Pollution happens when the environment is contaminated, or dirtied, by waste, chemicals, and other harmful substances. There are three main forms of pollution: air, water, and land.
Wildfires, volcanoes, and industrial chemicals cause some air pollution. But most air pollution comes from burning fossil fuels. These include coal, oil, and natural gas. Factories, electrical plants, and automobiles burn these fuels for power. The burning of fossil fuels may release solid particles, such as ash and soot, into the air. It also may release harmful gases. This type of pollution may be seen in the form of smog over big cities. Smog-filled air is hazy, or hard to see through.
Air pollution may cause such diseases as cancer and asthma. Air pollution also leads to acid rain, or polluted rain that can harm living things. In addition, air pollution may be a cause of global warming, which is a steady rise in Earth’s average temperature. Finally, air pollution damages a part of the atmosphere called the ozone layer. The ozone layer is important because it protects Earth against harmful rays from the Sun.
Some causes 
of water pollution are easy to see. People dump garbage and sewage into creeks, rivers, ponds, lakes, and oceans. Factories or cities sometimes release oils, poisonous chemicals, and other wastes into water.
Other causes of water pollution are not so direct. The use of chemicals in farming is one example. Farmers use some chemicals as fertilizers, or substances added to soil to help crops grow. They use other chemicals as pesticides or herbicides. These are substances that kill pests or weeds. When these chemicals seep into the ground, they may make the groundwater unfit to drink. Chemicals also may drain into rivers and lakes, where they can harm fish and other forms of life.
Littering, or tossing garbage on the ground, is a form of land pollution. Litter is unpleasant to look at. It also can destroy the habitats, or homes, of plants and animals.
The buildup of dangerous chemicals in the ground is another form of land pollution. The chemicals may come from farms or factories. These chemicals can spread to plants and animals. They may even harm people who eat the contaminated plants and animals.
Many governments, environmental groups, and ordinary people are working to control pollution. Governments have passed laws to keep people from releasing dangerous chemicals into the environment. Some companies and people are trying to use fewer fossil fuels. Instead they are getting power from sunlight, wind, water, and other energy sources that produce less pollution. Many communities use recycling to reduce pollution. Recycling is the process of making new products out of used paper, plastic, glass, aluminum, or other materials.
