Aerosols are defined as pressurized package disperse phase systems in which solid particles or liquid droplets constitute the disperse phase and gas (propellant) the continuous phase. Compressed gases, such as carbon dioxide and nitrogen, and liquefied gases such as halogen derivative of some saturated hydrocarbons (methane, ethane, propane, etc.) can be used as propellants. The material to be propelled may be solid or liquid. Aerosol systems are classified into two-phase systems and three-phase systems and are meant for local or systemic action.