Aerosols are also called "Pressurized dosage forms', and can be defined as dispersed phase system containing very fine solid particles or liquid droplets as a dispersed phase and gas as dispersion medium or continuous phase. The aerosols upon actuation emit a fine dispersion of liquid and / or solid material in gaseous medium.
Advantages of Aerosols:
- The required quantity of medicament can easily be withdrawn from the package without contamination and deliver directly to the affected part of the body such as wound and burnt skin.
- The product is well protected by any physical or chemical degradation.
- The sterility of the product is maintained up to its shelf-life.
- The application of the topical preparation is uniform and thin, without touching the affected area. This reduces the irritation caused by mechanical (finger-tip) application.
- The medicament can be given by oral inhalation hence suitable dosage form for bronchial diseases.
- A fine mist of inhalation directly goes to the bronchi avoiding GIT. Therefore, no chance of metabolism of medicament in GIT.
- The rapid volatilization of the propellant provides a cooling refreshing effect.
Disadvantages of Aerosols:
- Aerosols are costly systems.
- Most of the propellants are toxic in nature. They may be harmful to body and also pollute the nature.
- A person may feel irritation on the injured skin due to cooing effect of highly volatile propellants.
- The formulation on aerosols is difficult to design especially when drug is not soluble in propellant.
Aerosol consists of two parts, the product concentrate and the propellant. The product concentrates contain many additives like antioxidant, surfactants and solvents, to prepare a stable and effective product. The propellant is usually a liquefied gas or a mixture of liquefied gases which may serve both the purpose of propellant and the solvent or vehicle for the product concentrate. Sometimes, non-liquefied compressed gases such as carbon dioxide, nitrogen, and nitrous oxide etc., may be employed as the propellant When propellant is sealed within in aerosol container with the product concentrate, an equilibrium is quickly established between that portion of propellant which remains liquefied and that which vaporizes and occupies the upper portion of the aerosol container.
The vapour pressure exerts pressure in all directions like walls of the container, the valve assembly, and the surface of the liquid phase, which is composed of the liquefied gas and the product concentrate. the pressure upon actuation of the aerosol valve forces the liquid phase in the dip tube to the atmosphere through the orifice of the valve. As the propellant meets the air, it immediately evaporates due to the drop in the pressure, leaving the product concentrate as air borne liquid droplets or dry particles, depending upon the formulation. As the liquid phase is removed from the container, the equilibrium is again established between propellants liquetled and vapour state. Thus, the pressure is maintained constantly during the expulsion of the product and hence the product may continuously be released at the constant rate and pressure.