Health Considerations due to indoor relative humidity

In general, it is agreed that too much humidity is as undesirable as too little. The effects of relative humidity (rh) appear to reflect the following pattern (see Figure 1):

• - bacteria increase at 30 percent or less and 60 percent or more rh.,
• - viruses increase at 50 percent or less and 70 percent or more rh.,
• - fungi increase at 60 percent or more rh.,
• - mites increase at 50 percent or more rh.,
• - respiratory infections increase at 50 percent or less and unknown above 50 percent rh.,
• - asthma difficulties increase at either less than or higher than 50 percent rh.,
• - chemical interactions increase at 30 percent or more rh.,
• - ozone production increases at 75 percent or less rh.,

Sterling, Arundel, and Sterling. "Criteria for Human Exposure to Humidity in Occupied buildings." ASHRAE Transactions. 1985, Vol. 91, Part 1)