A metered-dose inhaler (MDI) is a propellant-driven delivery mechanism for inhalation of asthma medications. MDIs are just one of three basic types of devices used to deliver inhaled medications.Nebulizers deliver fine liquid mists of medication through a "mask" that fits over the nose and mouth, using air or oxygen under pressure. They are frequently used to treat people with asthma who cannot use an inhaler, including infants, young children and acutely ill patients of all ages.Rotary inhalers and dry powder inhalers have been introduced to deliver asthma medication without using the propellant chlorofluorocarbon (CFC), which damages the earth’s ozone layer.