Air refers to the Earth's atmosphere. Air is a mixture of many gases and dust particles. It is the clear gas in which living things live and breathe. It has an indefinite shape and volume. It has mass and weight, because it is matter. The weight of air creates atmospheric pressure. There is no air in outer space.
Animals live and need to breathe the oxygen in the air. In breathing, the lungs put oxygen into the blood, and send back carbon dioxide to the air. Plants need the carbon dioxide in the air to live. They give off the oxygen that we breathe. Without it we die of asphyxia.
Air can be polluted by some gases (such as carbon monoxide, hydrocarbons, and nitrogen oxides), smoke, and ash. This air pollution causes various problems including smog, acid rain and global warming. It can damage people's health and the environment.
Since early times, air has been used to create technology. Ships moved with sails and windmills used the mechanical motion of air. Aircraft use propellers to move air over a wing, which allows them to fly. Pneumatics use air pressure to move things. Since the late 1900s, air power is also used to generate electricity.
Air is invisible: it cannot be seen by the eye, though a shimmering in hot air can be seen.
Air is one of the four classical elements in Greek theory. It was considered an intervening element, somewhere between fire and water, and the driving force for the birth of the cosmos.
- "composition of air".
- Zimmer, Carl (3 October 2013). Earth’s Oxygen: a mystery easy to take for granted. New York Times.