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.

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References

Until the early 1990s, metered dose inhalers (MDIs) that contained chlorofluorocarbons (CFCs) as propellant were the most common way to administer inhaled therapy for asthma and chronic obstructive pulmonary disease (COPD). In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer included the phasing out of CFCs,1 warranting the development of new ways to deliver inhaled therapy for asthma and COPD. This included dry-powder inhalers (DPIs), CFC-free MDIs that used hydrofluorocarbons (HFCs) as a propellant and, aqueous/soft mist inhalers.

The hydrofluoroalkane propellants used in MDIs are potent GHGs, up to 3350 times worse than carbon dioxide (CO2). According to UK data, one patient’s inhalers for 1 year (3 salbutamol inhalers at 28 kg CO2 equivalent [CO2e] each and 12 controller inhalers at 19 kg CO2e each) are equivalent to driving a car 3200 km. These emissions are 10 to 37 times that of dry-powder inhalers (DPIs) and soft-mist inhalers (e.g., Respimat), which produce less than 1 kg CO2e/inhaler.