An aerosol spray is a canister holding a liquid under pressure from a pressurised vapour in equilibrium with a liquid which often also dissolves the payload (see propellant, below). When a valve is opened, the liquid is forced out of a small hole and emerges as an aerosol, or mist. As gas expands to drive out the payload, some propellant evaporates inside the can to maintain an even pressure. Outside the can, the droplets of propellant evaporate rapidly, leaving the payload suspended as very fine particles or droplets. Typical liquids dispensed in this way are insecticides, deodorants and paints. An atomiser is a similar device that is pressurised by a hand-operated pump rather than by stored gas.
The modern aerosol spray can was invented in 1926 by Erik Rotheim, a Norwegian engineer. Yet only in 1941 the Americans Lyle Goodhue and William Sullivan turned it into a useful device, used by military to spray the malaria mosquito during World War II.
If the can was simply filled with compressed gas, either it would need to be at a dangerously high pressure, or the amount of gas in the can would be small, and it would soon run out. Hence usually the gas is the vapour of a liquid with boiling point slightly lower than room temperature. This means that inside the pressurised can, the vapour can exist in equilibrium with its bulk liquid at a pressure that is higher than atmospheric pressure (and thus able to expel the payload), but not dangerously high; yet, as gas escapes, it is immediately replaced by more liquid evaporating. Since the propellant exists in liquid form in the can, it is desirable that it also be miscible with the payload, or dissolve it.
Chlorofluorocarbons (CFCs) were once often used, but since the Montreal Protocol entered into force in 1989 they have been replaced in nearly every country on Earth due to the negative effects CFCs have on Earth's atmospheric ozone layer. The most common replacements are mixtures of volatile hydrocarbons, typically propane, n-butane and isobutane. Dimethyl ether (DME) and methylethyl ether are also used. All these have the principle disadvantage of being quite flammable. Nitrous oxide is also used as a propellant to deliver foodstuffs (e.g. whipped cream). Medicinal aerosols such as asthma inhalers use hydrofluoroalkanes (HFA): either HFA 134a (1,1,1,2,-tetrafluoroethane) or HFA 227 (1,1,1,2,3,3,3-heptafluoropropane) or a combination of the two.