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The reactivity of ozone causes health problems because it damages lung tissue, reduces lung function, and sensitizes the lungs to other irritants. Ozone has long been known to increase the incidence of asthma attacks in asthmatics.  The general mechanism is fairly simple, though the exact details of how ozone or any other pollutant causes these problems are considerably more complex.

As air enters the lungs, it passes through the trachea and then into passages called bronchi that divide into smaller and more numerous passageways. Farther down in the lung, the passages only split, and do not become smaller. At the ends of the last bronchi are tiny sacs called alveoli, which increase in volume when a breath is taken. The alveoli exchange incoming oxygen for outgoing carbon dioxide, so they are home to the most crucial part of the breathing process.  If the alveoli cannot get fresh air to replace the carbon dioxide, you cannot breathe. The bronchi that lead to those sacs are very narrow. Here, ozone and other lung irritants have their worst effects.

Ozone and fine particles irritate the lining of these small passageways, making them swell and secrete extra mucus.  The swelling narrows the tiny passageway, letting very little air through to the alveoli, and dramatically reducing the lung’s capacity. The lung’s capacity drops further in asthmatics when the muscles surrounding the bronchi contract and irritated tissues respond by producing more mucus. Muscle spasms and excess mucus further narrow the bronchial passages, making it still more difficult to breathe.

Ozone exposure also makes the lungs more vulnerable to lung diseases such as pneumonia and bronchitis. Ozone not only affects people with impaired respiratory systems, such as asthmatics, but healthy adults and children as well.  Exposure to ozone for several hours at relatively low concentrations significantly reduces lung function and induces respiratory inflammation in normal, healthy people during exercise. This decrease in lung function is generally accompanied by symptoms such as chest pain, coughing, sneezing, and pulmonary congestion. Recent research in Southern California strongly suggests that, in addition to exacerbating existing asthma, ozone also cause asthma in children. Longer-term exposure to ozone can also lead to scarring of the lung tissue and permanent reductions in lung capacity.