One of the most crucial components of a MDI is its propellant. The propellant provides the force to generate the aerosol cloud and is also the medium in which the active component must be suspended or dissolved. Propellants in MDIs typically make up more than 99% of the delivered dose,  so it is the properties of the propellant that 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, they must:
A version of this article appears in print on October 13, 2013, on page A 1 of the New York edition with the headline: The Soaring Cost of a Simple Breath. Get Free Email Alerts on These Topics Asthma Drugs (Pharmaceuticals) Prices (Fares, Fees and Rates) Series Inside Health » Too Hot to Handle Arts » The Harmony of Liberty Opinion » Should Beach Privatization Be Allowed? Room for Debate asks whether shorefront homeowners should have to open their land to all comers.