1000 places in the United States to see if the air is dirty or clean.


Five pollutants are used by the EPA to determine the AQI index. Ozone is described in this page. Information on the other pollutants is "coming soon"!

  • Ground-level ozone
  • Particulate matter
  • Carbon monoxide
  • Sulfur dioxide
  • Nitrogen dioxide


You can't see or smell ozone in the air. Ozone can be good or bad. It all depends on where it is. Ozone is good when it is high up in our atmosphere. It protects us from sunburn. Ozone is bad when it is near the ground where we can breathe it in. It is formed when chemicals coming out of cars and factories are cooked by the hot sun. Ozone is more of a problem in the summer.
See how ozone is formed!
See how ozone is formed
(Flash plug-in required)
When you breathe in ground-level ozone, it can make the lining of your lungs red and swollen just like the sun can burn your skin. Breathing in ozone can make you cough. It can also make it harder for you to breathe. Ozone might even make it hurt to take a breath of air.

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