Inside a standard inhaler, you’ll find a small pressurized canister filled with medication, a liquid propellant that pushes the drug out, and a handful of inactive ingredients that keep the formula stable. The hardware surrounding that mixture, including a precision valve and a shaped mouthpiece, is just as important as the chemicals themselves. Here’s a closer look at every layer, from the metal canister to the mist that reaches your lungs.
The Hardware: Canister, Valve, and Mouthpiece
A metered-dose inhaler (MDI), the most common type, is built from four main parts. The canister is a small aluminum cylinder that holds enough pressurized formula for the labeled number of doses. Its inner walls are often coated with materials like anodized aluminum, epoxy-phenolic resin, or even a submicron layer of fused silica glass to prevent the medication from sticking to the metal over time.
Seated at the bottom of the canister is a metering valve. This is the component that makes each puff consistent. A tiny chamber inside the valve fills with a precise volume of the liquid formulation. When you press the canister down, that chamber opens toward the mouthpiece and seals off from the reservoir above, so you get the same measured dose every time. Many valves include a metal retaining cup that encloses the moving parts responsible for forming and releasing each dose.
The plastic actuator (the L-shaped housing you hold) serves two jobs. It contains a narrow nozzle that forces the liquid through a small orifice, breaking it into a fine aerosol mist. And it forms the mouthpiece you breathe through, directing that mist toward your throat and lungs. Most modern inhalers also include a dose counter, a small mechanical or digital indicator on the back of the actuator that counts down so you know when the canister is running low.
The Propellant: What Pushes the Drug Out
The liquid that fills most of the canister isn’t medication. It’s propellant, a substance that exists as a liquid under pressure but instantly becomes a gas when released into the air. That rapid expansion is what breaks the formulation into breathable droplets.
Until 2008, inhalers in the United States used chlorofluorocarbon (CFC) propellants. Those were phased out under the Montreal Protocol because they damage the ozone layer. Today’s inhalers use hydrofluoroalkane (HFA) propellants, specifically HFA-134a and HFA-227. These don’t deplete ozone and produce a softer, slightly warmer spray than the old CFC versions.
Manufacturers sometimes blend the two HFA propellants together. By adjusting the ratio, they can fine-tune two properties that affect how well the drug reaches your lungs: the density of the liquid (which determines how evenly suspended drug particles stay in the canister) and the vapor pressure (which influences how small the aerosolized droplets are). Higher vapor pressure generally produces smaller droplets, which travel deeper into the airways.
The Active Medication
What drug is inside depends entirely on the inhaler’s purpose. There are two broad categories.
Rescue inhalers contain short-acting bronchodilators. These are medications that relax the muscles wrapped around your airways, opening them within minutes. They’re designed for quick relief during an asthma attack or a sudden bout of breathlessness.
Controller inhalers are taken daily to prevent symptoms. The most common contain inhaled corticosteroids, which reduce the chronic inflammation that narrows airways over time. Some controller inhalers combine a corticosteroid with a long-acting bronchodilator, a medication that keeps airways relaxed for 12 hours or more. Long-acting bronchodilators are always paired with a corticosteroid because using them alone has been linked to an increased risk of severe asthma attacks.
Inside the canister, the medication is either dissolved directly in the liquid propellant (a solution) or suspended as microscopic solid particles floating in it (a suspension). Solution-based inhalers don’t need shaking before use. Suspension-based inhalers do, because the drug particles settle to the bottom over time, and shaking redistributes them for an accurate dose.
Inactive Ingredients That Keep It Stable
Beyond the drug and propellant, MDI formulations include small amounts of inactive ingredients. The two most common are ethanol and oleic acid. Ethanol acts as a co-solvent, helping dissolve the drug more evenly in the propellant. Oleic acid serves as a surfactant, a substance that reduces clumping so drug particles stay uniformly dispersed and don’t stick to the canister walls or valve components. The quantities are tiny: commercial formulations typically contain well under 3% ethanol and fractions of a percent of oleic acid by weight.
Dry Powder Inhalers: A Different Approach
Not all inhalers are pressurized canisters. Dry powder inhalers (DPIs) contain no propellant at all. Instead, they hold a fine powder that you pull into your lungs with a fast, deep breath. Your own airflow does the work that propellant does in an MDI.
The powder inside is not pure medication. The actual drug dose in a DPI can be remarkably small, sometimes as little as 20 to 500 micrograms of a corticosteroid. That’s far too little to handle, meter, or flow through a device reliably on its own. So the drug particles, ground down to just 1 to 5 micrometers in diameter, are blended with much larger carrier particles that add bulk and improve flow. Lactose monohydrate is by far the most common carrier, though glucose and mannitol are also approved for use. When you inhale, the force of your breath separates the tiny drug particles from the lactose carriers. The drug particles are small enough to reach deep into your lungs, while the larger lactose particles impact the back of your throat and are swallowed harmlessly.
This is worth knowing if you have a severe lactose allergy, though the amount involved is extremely small and the lactose used is highly purified.
Soft Mist Inhalers: No Propellant, No Powder
A third type, the soft mist inhaler, takes yet another approach. These devices hold an aqueous drug solution, essentially medication dissolved in water or a water-ethanol mixture. A spring-loaded mechanism inside the device forces the liquid through an extremely fine nozzle, producing a slow-moving mist that lingers in the air longer than the fast blast from an MDI. This gives you more time to coordinate your breath with the spray.
Because there’s no propellant to keep the solution sterile, soft mist inhalers include either ethanol (which doubles as a preservative) or a small amount of an antimicrobial preservative in the liquid. The absence of HFA propellants also means these devices have no direct greenhouse gas emissions, which has made them increasingly popular.
Why the Design Matters for Your Lungs
Every component inside an inhaler exists to solve one problem: getting a precise, tiny dose of medication past your mouth and throat and into the lower airways where it’s needed. The propellant creates droplets small enough to stay airborne. The valve ensures you get the right amount. The carrier particles in a DPI prevent the dose from clumping. The nozzle geometry controls droplet size. Even the coatings on the inner canister wall matter, because if medication sticks to the metal, your dose comes up short.
If your inhaler feels like it’s spraying but not helping, the issue often traces back to one of these components. A clogged nozzle, a canister that’s past its labeled number of doses, or forgetting to shake a suspension-based MDI can all mean the drug isn’t reaching your lungs in the right amount or particle size.