1 . Pollution

Pollution happens when the environment is dirtied, by waste, chemicals, and other harmful substances(物质).Pollution is a problem all over the world. But it is especially bad in large cities with a lot of industries and cars.    1    

Wildfires, volcanoes, and industrial chemicals cause some air pollution. But most air pollution comes from burning fossil fuels (矿物燃料）These include coal, oil and natural gas. The burning of fossil fuels may release harmful gases. Air pollution may cause such diseases cancer and asthma. It also leads to polluted rain that can harm living things    2     Finally, air pollution damages the ozone layer(臭氧层)which is important because it protects Earth against harmful rays from the sun.

Causes of water pollution are easy to see. People dump(倾倒) garbage and dirty water into river, lakes and oceans. Factories or cities sometimes release poisonous chemicals, and other wastes into water. These chemicals may make the groundwater unfit to drink.    3    

Littering, or throwing garbage on the ground, is a form of land pollution. Litter can destroy the habitats 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.    4     They may even harm people who eat the polluted 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     5     Instead they are getting power from the sun, wind, water, and other energy sources to produce less pollution. Many communities use recycling to reduce pollution.