A proportion of the active ingredients(s) is usually deposited on the inner surface of the actuator,

pressure in a seated container and are released as a fine mist of spray upon activation of a suitable valve system They are intended either for inhalation for local action in the lungs or for systemic absorption through the alveoli or for topical application to [he skin or various body onf ices Inhalation aerosols are metered dose preparations which provide controlled amounts of the active ingredient(s) The basic components of an aerosol system are the container, the propellant. the concentrate containing the active ingredient(s), the valve and the actuator Aerosols are of two types, the two-phase system consisting of gas and bquid or the three-phase system consisting of gas, liquid and solid or liquid The two-phase aerosol comprises a solution of active ingredient(s) in liquefied propellant and the vaporised propellant. The solvent is usually the propellant or a mixture of the propellant and co-solvents such as ethanol.propylene glycol and polyethylene pjycote The three-phase aerosol consists of a suspension or emulsion of the active ingredient(s) and the vaporised propellants in the suspension the ingredient(s) may be dispersed in the propellant system with the aid of suitable pharmaceutical aids such as wetting agents, solubilismg agents, emulsifying agents, suspending agents and lubricating agents to preveni clogging of valves Foam aerosols contain an emulsion of the active ingredient(s). surface-active agents, aqueous or non-aqueous liquids and the propellents Active ingredients For satisfactory bioavailability the active ingredient(s) should have the majonty of particles under 10 gin in size in the case of inhalation aerosols and not more than 100 gm for other types of aerosols Propellants Propellants perform the essential function of expelling the material from the container by supplying the necessary pressure within the aerosol system They are liquefied or compounded gases fiavmg vapour pressures exceeding atmospheric pressure The commonly used propellants in aerosol systems are hydrocarbons, especially the fIuorochloro-derivatives of methane and ettane. the butanes and pentanes and compressed gases such as nitrogen and carbon dioxide Mixtures of propellnts are often employed to obtain the necessary delivery and spray characteristics of the aerosol Vafces The vatve regulates Uie flow of the active inp/edientfs) and propeltent from the container and determines the spray charactensto of the aerosol It must be manufactured from materials which are inert to the contents of the aerosol The common^ used matenais are rubber, plastic, aluminium and stainless steel For topical products vatves capable of providing continuous spray operation are suitable However, products for oral or nasal inhalation require metered-dose vafves which ensure delivery of a uniform quantity of spray and an accurate dose of the active ingredient(s), both within specified tolerances, with each activation of the valve Metered valves may need priming before use if the aerosol packages have not been stored properly or have not been used for long periods of time Actuators The actuator or adaptor which is fitted to the aerosol valve stem is a device which on depresston or other movement opens the valve and directs the spray to the desired area The design of the actuator which incorporates an orifice of varying size and shape and expansion chamber is very important in influencing the physical charactenstics of the spray or foam, particularly in the case of inhalation aerosols, where the active ingredent(s) must be delivered in proper particule size range A proportion of the active ingredients(s) is usually deposited on the inner surface of the actuator, the amount available is therefore less than the amount released by actuation of the valve Containers Aerosol containers are made of metal (stainless steel, aluminium or tin-plated steel), glass or plastic or acombination of these materials The containers must be so designed that they provide the maximum in pressure safety and impact resistance Preparation of aerosols Aerosols are commonly prepared by filling under pressure and sometimes by filling after refrigeration to temperatures below 0* infilling under pressure, the requisite volume of the concentrate of the active ingredient(s) is filled in the container and either the propellant is forced under pressure through the valve orifice after the valve is sealed, or the propellant is allowed to flow under The valve cap and (he valve assembly is sealed in either case, the air in the container must be evacuated by means of vacuum or displacement with a small amount of the propellant Aerosol products should be manufactured under stritly controlled conditions and subjected to process controls which include propellant and medicament fill weights, pressure test and leak test of the finished product Inhalation aerosols should be manufactured in conditions designed to minimise mcrobial