Ozone is a gas that you cannot see or smell. Ozone occurs naturally in the sky about 10 to 30 miles above the Earth’s surface. Sometimes, this ozone is called “good ozone” because it forms a layer that protects life on earth from the sun’s harmful rays. Ground-level ozone can be harmful for your health and the environment.
Ground-level ozone forms when pollutants from cars and trucks, power plants, factories and other sources come in contact with each other in heat and sunlight. Factors such as weather conditions and intensity of sunlight also play a part in how ozone is formed. Ground-level ozone is one of the biggest parts of smog, and it is usually worse in the summer months.
Many urban areas tend to have higher levels of ground-level ozone but rural areas have ground-level ozone, too. Wind carries ozone and the pollutants that form it hundreds of miles from their original sources, and rural areas have sources of ozone that contribute to this problem.
Particle pollution, or particulate matter, consists of particles that are in the air, including dust, dirt, soot and smoke, and little drops of liquid. Some particles, such as soot or smoke, are large or dark enough to be seen. Other particles are so small that you cannot see them.
Particle pollution can come from primary or secondary sources. A primary source, such as wood stoves or forest fires, lets off particle pollution directly. A secondary source lets off gases that react and form particles. Examples of secondary sources are coal fires and power plants. Particle pollution also comes from motor vehicles, factories, and construction sites. These can be primary or secondary sources. Particle pollution can be a problem at different times of the year, depending on where you live.
Particle pollution includes:
- coarse particles that are between 2.5 and 10 micrometers
- fine particles that are between 0.1 micrometers and 2.5 micrometers (also known as PM2.5)
- ultrafine particles that are smaller than 0.1 micrometers.
Particles bigger than 10 micrometers can irritate your eyes, nose and throat but do not usually reach your lungs. Ten micrometers is about seven times thinner than one human hair.
Fine and ultrafine particles are the size of most concern because they are most likely to cause health problems. Their small size allows them to get into the deep part of your lungs and even into your blood.