What are aerosols?

Aerosols aren't aerosols at all. No, really, let's be clear about this. An aerosol is really the cloud of liquid and gas that comes out of an aerosol can, not the can itself. In fact, to be strictly correct about it, an aerosol is a fine mist of liquid or lots of solid particles widely and evenly dispersed throughout a gas. So clouds, fog, and steam from your kettle are all examples of aerosols, because they're made up ofwater droplets dispersed through a much bigger volume of air. Smoke is an aerosol too, though unlike those other examples (which are liquids dispersed in gases) it's made up of solid particles of unburned carbon mixed through a cloud of warm, rising air. Evencandles make aerosols: the smoky steam swirling above a candle flame consists of soot and water vapor dispersed through hot air.

Photo: An aerosol emerging from an aerosol can.

Now you know what an aerosol is, you can see what an aerosol can is all about: it's a mechanism designed to turn a liquid, such as paint or polish, into a finely dispersed mist. Let's have a closer look at how it works.

How do aerosol cans work?

If you've ever read the back of an aerosol can, you'll have noticed messages such as "pressurized container" and "contents stored under pressure". What's that all about? To ensure that something like paint comes out evenly when you press the button on the top of an aerosol can, the manufacturers have to squeeze the contents inside with a pump or compressor (a bit like inflating a bicycle tire). Typically, the contents of an aerosol are stored at 2-8 times normal atmospheric pressure. That's why aerosols really rush out when you press their buttons. It's also why aerosols feel really cold when you spray them on your body. If you let a gas escape from 8 times its normal pressure into the air, it expands enormously and cools down drastically. (Gases cool when you let them expand because the heat energy their atoms or molecules contain is spread over a much bigger volume.)

Photo: Many aerosols contain flammable propellants. Always read and follow the warnings on the back of the can. Don't point aerosols into your face and don't breathe in the spray.

Now we can't easily pressurize liquids, so just pumping something like liquid paint into a can isn't going to make an aerosol that actually works. Fortunately, we can pressurize gases very easily. So, in practice, aerosol cans contain two different substances: the liquid product you're interested in releasing (the paint, detergent, hairspray, or whatever it might be) and a pressurized gas called a propellant that helps to push the liquid product into the air and turn it into an aerosol cloud. The propellant gas usually turns into a liquid when it's forced inside the can at high pressure during manufacturing. That makes the propellant and the product mix together (and you can help to ensure they do so by shaking the can before you use it). The propellant turns back to a gas when you push the nozzle and the pressure is released. It disappears harmlessly into the air leaving behind the product you're really interested in.

Until the 1980s, chlorofluorocarbons (CFCs) were widely used as the propellants in aerosol cans, but they were banned after scientists discovered conclusively that they damaged Earth's ozone layer. (No wonder, really, when you consider that something like 10 billion aerosol cans are used and thrown away each year.) Now other chemicals are used as propellants instead, including the gases propane and butane. Although these gases don't damage the ozone layer, they do have other drawbacks: they can be harmful to inhale and they are highly flammable.

Photo: CFC chemicals used as propellants in aerosols (and also used as coolants in air conditioners and refrigerators) helped to create a huge hole in Earth's protective ozone layer, shown here in a satellite photo taken in 1998. CFCs were banned in most countries by an international agreement called the Montreal Protocol in the 1990s. Picture by NASA Goddard Space Flight Center courtesy ofGreat Images in NASA

Aerosol cans are made in two main ways. Some are fashioned from a thin sheet of steel, wrapped into a cylinder, that has a top and a bottom welded on later. The inside of a steel can is given a special coating to stop it rusting or reacting with the product or the propellant. Other cans are made by pressing a small lump of aluminium through a ring-shaped tool, called a die, so a cylinder forms from a single piece of metal.