In public buildings as well as individual homes, it is sometimes desirable to alter extremes of relative humidity so that living spaces are more comfortable. For human comfort, optimum relative humidity lies between about 30% and 50%. When indoor air is exceptionally dry, a humidifier may be used to add water vapor to the air, but when indoor air is excessively muggy, a dehumidifier may be used to remove water vapor from the air.
As cold, dry winter air is drawn indoors and heated by a furnace, its relative humidity declines--sometimes to uncomfortably low levels. Suppose, for example, that outdoor air has a temperature of -20°C (-5°F) and relative humidity of 50%. If that air were brought indoors and heated to 21°C (70°F), its relative humidity drops to about 2%--well below the optimal winter indoor relative humidity of 30% to 50%. At such very low relative humidity, people often feel discomfort caused by dry skin and irritation of the mucous membranes in their nose and throat. Also, wood furniture dries out and may crack and become unjointed. Discharge of static electricity (e.g., after walking across a carpet and touching a metal doorknob) can be a nuisance in dry rooms.
relative humidity of 50%. If that air is brought indoors and heated to 21°C
(70°F), its relative humidity drops to about 2%--well below the optimal
winter indoor relative humidity of 30% to 50%. At such very low relative
humidity, people often feel discomfort caused by dry skin and irritation of the
mucous membranes in their nose and throat. Also, wood furniture dries out and
may crack and become unjointed. Discharge of static electricity (e.g., after
walking across a carpet and touching a metal doorknob) can be a nuisance in dry
rooms. </P>
<P>One remedy for excessively low indoor relative humidity is a device known as
a humidifier. A humidifier elevates the relative humidity of indoor air to more
comfortable levels by evaporating water into the air (increasing the vapor
pressure). A common humidifier design consists of a wheel that continually
rotates a porous belt (or pads) into and out of a reservoir of water. A fan
blows air through the wetted belt, water evaporates into the air stream, and
the more humid air circulates through the dwelling.</P>
<P>On the other hand, a dehumidifier may be desirable during warm muggy summer
days. In addition to discomfort caused by reduced evaporative cooling, high
values of indoor humidity can cause increased incidence of mold and mites. A
dehumidifier lowers the relative humidity of indoor air to more comfortable
levels by inducing condensation of water vapor (decreasing the vapor pressure).
In a standard dehumidifier, a fan draws humid air past a cold refrigerated
coil. Air in contact with the coil is chilled to saturation, water vapor
condenses on the coils and drips from the coils into a collection reservoir or
through a hose into a drain. Most dehumidifiers function best when the air
temperature is at or above 18°C
One remedy for excessively low indoor relative humidity is a device known as a humidifier. A humidifier elevates the relative humidity of indoor air to more comfortable levels by evaporating water into the air (increasing the vapor pressure). A common humidifier design consists of a wheel that continually rotates a porous belt (or pads) into and out of a reservoir of water. A fan blows air through the wetted belt, water evaporates into the air stream, and the more humid air circulates through the dwelling.
On the other hand, a dehumidifier may be desirable during warm muggy summer days. In addition to discomfort caused by reduced evaporative cooling, high values of indoor humidity can cause increased incidence of mold and mites. A dehumidifier lowers the relative humidity of indoor air to more comfortable levels by inducing condensation of water vapor (decreasing the vapor pressure). In a standard dehumidifier, a fan draws humid air past a cold refrigerated coil. Air in contact with the coil is chilled to saturation, water vapor condenses on the coils and drips from the coils into a collection reservoir or through a hose into a drain. Most dehumidifiers function best when the air temperature is at or above 18°C (65°F).
Click on forecast map for locations across the country where today's indoor relative humidity is expected to be too high or too low in the absence of any controls on indoor humidity.
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Prepared by AMS WES Central Staff and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.