Humidity, the water-vapor content of the air. Absolute humidity is the weight of the water in a given volume of air. Relative humidity, the figure given in weather reports, is the percentage of saturation. At a given temperature and pressure, the air can hold a certain amount of water and no more. Air holding the maximum amount of water is saturated, and its relative humidity is 100 per cent. Air at the same temperature and pressure holding half as much water is half saturated and has a relative humidity of 50 per cent. Humidity, either relative or absolute, is measured with a hygrometer.

Warm air can hold more water than cold air. When warm air is cooled, its water content saturates it at a certain temperature, called the dew point.

Effects of Humidity

The humidity level of a building affects the comfort and health of the occupants. The desirable range is 40 to 50 per cent relative humidity for rooms at 70 F. (21 C.). The U.S. National Weather Service uses a scale called the Heat Index to indicate the health risk posed by a given combination of temperature and humidity.

When the relative humidity is high a person feels sticky because the evaporation of perspiration is retarded. High humidity for an extended period will cause mildew. In basements, high humidity causes beads of water to form on exterior walls. This is because the walls are cooler than the air, causing the moisture-laden air to reach its dew point.

Low relative humidity causes a person to feel chilled —even at 70 F.— because perspiration evaporates at a rapid rate. Low humidity causes dryness of the skin or throat and may aggravate colds and other respiratory ailments. Low humidity is most likely to occur in winter, particularly when the house is being heated.

Controlling Humidity

Indoor humidity can be decreased by an air conditioner or dehumidifier. A simple dehumidifier consists of a bag of calcium chloride, which absorbs moisture from the air. A mechanical dehumidifier operates on the same principle as an air conditioner but does not cool the air.

Indoor humidity can be increased in several ways. A simple method is to place a pan of water on a hot radiator. The heat evaporates the water, thus adding moisture to the air. A more effective method is to use a humidifier. One common type evaporates water at a relatively rapid rate by blowing air through a wetted porous or mesh material. Humidifiers that are attached to a furnace are usually of this type. Some types of humidifiers increase the humidity by blowing a fine mist of water into the air; in an ultrasonic humidifier, the mist is formed by a rapidly vibrating element that breaks up water into tiny droplets. A steam vaporizer, often used for relieving respiratory ailments, produces a flow of warm, moist air by heating water electrically.