In the wake of Covid-19 an app was developed that turned out to be very efficient at diagnosing the infection by the quality of its cough. This continues a long tradition among older physicians of diagnosing different chest infections by the type of cough: they could use it to differentiate different pneumonias, bronchitis, tracheitis, pharyngitis, pleurisy, bronchial asthma and tuberculosis. Often a stethoscope was for confirmation purposes only.

Actually anyone can apply the basic principles and can get quite clever in choosing the best cough remedy. The first question was whether the cough was ‘wet’ or ‘dry’. A wet cough brings up phlegm, sputum or other fluids. A dry cough does not bring up much, or maybe just tacky globs. The difference goes to the heart of lung defences and how to support them.

Let’s first introduce the ‘mucociliary escalator’: a supremely elegant protective function. The lining of the respiratory mucosa includes many cells that have ‘cilia’, a fine whip-like extension on each cell. The ciliated cells flex together so that from above the mucosa looks like a field of wheat with waves of wind moving across it. In this case the waves move towards the throat, up the bronchial airways (and from the nasal cavity they move downwards). The other large group of mucosal cells are ‘goblet cells’ that produce mucus. This is borne along by the waves of cilia. It is also sticky to passing inhaled particles. As you inhale the structure of your airways spins the air into a vortex down the tubes: the centrifugal force so generated throws dust, smoke, pathogens, pollen, house mite debris and other airborne particles onto the sticky walls like flypaper. This is then moved towards the throat where it is constantly swallowed. The stomach then sterilises and breaks it all down.

This wonderful mucociliary escalator works constantly to keep the lungs clear and mostly without us being aware of it. However if it becomes overloaded, then the back-up defence is called upon. This is the cough. The escalator can suffer in two types of way.

  • It reacts to irritant or infected inhalants by generating more mucus, which then overloads the cilia and accumulates in the airways. Coughing is required to heave this stuff out and is then ‘productive’ or wet. This generally is the case with bronchitis, bacterial or fungal chest infections.
  • By contrast the reaction involves drying out the mucus. The escalator fails because the fluid ‘flypaper’ is compromised. The resulting irritation leads to a ‘non-productive’ or dry cough. Most commonly it is the sign of many childhood infections, and above all anything with an allergic or asthmatic element. It can also be is a sign of pneumonia, pleurisy, tracheitis and was a classic feature of tuberculosis (where the old word was that the airways had become so dry the body needed blood to lubricate them). It is also a symptom of lung cancers. So there is a much wider range of potential problems than those leading to wet coughs and further clues will need to be sought to narrow down the cause.

‘Expectorants’ are remedies which help a cough on its way so that it finishes the job. In the case of a wet cough heating and drying remedies were sought. The classics were ginger and cinnamon (still best taken together as a hot tea), and other warming spices like fennel, aniseed and raw garlic. Stronger ‘stimulating expectorants” included ipecac (Cephaelis ipecacuanha), squills (Scilla maritima) and from North America lobelia (Lobelia inflata): interestingly these are also emetic in larger doses and it appears that their effects use the same reflex mechanism in stimulating airways contractions.

In the case of a dry cough further work may be required to narrow down the cause. In the case of dry coughs not complicated by other diseases, the obvious choice was a ‘moistening’ expectorant. This usually included plants with high levels of mucilage like marshmallow, the plantains (Plantago spp), and slippery elm. Interestingly these get quickly broken down in the stomach, so their effect seems to be by reflex from the upper digestion, in the same way that emetics are stimulating expectorants.

We can finish this post by pointing out that licorice root can support both type of cough and justifiably features as an ingredient in many herbal cough prescriptions.