Ozone (O₃). is a gas that forms in the atmosphere when three atoms of oxygen are combined. It is not emitted directly into the air, but is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Ozone has the same chemical structure whether it occurs high above the earth at stratosphere or at ground level and can be beneficial or harmful, depending on its location . Ozone occurs in two layers of the atmosphere. The layer surrounding the earth's surface is the troposphere. Here the ground level or harmful ozone is an air pollutant that damages human health. It is a key ingredient of urban smog. The stratospheric or beneficial ozone layer extends upward from about 10 to 30 miles and protects life on earth from sun's harmful ultraviolet rays (UV-b).

Motor vehicle exhaust and industrial emissions, gasoline vapours and chemical solvents are some of the major sources of NOx and VOC, also known as ozone precursors. Strong sunlight and hot weather cause ground level ozone to form harmful concentrations in the air. Many urban areas are subject to high ozone levels, as winds carry NOx emissions hundreds of miles away from their original sources. Ozone concentration can vary from year to year. Changing weather patterns (especially the number of hot, sunny days), periods of air stagnation, and other factors contribute to ozone formation .

Health Effects

Repeated exposure to ozone pollution (Table 14) may cause permanent damage to the lungs. Even when ozone is present in low levels, inhaling it triggers a variety of health problems including chest pain, coughing, nausea, throat irritation and congestion. It also worsens bronchitis, heart disease, emphysema, asthma and reduces lung capacity. Ozone can irritate respiratory system, causing coughing, feel an irritation in throat and/or experience an uncomfortable sensation in chest. Ozone can reduce lung function and make it more difficult to breath as deeply and vigorously as one normally would. When it happens, then breathing becomes uncomfortable. Ozone can aggravate asthma, when ambient ozone levels are high. More people with asthma have attacks that require a doctor's attention or the use of additional medication. One reason this happens is that ozone makes people more sensitive to allergens, which are the most common triggers for asthma attacks. Also, asthmatics are more severely affected by the reduced lung function and irritation in the respiratory system. Ozone can inflame and damage lung cells. Within few days of ozone exposure the damaged cells are replaced and the old cells are shed. Ozone may aggravate chronic lung diseases such as emphysema and bronchitis and reduce the immune system's ability to fight off bacterial infections in the respiratory system.

Table 14 Pulmonary Function Response to Various Ozone Exposures

FEV1- Forced Expiratory volume in one second

Ozone may cause permanent lung damage. Repeated short-term ozone damage to children's developing lungs may lead to reduced lung function in adulthood. In adults, ozone exposure may accelerate the natural decline in lung function that occurs as part of the normal aging process. Healthy people also experience difficulty in breathing when exposed to ozone pollution.