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Influence of humidity on spread of viruses and bacteria, what about it?

Humidity and viruses

Low humidity affects both comfort and people's well-being/health. But what is this really like?

In winter, when the relative humidity of the air decreases in closed and heated rooms, because the lungs can absorb less oxygen from the inhaled air, respiratory problems and conditions that adversely affect the eyes, skin, nose and mouth increase. Low humidity adversely affects mucous membranes. This can lead to irritation of the eyes, nose and throat, as well as reduced resistance to infection. It also increases the risk of disease because many viruses live longer in dry air. The observations related to growth and diffusion of biological nature indicates closer values than if we consider only the effect on the human body. We are talking mainly about mold, mites, viruses and bacteria. For each of these infections, the relative humidity has a specific influence which should be considered individually.

In general we can say that when the relative humidity exceeds 50% there is an increasing proliferation of all the above mentioned contaminants. Surprisingly, in some cases a specific malignancy occurs even at lower values of relative humidity. This is due not only to the reduced resistance of the respiratory tract but also to the droplets that people spread by talking, sneezing and coughing. The size of these droplets is such that they touch the ground at about 1m distance. However, in a dry environment, the droplets quickly evaporate to a size of 0.5 to 5µm, turning into an aerosol that remains suspended in the air and can spread much further. They contain pathogens and can penetrate deep into the respiratory tract due to their size.

Thus, low humidity levels facilitate the spread of bacteria and viruses in the atmosphere, while excessive humidity levels further increase the risk of spreading viruses and bacteria. To prevent biological contaminants from multiplying and spreading, relative humidity, ideally, should be kept between 40% and 60% say several studies. Precise humidity control reduces the problems caused by a dry environment. Comfort is increased and equipment is protected. This creates a safer healthier workplace for staff and visitors.


If we ask the GGD for advice we get different values (images above).

Drenthe 40-60%, IJselland 30-70% ideal 50%, Zeeland 40-65%, Rotterdam Winter 30-55% Summer 55-70%. 30-70% is considered acceptable. GGD Booklet the fresh elementary school 30-70% but preferably 40-60%. GGD Groningen 40-60% at 20˚°C (incidentally the only one to attach a temperature to it). GGD i.s.m. Adem Gezond 40-60% (tips for people with Asthma and COPD)

It is striking that 40-60% is often mentioned as ideal but nevertheless, despite all available data and research, there is no Dutch health advisory value for relative humidity.

Incidentally, these values are also requested by our government for correctional facilities. Indeed, offenders are entitled to compensation if the air in their cells is not of sufficient and/or adequate quality. So personally, I find it strange that they do not wish to advise this for the rest of our society such as the vulnerable people staying in nursing and care homes or our children in daycare centers or schools.

In the Netherlands it is stated that the employer is responsible for a healthy indoor climate but with the exception of the amount of fresh air, and temperature, among other things, there is no mention of the influence of the relative humidity in our buildings.

In addition, researchers claim that dust in the home is a culprit. This is often cited by opponents of humidification in a building. With dry air, dust is more likely to be released and float in the air which makes sense. However, we must question the extent to which these studies are reliable. All studies done on the influence of dust have all quoted from the same sources (17 of them)  

Further research by TU Delft showed that of the 17 there were 14 that could not stand the scientific test of criticism.  (Analysis own research TU Delft).

STULZ offers climate solutions with products in precision cooling, comfort cooling, humidification and heat pumps and air treatment. In doing so, STULZ carries several A-quality brands such as Mitsubishi Heavy Industries, CAREL and REMKO.


Various departments GGD, TU Delft, Stichting Adem gezond (formerly Asthma fund), RIVM report 609021043

A detailed conclusion on the influence of relative humidity on pollution growth (E.M. Sterling, A. Rundel, T.D. Sterling, Criteria for human exposure to humidity in occupied buildings, ASHREA Trans. 91, 611-621, 1985) is based on the observation of a lower incidence of respiratory illness in buildings where relative humidity was regulated.

Bert Leffers

Humidification specialist at STULZ GROUP B.V. Author of several white papers and articles on adiabatic cooling, humidification and humidity.