Propellants can be any one gas, or mixture of gases, that are compressed or liquefied. All aerosol products use propellants to provide the pressure that dispenses the contents of the container. Some containers utilize barriers, such as a bag or piston, to separate the propellant from the product that is dispensed. Most aerosols, however, require the propellant to interact with the other ingredients to deliver the unique and desirable characteristics that cannot be achieved with other packaging types.