Dissecting an asthma inhaler with a dose counter

Back in the 90s, before Le Chiffre made asthma inhalers cool,1 my mother taught me how to tell if my Proventil canister was empty. If there was any of that sweet sweet albuterol left, it would float vertically in water, but if it went horizontal it was empty.

Apparently, in the years since I was a wheezyboy, inhaler technology has advanced a bit. Modern ones have clockwork contrivances to tell you how much is left, which I have to admit is convenient.

Photo of part of a Symbicort inhaler, showing the dose counter

I got curious about its inner workings, so I pried the whole thing apart and took pictures.

Photo of Symbicort canister inside the inhaler

Here’s the canister that holds the medicine and a propellant.2 The left part of the canister in this picture contains a valve that dispenses a consistent amount of medication each time the white stem is pushed into the body of the canister.

Diagram of inhaler components
Figure copyright Federico Lavorini, CC BY 3.0, from The Challenge of Delivering Therapeutic Aerosols to Asthma Patients

The other essential mechanical component of the device is the mouthpiece with its nozzle, which expands the squirt of medication into an aerosol so that it can make it all the way into your lungs. In this photo we’re looking straight down the nozzle.

Looking into the jet nozzle

Again back in the 90s, when my inhaler was tumbling around my green and purple nylon fanny pack, it would probably actuate uselessly sometimes. These days, the mouthpiece cover prevents the actuation from happening by blocking it:

Picture of a piece of plastic blocking another one

Since they’re using the mouthpiece cover for that, it has to latch on so it doesn’t just fall off. They include unlatching instructions:

Mouthpiece cover with a sticker saying "Squeeze Sides gently then Pull Out"

Hah. Okay, so now let’s look at the dose counter.

Face of the dose counter, with 120 doses

It’s normally jammed onto the medication container:

Dose counter attached to the canister

How does it know when you’ve done a puff? The red body of the inhaler has a tab that sticks out, and it pushes a lever inside the grey cap.

Picture of tab and slot

Pushing the lever turns this worm gear a partial turn.

Picture showing slot and worm gear

That’s the underside, here’s the top:

Picture of the dial of the counter with the face removed Picture of the face, dial, and the rest of the cap

The dial has rack-style teeth around its edge, which are moved by the worm gear. The black gunk is graphite, I was trying to turn it with a pencil.

Picture of how the dial works with the worm gear

How exactly does pushing in the red tab turn the worm gear? The tab pushes a pin, which turns this ratchet wheel:

Picture of ratchet wheel

Here I’ve taken out that pin, the spring that holds it down, and the ratchet - worm gear spindle.

Picture of the three tiny parts

The pin has a dangly bit, does it count as a flexture? Anyway it looks like a devil’s tail.

Lil pointy bit

Here are all the parts laid out:

All the parts of the inhaler

Finally, here are some close up details of random parts.












Life is good with asthma inhalers! Better living through chemistry! Thanks for playing, thanks for breathing, thanks for reading!

  1. Le Chiffre is seen to be using his MDI inhaler device too fast for good lung deposition and does not often shake the inhaler before use. This will only result in deterioration of his symptoms and poor asthma control during stressful situations.

    -Medicines Academy 

  2. The propellants of these metered-dose inhalers are often potent greenhouse gases! Inhalers accounted for 4% of the greenhouse gas emissions of the entire UK National Health Service in a recent year. My doctor recently switched me to a different control inhaler, which uses powders - there’s more plastic waste, but it must be better for climate.