All living things need air to breathe. Animals and plants would die without it. Air is also used to inflate tires and balloons, and it is what holds airplanes and kites above the ground. It surrounds the Earth as the atmosphere. Air has no color, taste, or odor. Although air is invisible, people can hear air when the wind howls and can feel air when the wind blows through their hair on a windy day.

What air is made of

Air consists of a mixture of gases. These gases consist of tiny particles called molecules. They are so small that they cannot be seen. One cubic inch of ordinary air may contain about 300 billion billion molecules.

Molecules are combinations of different chemical elements. In air the most common of these elements are nitrogen and oxygen. Oxygen is an important component of air because humans and other animals breathe it. Pure oxygen, however, burns easily. Fire would start every time there was a lightning strike or a spark. Nitrogen, which is more abundant in the air than oxygen, thins out the oxygen and makes it less flammable. Air also contains small amounts of argon, neon, helium, and methane.

Through the process called respiration, humans and other animals take in oxygen and breathe out carbon dioxide. Carbon dioxide is a molecule made of oxygen and carbon. Plants take in carbon dioxide from the air to make food through a process known as photosynthesis. They break down the molecule, use the carbon, and then release the oxygen into the air. In this way the carbon dioxide and oxygen in the air are constantly used and replaced.

Air also contains water vapor, dust, pollen, bacteria, and spores. The water vapor is the source of all precipitation, such as rain and snow.

Compressed air

Although there are billions of molecules in air, there is a lot of space between the molecules. Because of this space, air can be compressed. To compress air, pressure is applied to squeeze the molecules closer together. The molecules in air are moving all the time, even when compressed. When the pressure is released, the molecules expand rapidly and the air has great force. Compressed air is used for paint spraying, for operating small tools such as drills, and in dry cleaning. Railroads use compressed air in air brakes, and bottling plants use it to operate filling machines.

Air pollution

Air pollution is a serious problem in societies with a lot of industry. Automobiles, factories, and chemical plants produce harmful gases and particles that poison the air. Air pollution can lead to health problems such as asthma and cancer. Environmental problems such as acid rain are caused by air pollution as well. Some countries have laws to limit the amount of pollution that can be released into the air. In 1970 the United States passed the Clean Air Act to limit the amount of pollution that can be in the air in the country. The act was updated in 1990.