Why aim for the lungs
Pulmonary delivery sends the drug as an aerosol into the airways. For asthma and COPD this is the obvious choice — the steroid or bronchodilator lands exactly on the inflamed or tightened airways, so a microgram dose does the work of a milligram pill while sparing the body. But the lung is also a tempting door to the bloodstream: its surface is the size of a tennis court, the membrane is a single cell thick, and blood flows past on the other side. Drugs absorbed here also dodge the first-pass effect.
The 1–5 micrometre window
Where a particle lands depends not on its actual size but on its [[aerodynamic-diameter|aerodynamic diameter]] — the diameter of a water droplet that would settle at the same speed, which folds together size, shape and density. Big particles (over about 5 µm) slam into the back of the throat by inertia and are swallowed. Very small particles (under about 1 µm) stay suspended in the air and are breathed back out. Only the middle band, roughly 1–5 µm, settles gently onto the small airways and alveoli where it is wanted.
Three machines for one job
The [[metered-dose-inhaler|metered-dose inhaler (MDI)]] sprays a fixed puff from a pressurised canister. It is small and reliable, but the spray comes out fast and the patient must press and breathe in at the same moment — hard to coordinate, especially for children and the elderly. The [[dry-powder-inhaler|dry-powder inhaler (DPI)]] instead holds the drug as a fine powder that the patient's own forceful breath pulls apart and carries down; there is nothing to coordinate, but a weak breath delivers a weak dose. The [[phc-nebulizer|nebulizer]] turns a liquid into a continuous mist the patient simply breathes over minutes — ideal for the very young, the very sick, or large doses, at the cost of size and time.
- MDI: shake, exhale fully, seal lips on the mouthpiece, then press and inhale slowly and deeply — coordination is the weak point.
- Add a spacer to an MDI: the puff fills a holding chamber first, so timing no longer matters and less drug hits the throat.
- DPI: do not exhale into it (moisture clumps the powder); inhale hard and fast to break the powder into respirable particles.
- Nebulizer: breathe normally through the mask or mouthpiece until the chamber runs dry — slow but forgiving of technique.