Ozone is a colorless gas that exists naturally in the atmosphere.
Ground level ozone is formed by a chemical reaction between volatile organic compounds and oxides of nitrogen in the presence of sunlight. According to Schaefer, sources of ground level ozone include auto, boat engine and lawnmower exhausts, industrial emissions and vapors released by some household products. From May through September, when the weather is hot and sunny with little or no wind, ozone levels can become unhealthy. High concentrations of ground level ozone can cause respiratory inflammation and distress, worsen asthma and increase susceptibility to lung infections. Its unhealthy effects can continue for days after exposure to high levels.