RH% (Relative Humidity) tells us how much water vapor is in the air, compared to how much it could hold at that temperature. It is expressed as a percentage. For example, 50% RH means the air is holding one half the water vapor it can hold, again dependent on temperature.
Rules of Thumb
As temperatures increase, the air expands and becomes drier, RH% decreases. As temperatures decrease, air becomes denser and wetter, RH% will increase. When thinking about air pressure, as air pressure decreases, air becomes drier and RH% increases. As air pressure increases, air again becomes wetter and RH% increases.
Comfort, Health and Safety
The most widely designated range for best human comfort is between 35 to 55%. This range will support the human immune system and not dry out eyes, noses and throats. Inside dew points are not usually reached and cool surfaces will not form condensation at typical temperatures. Levels below 30% cause irritation to eyes, dry out skin and will shrink mucous membranes reducing a person’s resistance toward upper respiratory illnesses. Lower levels down to 15% or less over time will cause DVT (deep vein thrombosis). RH% over 60% will support fungal growth on first condensing surfaces.
Impact on efficacy of UV (ultraviolet) 254 nm emissions, UVC PCO (Ultraviolet Photocatalytic Oxidation) and UVGI (Ultraviolet Germicidal Irradiation)
Past research performed to measure the effect of temperature and RH% on PRRS (airborne porcine and reproductive syndrome) virus showed that PRRS virus was more susceptible to Ultraviolet as temperatures decreased, most susceptible to UVC inactivation at RH% between 25 to 79%, less susceptible at < 24% and least susceptible at > 80%.
Another Study performed on three pathogenic airborne bacteria which entailed passing bioaerosols horizontally into a UV device, showed that increased UV resistance with relative humidity is likely associated with the hygroscopicity and increased aerodynamic diameters at high RH%. UVGI effectiveness was significantly increased with decreasing RH%.
Research published by 2020 UHOO in Singapore reported that virus particles are most inactive at 50% RH. Real time testing performed by AIAQS on Active and Passive technology combining UVC, BPI and activated oxygen showed that the combined efficacy was diminished as much as 20% at RH% exceeding 70%.