DPIs rely on the patient’s own inhalation energy to disperse and aerosolize the powder, so several factors determine how well they work. The inhaled flow the patient can generate matters because DPIs require a sufficient inspiratory force to break up the powder into fine particles that can reach the lungs. If the patient can’t inhale strongly enough, de-aggregation and emission of the dose drop, reducing effectiveness. The device’s intrinsic resistance and the resulting inspiratory flow rate matter because different DPIs are designed to work best at different flow rates. A high-resistance device needs a stronger inhalation to reach the same flow, while a low-resistance one may be easier to use but still depends on achieving an adequate flow to properly de-aggregate the powder. This interaction between device design and inhalation effort shapes how much drug is emitted and deposited. Humidity and moisture exposure matter because dry powders can clump or stick when damp. Moisture reduces powder flowability, alters particle behavior, and lowers the emitted dose, leading to inconsistent dosing and less drug reaching the lungs. Since all three factors influence dose delivery and deposition, all of them affect DPI use.

Inertial impaction happens when particles have enough momentum that they can’t perfectly follow the air as it changes direction at bends or branch points in the airways. When you inhale with a higher flow rate, the air moves faster, increasing the particle momentum and making it harder for larger particles to turn with the airstream. They end up striking the airway walls and depositing there, often in the upper airways rather than reaching deeper lungs. So a higher inspiratory flow rate favors inertial impaction. Lowering the flow rate reduces this effect because the air isn’t rushing as much, allowing particles to better follow the stream and pass deeper. Smaller particle size also reduces inertial impaction since tiny particles can follow the air more readily. Breath-holding tends to enhance deposition by sedimentation and diffusion rather than inertia, so it doesn’t specifically promote inertial impaction.

The main concept is that particle inertia causes deposition when a particle can’t bend its path with the airflow around airway bends. Large or fast-moving particles carry momentum that keeps them moving in a straight line as the airstream curves at branch points in the airways. Because they can’t follow the turning flow, they strike the airway walls and deposit there. This is inertial impaction, and it is most impactful for bigger particles or higher flow rates, which is why upper airway deposition is common with these conditions. Sedimentation relies on gravity pulling particles down during slower or longer residence times, and Brownian diffusion involves random molecular motion that matters most for very small particles over time. Interception, meanwhile, happens when a particle following a streamline comes within a particle radius of a surface and is effectively “caught” by geometry, not primarily by inertia. So the mechanism driven by inertia—the straight-line tendency of heavier particles that prevents them from following curved airways—is inertial impaction.

Nebulizer performance hinges on how the device creates and delivers the aerosol, which depends on the device design and how it’s operated, the drive conditions, the gas properties, environmental factors, and the drug formulation itself. The design determines how droplets form and how efficiently the liquid is converted to aerosol; the flow or driving gas rate sets how much aerosol is produced and how consistently it’s delivered; the gas source and its density affect atomization, evaporation, and momentum transfer; humidity and temperature influence moisture content and liquid properties that alter droplet formation; and the formulation’s characteristics—viscosity, surface tension, and concentration—shape how the liquid breaks into droplets and what the final particle size distribution will be. All of these factors together determine the output, droplet size, and deposition in the airways. In contrast, things like color, weight, brand of supplier, or user color preference don’t affect the physics of aerosol generation and thus don’t impact performance.

Teaching how to use a pressurized metered-dose inhaler needs enough time to cover both the steps and the user’s ability to perform them correctly. Thirty minutes gives room for a clear demonstration, followed by the patient practicing the technique and receiving feedback, plus a check that they can return-demonstrate the correct method on their own. This time also accommodates key points: coordinating actuation with inhalation, timing a proper breath-hold after inhalation, using a spacer if prescribed, priming if needed, and basic care like cleaning and storage. When technique isn’t mastered, medication delivery to the lungs drops significantly, so dedicating about half an hour for initial education and technique assessment helps ensure effectiveness and safety. Real-world timing may vary, but thirty minutes is a common target for comprehensive pMDI education.