A humidifier should be set between 45 and 50 percent relative humidity. The variance in humidity levels is based on how dry or how moist a person wants the air to be.
Humidity is the measure of how saturated the air is with water. When the humidity is too high, such as more than 50 percent, harmful particles like mold and bacteria are able to grow and propagate quickly. This can be bad for respiratory health as well as the home's structural integrity. When the humidity is too low, such as less than 30 percent, the air will seem dry, which can cause irritation and breathing problems, along with damaging a home's foundation.