Lessons From Nature On Indoor Water Vapor Management
What does indoor air quality have in common with the blacklegged tick?
The debate about the need (or lack of a need) for humidification in buildings rages on. I recently had an interesting conversation with an engineer who specializes in desiccant technology, in particular, dehumidification for operating rooms where the temperature is lowered to keep clinicians wearing sterile gowns comfortable. Since I am a proponent of balanced indoor-air hydration (humidification) to support patient healing and to decrease transmission of infectious bio-aerosols, our chat was somewhat tense.
The dislike of humidifiers, as far as I can tell, results from concerns about unhygienic equipment, condensation and mold growth in walls and ductwork, and excess energy consumption.
And yet, water is critical to all life! In biology, successful vapor pressure management exists in even the most extreme climactic conditions, ranging from insects living in the desert to fish swimming in fresh water. Remarkably, biological strategies for internal water homeostasis are beautifully energy and material efficient. What elegant model can Mother Nature show us now?
There are many examples, but a particularity interesting one is the blacklegged tick, Ixodes scapularis. This tick must avoid death due to desiccation for several months between blood-feedings on hosts. Then, after the meal, the large volume of ingested liquid must be rapidly managed to excrete excess moisture. How is their life maintained with these wild swings of water availability?
The secret to Ixodid tick survival is the osmoregulation of their saliva. During extended fasting periods, their salivary glands secrete a highly hygroscopic solution rich in Na+, K+, and Cl− onto the surface of their mouth. This solution quickly dries into hydrophilic crystals which absorb water vapor from unsaturated air because of their high osmotic pressure. Once the crystals are saturated, the tick draws the now hydrated secretion back into its body.
Biological solutions for water management with potential applications to buildings exist not only for humidity control, but also for cooling and water conservation. For example, to manage evaporation rates, some plants have guard cells with osmotic pressure sensors that open and close for optimal gas and vapor exchange as needed. In arid regions, desert plants and beetles have ribbed surfaces that can swell and shrink, creating self-shaded regions that reduce irradiation and water loss from transpiration. In other settings where humidity is too high, evaporation rates can be increased by airflow over moist and vibrating membranes.
Going back to my cautious conversation with the desiccant engineer, I did not bring up tick saliva, but I did ask him what rh range he thinks is best in operating suites. He stated, “forty to sixty percent.” I laughed, because these are the same limits I recommend. In fact, this rh scope has been known to be the healthiest since 1985 when Sterling, Avundel and Sterling published their well-known graphic for ASHRAE.
While humans clearly are not desiccated or swollen ticks, all organisms from insects to mammals need balanced water in both the vapor and liquid state to live. Once again, we can learn a few things from Mother Nature to inspire innovation in the development of desiccants, humidifiers, building envelope design, and other HVAC equipment.