What is ozone?
Ozone is an invisible gas made of three oxygen atoms (O3). Ozone is often referred to as smog. Ozone forms when two groups of gases have a chemical reaction in the air triggered by sunlight and heat. The two groups of gases—hydrocarbon vapors and nitrogen oxides—come from many sources around us. [1]

Why is ozone harmful?
Ozone reacts chemically ("oxidizes") with internal body tissues, such as those in the lung. Think of it as a "sunburn" on the lungs. Ozone irritates and inflames the respiratory system at levels frequently found across the nation during the summer months. Breathing ozone may lead to:

• shortness of breath, chest pain
• inflammation of the lung lining, wheezing and coughing
• increased risk of asthma attacks, need for medical treatment and for hospitalization for people with lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD) [1]
• premature death [2]

Who are most at risk?
People at greatest risk include:

• people with lung disease, especially chronic lung diseases such as asthma and COPD [4]
• children, because their airways are smaller, their respiratory defenses are not fully developed, and their higher breathing rates increase their exposure [5]
• people who work or exercise outdoors [6]
• senior citizens [6]
• "responders"--otherwise healthy individuals who experience health effects at lower levels of exposure than the average person.

Where does ozone come from?
The two gases that react to form ozone come from many different sources. Because ozone forms in the air, it often shows up downwind from the sources of the gases that create it.

• Hydrocarbon vapors, or Volatile Organic Compounds (VOCs), come from cars, trucks, buses and other motor vehicles, small engines, chemical plants, refineries, factories, gas stations, paint and other sources
• Nitrogen oxides come from sources that burn fossil fuels such as power plants, industrial boilers, motor vehicles, locomotives, and ships. 

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