It is obvious that high humidity levels in buildings should be avoided, among others, to reduce the risk of mold. But what about the low levels? It is recommended to reduce pollution sources, ventilate, and keep buildings dry to reach a high level of air quality. But how low is acceptable? Thermal comfort requirements define the range of relative humidity between 30% to 70%, whereas 40% to 60%  is recommended concerning health risk and infection control. EN Standard 16798 defines the level for humidification to be when relative humidity is between 20% and 30%, depending on the building category. What, then, is the right low relative humidity level, and what is the dry air? This short talk will attempt to answer these questions by briefly summarizing the research evidence and discussing different humidification solutions. A tentative acceptable level of low relative humidity will be proposed. 

Ph.D., Prof. Pawel Wargocki  ISIAQ Fellow, ASHRAE Fellow, REHVA Fellow International Centre for Indoor Environment and Energy Department of Environmental and Resource Engineering, DTU SUSTAIN Technical University of Denmark pawar@dtu.dk