Common pollutants made by humans come from burning the carbon in coal, oil or gas for power or heat. Oxides - combinations of oxygen with other atoms - are formed in burning ANYTHING that causes bad effects when in the air can be called a pollutant. Some gases people think of as pollution are natural; others are man-made, and humans sometimes put more of a natural pollutant into the air. The Earth's air is a mixture of gases. The most important is oxygen, on which life itself depends; about one-fifth of the air is oxygen. The most common is nitrogen, nearly four-fifths of the Earth's air. Some plants can make proteins from the nitrogen, but mostly it doesn't do anything except take up space. Other kinds of gases make up about 1 one-hundredth of the air. Hot springs and volcanoes often put smelly compounds containing sulphur into the air. A mixture of gases and dust from the 1991 eruption of volcano Mt Pinatubo in the Philippines seems to have cooled the Earth by blocking some heat from the sun. Common pollutants made by humans come from burning the carbon in coal, oil or gas for power or heat. Oxides - combinations of oxygen with other atoms - are formed in burning. One oxide of carbon, carbon monoxide, is poisonous. The other common oxide of carbon, carbon dioxide, is likely to make the Earth warmer if enough of it gets into the air. Oxides formed from the sulphur and nitrogen usually found in fuels cause acid rain. Unburned bits of carbon - sometimes in solid pieces called soot - help form the smog you can see in many large cities. Pollution can eat into building materials and destroy them. Acid rain does a lot of this damage. Acids are chemical compounds. A mild acid is lemon juice. Acid rain is created when chemicals such as sulphur dioxide are in the air; then, rain comes down as sulphuric acid. Temples built in ancient Greece lasted for more than 2,500 years, until acid rain began to eat into the marble stone. Vehicle fumes also can dirty older buildings made of stone. The stone is very hard but porous, which means full of holes and like a sponge. The stone soaks up the dirt. Most water pollution by animals is from their own body waste. That's true of birds, for example, such as Canada geese. But usually that pollution, or dirt, is easy for water to break down. It's a problem when there's too much goose manure in one area, which changes the life and plants in the water. That usually means humans are responsible. For example, people may plant grain that would draw more geese than usual to a particular place. There are other ways in which water becomes dirty: In oceans, squid shoot off ink as a way to defend themselves. On land, beavers sometimes put rubbish in the water when making a beaver dam, which changes the water flow, too. It's generally considered pollution when any of these natural materials is present in such a big quantity that it hurts the quality of the water. When pollution is caused by man, for instance, in an oil spill like the Exxon one, many animals and especially birds get killed when oil gets on them. When a bird is covered with oil, the feathers get matted down and cannot trap air to keep the bird warm. Like the down coats many people wear in winter, downy feathers help keep birds warm by trapping air around their bodies and keeping their skin dry. When the feathers get all stuck together, water is able to seep through to the bird's skin, making the bird even colder. Fluffy feathers also help birds float, swim and fly. With oily feathers, they may drown or starve because they cannot move around to get food. Or they may get killed by other animals because they cannot get away fast enough. When a bird gets dirty, it tries to clean its feathers with its beak. In the process, a bird that is covered with oil may eat the oil, get sick and die.