At the earth's surface, ozone is harmful to crops, forests, building materials, and the health of humans and animals. Warm and sunny days with little wind and no rainfall contribute to ozone formation. When these conditions occur over several days, ozone can continue to build up to unhealthy levels, especially in cities. Ozone can be blown long distances downwind of urban areas. Ozone levels are usually higher in the afternoon and early evening after traffic and industry have released large amounts of ozone-forming materials into the air.
Ozone can be a respiratory irritant at levels often found in many cities during the summer months.
It not only affects people with existing breathing problems, but also can affect healthy children and adults. Persons especially sensitive to ozone exposure are the elderly, infants and children, asthmatics, chronic respiratory patients, pregnant women, smokers, persons with a history of chest discomfort or respiratory allergies, and persons with lung cancer, cardiovascular disease or immune system deficiency. At very low ozone levels, sensitive persons may experience irritation of the eyes, nose, sinuses, throat, and lungs resulting in watery eyes, runny nose, and coughing. At higher ozone levels, persons may experience chest tightness, shortness of breath, pain when breathing deeply, and wheezing. Other less common symptoms are blurred vision, headache, nausea, vomiting, and fatigue.