User:Halcharbonneau/Metered-dose inhaler

Propellants
One of the most crucial components of an MDI is its propellant. The propellant provides the force to generate the aerosol cloud and is also the medium where the active component must be suspended or dissolved. Propellants in MDIs typically make up more than 99% of the delivered dose, so the properties of the propellant dominate more than any other individual factor. This is often overlooked in literature and in industry because so few propellants are used, and their contribution is often taken for granted. Suitable propellants must pass a stringent set of criteria, and they must:


 * have a boiling point in the range of -100 to +30 °C
 * have a density of approximately 1.2 to 1.5 g cm−3 (approximately that of the drug to be suspended or dissolved)
 * have a vapour pressure of 40 to 80 psig
 * have no toxicity to the patient
 * be non-flammable
 * be able to dissolve common additives (active ingredients should be either fully soluble or fully insoluble)

Chlorofluorocarbons (CFCs)
In the early days of MDIs, the most commonly used propellants were the chlorofluorocarbons CFC-11, CFC-12 and CFC-114.

In 2008, the Food and Drug Administration announced that inhalers using chlorofluorocarbons as a propellant, such as Primatene Mist, could no longer be manufactured or sold as of 2012. This followed from U.S. decision to agree to the 1987 Montreal Protocol on Substances that deplete the ozone layer that developed new inhalers, including dry-powder inhalers (DPIs), CFC-free MDIs that use hydrofluorocarbons (HFCs) as a propellant and, aqueous/soft mist inhalers.

Hydrofluorocarbons
Hydrofluorocarbon propellants have replaced CFC propellants. Concerns about the use of hydrofluorocarbon propellants have, however, since arisen since these compounds are potent greenhouse gases; propellants released during the use of a single inhaler result in a greenhouse footprint equivalent to greenhouse gases released during a 180-mile car journey.