We all know "dry air" - but what is it that we perceive and exactly how dry air affects people is less known. Nonetheless research indicates that a well-balanced indoor air humidity can alleviate some of the problems with perceived dry air.
Dry air is continually among the most abundant and common complaints about perceived indoor air quality, beating stuffy air, dust or dirt, and draught. Clearly, dry air affects us to a high degree. But humans have no receptors for humidity (nor lack of humidity). Thus, what we perceive as “dry air” actually is, is unclear.
The mystery of dry indoor air
In a paper called “The mystery of dry indoor air – An overview”, published in Environment International 121 (2018), author Peder Wolkoff thoroughly goes through existing research on dry air and how it affects people in indoor environments. Dr. Wolkoff is a senior researcher and, among other things, a former research professor in indoor environmental science at the National Research Centre for the Working Environment in Denmark.
What is dry air?
A number of hypotheses have been formed to explain what we perceive as dry air, including indoor air pollutants, high dust levels, diminished mucociliary clearance (especially at higher ages), low temperatures, psychological factors, disease or medication side effects, or even high levels of sulphur dioxide. Other researchers have suggested that perception of dry air might be confused with perception of a dry nose, dry throat or dry skin. Possibly, perceived dry air could in part come from a cooling sensation, caused by water evaporation from extended exposure to low indoor air humidity (IAH). The most likely explanation, however, is that perceived dry air is caused by a combination of low indoor air humidity, sensory irritants, particles and bioaerosols.
Well-balanced indoor air humidity can lower exposure to microbes
Elevated IAH lowers the concentration of air-borne particles that can impact the nose and airways, especially from hard surfaces like hardwood and vinyl flooring. Floors are an important reservoir of bacteria and virus, and this suggests that elevated IAH may lower the exposure to microbes. However, detailed knowledge on the mechanisms is lacking and more research is needed on the subject. IAH is also the perhaps most important factor that influences the mucociliary clearance – low IAH likely enhances the susceptibility of the mucous membranes towards sensory irritants, oxidants, particles and bioaerosols.
Exposure to low levels of relative humidity can result in desiccation of eyes and mucous membranes
Other studies show that low IAH and ozone were associated with dry eye symptoms and dry eye diseases. A reasonable hypothesis is that low IAH alters the eye tear film stability, making us more vulnerable to aggressive chemicals and particles. Prolonged exposure to low IAH can result in desiccation of eyes and mucous membranes, which naturally causes problems. Seasonal variations in air humidity can exacerbate this. That an increase in IAH alleviates both the perception of dry air, and symptoms of dry eyes and upper airways, has been shown in several studies.
Elevation of IAH may be beneficial for example by retaining the humidity balance in mucous membranes and the eye tear film; by helping to lessen resuspension of particles from floor surfaces; by increasing the size and weight of hygroscopic particles, which increases the deposition rate on floor surfaces; and by decreasing the infectivity of influenza and other viruses.
In short, the combination of low indoor air humidity, sensory irritants, particles and bioaerosol is likely what people are referring to when the complain about the air being too dry. This combination can lead to desiccation of eye, mucous membranes and diminished mucociliary clearance. This makes it important to evaluate and control the humidity levels indoors to create healthy indoor environments