The Sterling Chart – optimum RH for health
The Sterling Chart below illustrates how RH affects health and well being. Colds, flu, sore throat, dry eyes, itchy and cracked skin are all symptoms that are usually prevalent in the cold dry months of the winter when the indoor RH is at its lowest. The increase in bacteria, viruses and ozone production (caused by static electricity) in low RH levels all have an adverse affect on health.
How does the outdoor temperature affect the indoor humidity level?
Admitting cold outdoor air into a space will lower the indoor RH. When the lower temperature outdoor air is brought indoors and heated, it loses moisture and reduces the overall RH. For example: if the outdoor temperature is 0°F and 50% RH and the air comes indoors and is heated to 70°F, the residual moisture after heating the outdoor air will only be about 3% RH. Even on a nice, sunny 35°F and 50% RH day, the residual indoor RH will only be about 14%.