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Smog is a form of air pollution.. Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfer dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions.

In the 1950s a new type of smog, known as photochemical smog, was first described. This forms when sunlight hits various pollutants in the air and forms a mix of inimical chemicals that can be very dangerous.

Smog is a problem in many cities and continues to harm human health. Ground-level ozone, sulfer dioxide, nitrogent dioxide and carbon monoxide are especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. Smog can inflame breathing passages, decreasing the lungs' working capacity, and causing shortness of breath, pain when inhaling deeply, wheezing, and coughing. Smog can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness.