You place the liquid asthma medicine into a cup that attaches to an air compressor. When you turn the compressor on, it changes the liquid into a mist. You then breathe this mist through a mouthpiece into your lungs. Nebulizers with masks can be used to give asthma medicines such as Pulmicort Respules to children who are too young to use a metered dose inhaler properly.
Sources: Merriam Webster Dictionary in Medline Plus, Allergy & Asthma Foundation of America


