There are five major components of air: nitrogen (78.0842%), oxygen (20.9463%), water vapor (about 1%), argon (0.93422%), and carbon dioxide (0.03811%). Trace components make up another 0.002%. Out of all these substances, the one that animals (including humans) need to survive is oxygen, while plants require carbon dioxide and nitrogen.
The respiration of animals consumes oxygen and produces carbon dioxide as a byproduct, while plants consume carbon dioxide and produce oxygen. The world's ecosystems depend on this balance. The components of air may be altered by human activity, such as the burning of fossil fuels, which has increased the proportion of carbon dioxide in the atmosphere.
Air has been a subject of study for scientists for hundreds of years. Like other gases, air behaves according to Boyle's Law, which states that the pressure and volume of a gas are inversely proportional in a closed system where the total quantity of gas and its temperature remain fixed. This means that a person can decrease the volume of air by compressing it, but its pressure will increase proportionally.
It is possible to pump air into an elastic membrane, like a balloon, to inflate it. Because the pressure exerted outwards by the air inside a balloon is roughly equivalent to the pressure exerted on the balloon by the outside, it remains inflated. This only holds true when the air pressure of the air originally put in the balloon is similar to the ambient air around it, however. If a balloon is filled with air from the upper atmosphere, then brought down to sea level, it will shrink, while if it filled with air from sea level and brought to a very high altitude, it will pop. This is what happens to balloons that are accidentally released into the sky.
