A comfortable level of humidity ranges between 20 and 60 percent. Although weather conditions determine humidity outdoors, human activities can add to higher humidity within a home.
A house with vapor barriers, windows and doors that fit tightly will hold in more heat and prevent the release of indoor moisture to the outside. Air conditioning, dehumidifiers, and fresh air intake ducts can reduce indoor humidity.
The heat from cooking produces a lot of water vapor and adds to indoor humidity levels. According to the Minnesota Blue Flame Gas Association, cooking for four people produces about 5 pints of water over a 24-hour period. Dishwashers also increase humidity.
A bath generates about 1/8 pint of water vapor, and showering releases about 1/2 pint. The Minnesota Blue Flame Gas Association reports that an extra 4 to 6 pints of water vapor can increase humidity within a 1,000-square-foot area up to 60 percent.
Moisture from a washing machine and clothes dryer also raises humidity levels. Proper venting with ducts that open to the outside, the use of exhaust fans, and opening windows will help moisture escape outside.
All houseplants produce water vapor. Indoor furnishings, which typically absorb some of that moisture, will release it into the air in drier weather.