It is salient to distinguish between ambient humidity (relative (RH) or absolute) near the breathing and ocular zone and phenomena caused by moisture-damage of the building construction and emissions therefrom. This paper summarises state-of-the-science knowledge about the impact of RH on a number of important health and comfort outcomes in indoor environments.Methods
The overview integrates studies about how ‘extended’ exposure to low RH (or absolute) impacts health, sensory effects in the eyes and airways (sensory symptoms), transmission and survival of influenza virus, work performance, sleep quality, and impact on the vocal cord. Effects by moisture damage of construction products are excluded.Results
There is evidence that humidified indoor air can positively impact eye symptomatology, but not sensory irritation in the airways. Low humidity favours transmission and survival of influenza virus and is associated with an increase of infections in institutions. Slightly elevated humidity appears also to improve sleep quality, while low humidity may impact the vocal cord negatively.Discussion
Reporting of ‘dry air’ or ‘dryness’ or dry eyes and airways continues to be a major complaint in office environments, despite the continued efforts to develop low emitting building materials and better ventilation strategies. Some researchers continue to argue that the complaints of ‘dry air’ (semantically misleading, since we do not have an organ sensing humidity) are associated with indoor pollutants. However, measured concentrations of common VOCs, in general, are orders of magnitude below thresholds for sensory irritation effects, perhaps with the exception of formaldehyde and acrolein. The perception ‘dry air’ appears to be composed of different perceptions and associated causes.Discussion
Epidemiologic studies and intervention studies have shown associations between low RH and complaint rates; furthermore, aggravation of the eye tear film stability by low RH may result in desiccation, hyperosmolarity and inflammatory reactions. Thus, the merged information about the impacts of VOCs and particles indoors versus low RH favours the latter as an important risk factor to consider for assessment of eye and airway complaints, sleep quality, and vira transmission and survival. The impact of low RH on voice reduction is less clear. ‘Dry air’ should be replaced with a meaningful indoor air quality descriptor.