Surprisingly, it is less well known that the opposite condition, excessively dry indoor air, can be as uncomfortable and debilitating for people as overly damp buildings. It is well documented that indoor air RH between 40% to 60% is optimal for people, yet maintaining indoor air RH is rarely mentioned in building guidelines or codes — except perhaps in nursery greenhouses. Even in hospital operating rooms where patients may have large surface areas of inner body tissues exposed, indoor air RH may be as low as 20%. This non-physiologic moisture level can lead to major problems for the vulnerable patient.

Why, then, is overly dry air not addressed as an indoor air contaminate? Perhaps the oversight is because consequences of excess moisture such as mold growth can be seen and smelled, while dryness is invisible, falling into the category of “out of sight, out of mind.” In addition, studies have shown that we (humans) cannot directly perceive low indoor RH and only complain of the air being “too dry” when physiological damage has already occurred — not when we encounter the injurious condition. Conversely, we can easily feel the consequences of high indoor air RH when temperature and humidity rise to the point of decreased convective and evaporative cooling of our skin and mucous membranes by inhaled air.

Despite building occupants’ delayed awareness of dry indoor air, the many health benefits of properly circulated and humidified indoor air makes this a critical parameter to maintain. The challenge of controlling moisture accumulation in the building envelope and indoor materials must be simultaneously addressed if our buildings are to be truly “healthy.”

How do the illnesses from overly dry indoor air compare to those from excess moisture and fungal growth in a building structure? What is the timing of exposure to these conditions and the consequent symptoms in people?

Resulting Clinical Illnesses

In contrast to vague comfort studies, results derived from clinical studies under controlled conditions show that dry air leeches moisture from eyes, skin, and respiratory tract mucosa. This can result in the following diseases.

Eye irritation occurs with breakdown of the aqueous phase of the tear film. The aqueous film provides essential humidification of the ocular surface, sliding of the eyelids, and clears pollutants. Consequently, tear film disorders can result in severe impairment of vision and eye injury.

Significant water loss from the skin with prolonged indoor RH below 30% causes an increase in dry, brittle, and cracked skin, and exacerbation of chronic atopic dermatitis and psoriasis.

Low RH can dry out and inflame the mucous membrane lining of the respiratory tract, increasing the risk of colds, the flu, and other infections. Some micro-organisms also survive longer, and spread more easily when humidity levels are low.