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Understanding why
It’s just physics: the lower the relative humidity, the more moisture the air will absorb from hygroscopic materials (wood, paper, leather, fruits and vegetables, etc.) and from the skin and mucous membranes of living beings. In other words: dry air dries us out!
That means your skin, eyes, nose, respiratory system, the latter our primary defense mechanism against airborne infection, suffer.
To ward off viral infections, your immune system requires the indoor environments you inhabit maintain a healthy level of relative humidity.
If we are exposed to relative humidity lower than 40% for an extended period, our mucous membranes dry out, resulting in damage to the cilia, miniscule hairs that filter the air we breathe, by moving to a well-orchestrated rhythm. They become literally ‘stuck’ in gooey mucous membrane, rendered incapable of performing their key function of removing and defending against pollutants, bacteria and airborne viruses. Left alone, these intruders are able to multiply in our respiratory tracts.
Respiratory disease is often the result of this situation, including the common cold, bronchitis and sinusitis, even contributing to increased risk of COVID-19 and influenza, as mounting epidemiological studies indicate.
What you can do: Humidity management
Medical research shows the optimal indoor air parameters for good health and comfort lie in the range of 21–22° C with relative humidity between 40–60%. These levels facilitate better protection, better concentration for work and education, along with improved leisure and sleep, and healthier looking skin, to mention just a few of the many benefits of properly managed relative humidity.
Furthermore we should always maintain 40-60%RH in public buildings like hospitals, schools and offices, throughout the year, besides surely, in our own homes-sweet-homes.