Canned air comes in a pressurized canister used to clean sensitive electronic equipment like stereos and computers. The contents of the canister are under such extreme pressure that they have become liquid, but a narrow nozzle directs the released gas into a strong stream of air. The air blows out dust and debris from tiny places like the spaces between keys on a keyboard. Since canned air has no moisture, added chemicals, or foreign particles, it is a safe and efficient method of cleaning such devices.
Computer technicians, photographers, and stereo enthusiasts have been using canned air for years to clear out accumulated dust from delicate areas. For instance, the insides of computers, especially around the fan, can build up so much dust that it begins to interfere with the cooling or functioning of circuitry. Particles cling to surfaces like lenses, film negatives, and enlargers but you can't use a cloth or feather-duster lest you scratch them. Electrical components can't be sprayed with anti-dust chemicals because they might damage the circuit or build up excess static electricity.
Canned air has been specifically designed to clean places that couldn't be cleaned with a vacuum, dust cloth, or ammonia-based spray cleaner. Its contents contain absolutely no moisture, so it isn't dampening your printer, scanner, camera, eyeglasses, keyboard, monitor, or hard drive as it cleans. While it's true that the air only sprays the dust back into the air, where it may resettle, it has an advantage over vacuums since it doesn't create a static charge.
You might be worried that if the atmosphere gets more polluted, we'll all be paying to live off of canned air. However, this useful product isn't like the air we breathe at all. It used to contain chlorofluorocarbons like many pressurized canisters, such as hairspray. After those gases were shown to damage the ozone in the atmosphere, most companies eliminated them from the cans. Now, the mixture is often made up of a lot of nitrogen, along with other harmless gases heavier than air, so that they'll be labeled "ozone safe."
The non-refillable containers look like small cans of oil lubricant. The contents are delivered through a long, cylindrical nozzle when you depress a valve with your index finger. The contents are nonflammable. It is possible that enough gas could displace oxygen in one area and cause lightheadedness, so make sure you work with adequate circulation. The canisters are no longer allowed on airplanes.
Another interesting aspect of canned air is its adiabatic cooling property. When gas is under such high pressure, around 70 psi, a release of pressure causes a notable drop in temperature. You may notice that the surrounding air and the canister itself gets cold during use. The rapid expansion of gas that previously occupied a small space into a large space means that it absorbs a lot of heat from the area as it spreads out. For this reason, do not ever spray canned air onto your skin, as it can quickly cause frostbite.