Science

Introduction
The contamination of the environment—land, water, and air—by waste, smoke, chemicals, and other harmful substances is called pollution. The most serious pollution occurs in areas with large cities and many factories.
Pollution is not a new problem. Cities of ancient times were often fouled by human wastes and debris. In the Middle Ages unsanitary conditions encouraged the spread of diseases such as the plague. Much has been done to improve sanitation and public health over the centuries. But since the Industrial Revolution, the problems of waste disposal have become more complicated. The growth of industry, the introduction of new technologies such as motor vehicles, and rapid increases in human populations have combined to create pollution problems on a level never before seen.
 
Air pollution
Clean air is essential to a healthy environment. Air is considered to be polluted when it contains certain substances in amounts high enough and for periods long enough to cause harm. Air can be polluted through such natural causes as volcanic eruptions and forest fires, which send smoke, ash, and gases into the atmosphere. These types of pollution may have not only local and regional effects but also long-lasting global ones. Nevertheless, only pollution caused by human activities, such as industry and transportation, can be controlled.
Most air pollution comes from the burning of substances called fossil fuels, such as coal and gasoline. Factories and automobiles burn these fuels for power, but they do not burn them completely. The unburned particles from the fuels include solids such as soot and ash as well as gases such as carbon monoxide and ozone. In many places smoke from factories and cars combines with naturally occurring fog to form smog. London, Los Angeles, Tokyo, and Mexico City are among the cities that have faced serious smog problems over the years.
Air pollution may affect humans directly, causing diseases such as cancer, bronchitis,...