It’s not the heat
This Accurite digital humidity and temperature meter reports when indoor humidity levels reach levels that are too high or too low. accurite.com
"It's not the heat, it's the humidity." That's a common expression we hear during a mid-summer heat wave. But it's just as irritating indoors in winter.
Low relative humidity in a heated indoor room can create dryness that leads to nosebleeds, skin irritation, difficulty breathing and damaging static electricity.
Humidity is the amount of water vapor in the air, while relative humidity (RH) is the ratio of moisture in the air to the maximum amount of moisture the air can hold. Hotter air can hold more moisture, so the amount of moisture the air can hold varies depending on air temperature.
A comfortable home environment is when the level of humidity stays between 25 percent and 55 percent RH. Over 55 percent RH is considered to be a high humidity level, as we sometimes see during summer.
The U.S. Environmental Protection Agency says, "Too much moisture in a home can lead to mold, mildew and other biological growth. This in turn can lead to a variety of health effects ranging from more common allergic reactions, to asthma attacks, and hypersensitivity pneumonitits."
For more information, refer to "A Brief Guide to Mold, Moisture, and Your Home" at epa.gov.