A Few Words About Fluids
The basic idea of an aerosol can is very simple: One fluid stored under high pressure is used to to propel another fluid out of a can. To understand how this works, you need to know a little about fluids and fluid pressure.
Aerosol cans come in all of shapes and sizes, housing all kinds of materials, but they all work on the same basic concept: One high-pressure fluid expands to force another fluid through a nozzle.


A fluid is any substance made up of free-flowing particles. This includes substances in a liquid state, such as the water from a faucet, as well as substances in a gaseous state, such as the air in the atmosphere.

The particles in a liquid are loosely bound together, but they move about with relative freedom. Since the particles are bound together, a liquid at a constant temperature has a fixed volume.

If you apply enough energy to a liquid (by heating it), the particles will vibrate so much that they break free of the forces that bind them together. The liquid changes into a gas, a fluid in which the particles can move about independently. This is the boiling process, and the temperature at which it occurs is referred to as a substance's boiling point. Different substances have different boiling points: For example, it takes a greater amount of heat to change water from a liquid into a gas than it does to change alcohol from liquid to gas.

The force of individual moving particles in a gas can add up to considerable pressure. Since the particles aren't bound together, a gas doesn't have a set volume like a liquid: The particles will keep pushing outward. In this way, a gas expands to fill any open space.

As the gas expands, its pressure decreases, since there are fewer particles in any given area to collide with anything. A gas applies much greater pressure when it is compressed into a relatively small space because there are many more particles moving around in a given area.


Why are aerosol cans bad for the environment?

The propellant used to force the aerosol out of the can, was a CFC, which damaged the ozone layer. They now use liquid petroleum gas, which still contributes to global warming, but is less of an issue than CFCs.

Accodring to the Consumer Aerosol Products Council:

American Aerosol Industry Reacts
As a result of Rowland and Molina’s discovery, American aerosol manufacturers took the lead in switching from CFC propellants to suitable alternatives. In fact, by 1978 when the U.S. EPA banned the use of CFC propellants, most of them had already voluntarily stopped using CFCs. An exception was made for some asthma inhalers, but they will also be CFC-free by December 31, 2008. This will complete the phase out of all CFC propellants in consumer aerosol products produced and sold in the U.S. Other ozone-depleting substances that were used in some aerosol formulations for non-propellant purposes also have been phased out according to the legislative timetable.