The optimum humidity level for humans is between 45 and 55 percent, according to Bright Hub Engineering. People can tolerate humidity from 20 to 60 percent, and the comfortable range for living conditions is 30 to 50 percent humidity, reports the Tampa Bay Times.
Humidity levels shouldn't exceed 50 percent for people suffering from respiratory disorders such as asthma, according to Bright Hub Engineering. At the opposite end, humidity below 30 percent can lead to dry conditions that cause dry, itchy skin. Another concern is that humidity over 50 percent is optimal for mold growth. Humidifiers and dehumidifiers help people moderate humidity levels in their homes.