Air and oxygen are sometimes used interchangeably, but the two things are actually different. Oxygen is a pure element, while the air people breathe is a mixture of elements. Although oxygen is necessary for survival, repeated exposure to the pure form could be harmful. The air that makes up Earth's atmosphere has a healthy balance of elements to sustain human life.
Oxygen is a pure gaseous element. Prolonged exposure to it can actually cause brain damage, although the element is sometimes used therapeutically. Most animals require a certain concentration of oxygen in their blood in order to thrive, and for this reason, the gas is used in emergency situations in which access to air is restricted. When used therapeutically, oxygen is usually mixed with other elements.
Oxygen is number eight on the periodic table, which means that it has eight electrons and eight protons. Only a handful of elements, including helium and hydrogen, are lighter. It has a cubic structure and is a totally colorless element.
The air that people breathe is a mixture that contains primarily oxygen and nitrogen. The nitrogen generally makes up approximately 78%, while oxygen makes up 21%. Other gases, including carbon dioxide, hydrogen, helium, argon, and neon, make up the remainder. These gases dissolve and are carried in the blood.
Anyone who has experienced radical pressure changes is familiar with the effects of dissolved nitrogen in the blood. While this element is normally carried in the blood without any ill effects, extreme depressurization can cause it to form gaseous bubbles. This causes extreme pain and can lead to death from a condition known to divers as the bends.
The term "air" is also used to refer to the Earth’s atmosphere as a whole, which makes life on Earth possible. Air forms the lower part of the atmosphere, and becomes thinner the further the observer is away from sea level. Thin air refers to that which is less dense than that at sea level. On Earth, people at high elevations experience thin air, which can cause shortness of breath and nausea. It has less oxygen, which means that the body must work harder and breathe more quickly to extract the necessary amount of oxygen.
Most creatures tend to take air for granted, because it is an abundant substance. However, increasing pollution has led to a decline in air quality, which has resulted in health problems for some animals. Air which is heavily contaminated with particles in high concentrations can damage lung tissue. Low quality can sometimes be physically seen in the form of smog, a heavy collection of hydrocarbons in the atmosphere.