The Hummel protocol
The scientific basis of standard mode
Where the protocol came from
In 2009 Thomas Hummel and colleagues at the Dresden smell and taste clinic published the study that remains the reference point for the whole field. The idea was almost embarrassingly simple: if a person sniffs a few fixed odours every day, deliberately, does their sense of smell come back faster than it would on its own?
Patients with smell loss of various origins — after infections, after head injury, with no identified cause — sniffed four odours twice a day for 10–20 seconds each, over twelve weeks. A control group did nothing. After twelve weeks the training group scored markedly better on smell tests.
The classic four were rose, lemon, clove and eucalyptus. Not an arbitrary choice: four distinct odour categories — floral, fruity, spicy and resinous.
Why it is built this way
The olfactory epithelium can regenerate: basal cells divide and mature into new neurons throughout life. But growing back is only half the job — a new neuron has to reach the olfactory bulb and connect in the right place. Training does not make neurons grow faster. It gives the brain a reason to parse the incoming signal again and again, and that is what drives the rewiring.
Hence three requirements you cannot cut:
Regularity beats duration. Two short sessions every day work better than one long session every third day. What matters is repetition, not dose.
Different categories, not different odours. Four citrus oils are essentially one stimulus. The point is to engage receptor groups that are unlike each other.
Length. Twelve weeks is the minimum the original work started from. Later studies found that results keep improving at 24, 32 weeks and beyond.
What was added later
Fifteen years have refined the protocol.
Changing the set. Swapping the four odours for a new group every twelve weeks produces better outcomes than keeping one fixed set. The logic is familiar from any training: a system adapts to a constant load.
Deliberate attention. Mechanically holding a vial under the nose works worse than trying to recall the odour, name it, tie it to a specific image from your past. Smell is tightly bound to memory, and that binding is a tool, not a poetic aside.
Starting early. The less time has passed since the loss, the larger the effect. That does not make it pointless to begin a year later — improvements are described in people with long-standing anosmia too, they just come more slowly.
How the app implements it
The standard mode follows the same scheme: four odours a day, deliberate attention to each, daily repetition. Every odour is split into four steps — look, inhale, remember, integrate — because visual support and memory work reinforce the classic protocol through multisensory integration.
The app prompts you when it is time to change the set, and keeps a sensation diary: it shows what you cannot notice from the inside, that an odour which was silent a month ago has started to give some response.
What the protocol does not do
Smell training is not a cure. It does not remove the cause of the loss and does not replace an ENT examination. If smell disappeared suddenly, if there is discharge, pain, or a head injury in your history — see a doctor first, and train alongside the treatment you are given, not instead of it.
And an honest note on expectations: some people recover partially, some not at all. Studies report a statistical improvement across a group, not a promise to each person. That is a reason to start, not a reason to expect a result by a date you set yourself.
Sources
- Hummel T. et al. Effects of olfactory training in patients with olfactory loss. Laryngoscope, 2009. PubMed — the original study the protocol grew from.
- Damm M. et al. Olfactory training is helpful in postinfectious olfactory loss: a randomized, controlled, multicenter study. Laryngoscope, 2014. PubMed — randomised multicentre confirmation.
- Altundag A. et al. Modified olfactory training in patients with postinfectious olfactory loss. Laryngoscope, 2015. PubMed — on changing the odour set every twelve weeks.
- Sorokowska A. et al. Effects of olfactory training: a meta-analysis. Rhinology, 2017. PubMed — a summary of the accumulated work.
