Liquefied gases are widely used as propellants for most of the aerosol products. These are materials that exist in the gaseous or vapor state at room temperature and atmospheric pressure and are capable of being liquefied easily by lowering the temperature below its boiling point or by increasing the pressure. The compounds chosen generally have boiling points below 70°F (21°C) with vapor pressure exerting between 14 and 85 psig at 70°F (21°C). They are relatively inert and nontoxic.

Two types of liquefied gases used are the chlorofluorocarbons (CFCs) and hydrofluorocarbons. Pharmaceutical aerosols were originally developed using CFCs 11, 12 and 114. Unfortunately, these propellants have an ozone depleting effect and the current global regulations require pharmaceutical aerosols to be formulated using non-ozone depleting propellants. Their uses in pharmaceutical aerosols have been practically eliminated except in MDIs. The alternatives to CFC propellants should meet certain criteria such as they should be nontoxic and non-flammable.