systems in which they live. Some chemical pollutants return to earth in the form of acid rain and snow. This acid rain will corrode statues and buildings, damage crops, forests and make the seas unsuitable for fish and other plant and animal life. The chemical pollution is also changing the earth's atmosphere in a way that it lets in more harmful radiation from the sun. At the same time, our polluted atmosphere is becoming a better insulator, preventing heat from escaping back into space and thus leading to a rise in global average temperatures. Scientists predict that the temperature increase, referred to as global warming, will affect world food supply, alter sea level, make weather more extreme and increase the spread of tropical disease. Most chemical pollution comes from one human activity: burning fossil fuels, natural gas, coal, and oil to power industrial processes and motor vehicles. Among the harmful chemical compounds this burning puts into the atmosphere are carbon dioxide, carbon monoxide, nitrogen oxides, sulphur dioxide, and tiny solid particles including lead from gasoline additives called particulates. Between 1900 and 1970, motor vehicle use rapidly expanded, and emissions of nitrogen oxides, some of the most damaging pollutants in vehicle exhaust, increased 690 percent. When fuels are incompletely burned, various chemicals called volatile organic chemicals also enter the air. Pollutants came also from other sources. For instance, decomposing garbage in landfills and solid waste disposal sites witch emits methane gas and many household products give off explosive organic chemicals. Some of these pollutants also come from natural sources. For example, forest fires emit particulates and explosive organic chemicals into the atmosphere. Ultra fine dust particles, dislodged by soil erosion when water and weather loosen layers of soil, increase airborne particulate levels. Volcanoes spew out sulphur dioxide and large amounts...
