"Metered dose pressurized aerosols and the ozone layer".

The term "aerosol" has a precise scientific definition as liquid or solid particles suspended in air [1], but has in the last few decades acquired a new non-scientific meaning as a self-contained, sprayable product in which the propellant force is supplied by liquefied gas [2]. The propellants used in these aerosol canisters are of several types, namely: a) chlorofluorocarbons (CFCs); b) hydrocarbons, such as propane and isobutane; c) mixtures of CFCs and hydrocarbons, which are used in many proprietary brands of household aerosol spray; and d) other substances including dimethyl ether, nitrogen and carbon dioxide. The oldest reference to a product propelled by the energy derived from one of its own constituents is in a US Patent of 1899 [2], utilizing methyl and ethyl chlorides as propellants to deliver paints and varnishes. Further Patents of 1903 and 1921 described devices for perfume and antiseptic spraying, using carbon dioxide as the propellant. CFCs were developed in the early 1930s as "safe" alternatives to ammonia in the refrigeration industry [3], being first used as propellants in a fire extinguisher in 1933, and subsequently in insecticide sprays during World War 11 [4]. Household sprays containing CFCs were introduced in the 1950 15) and s ix years later, Riker Laboratories developed Lite first pressurized metered dose inhaler (MDI) for asthma tlterapy [6), containing the adrenergic agents isoprenaline (isoproterenol) or adrenaline (cp in ephrine) . Subsequently, CFC-powered MD Is have become the mainstay of asthma therapy, for delivery of selective bcta-agonists, topical con icosteroids and anti-allergic agents. The propellants used in MDis are CFCs 11 (CCll). 12 (CCL2F2) and 114 (CClF2CClF2); mixtures of two or three of these substances being selected, in order to achieve the desired vapour pressure and spray characteristics. The numbering system for CFCs is a code which indicates the number of carbon, hydrogen, chlorine and fluorine atoms present in the molecule. CFCs are also known as Freons and Arctons, these being trade names imposed by two of the leading chemical companies. Although most CFCs are gases under normal ambient conditions, they can be liquefied either by cooling or by pressurization. Each CFC has a characteristic boiling