Relative humidity levels can affect the release rate of many indoor air pollutants, their
concentrations in the air, and the potential growth of mold organisms. Keeping the Relative Humidity (RH) in the
ideal range will help indoor air pollution prevention efforts. The following chart shows you what happens at
different levels of RH. Note that humidity levels from 0% to 100% are shown on the bottom horizontal line.
Relative humidity levels are often ignored in homes despite the fact that unsuitable levels can
be unhealthy, increase heating and cooling costs, damage building components, and affect comfort levels.
Research has shown that high-relative humidity levels also support the growth of dust mites,
mold count levels and bacteria levels that can lead to increased allergy symptoms and reduce indoor air quality
(IAQ). However, little to no measured data is available on actual indoor humidity levels in homes across the United
The key to controlling relative humidity levels in a home is to maintain a moisture balance.
Yes, some moisture in the air is necessary for human health and comfort, but too much moisture can cause problems.
A moisture balance is maintained by making sure a home has exterior drainage away from the house in all directions,
a properly sized heating and cooling system, vapor diffusion protection (through the installation of vapor
retarders where needed), as well as adequately planned ventilation, especially at points where moisture is
produced. This means that a kitchen stove needs an exhaust fan over it, which should be ducted to the outside. A
circulating fan may remove some cooking odors from the air, but it will not remove moisture. A bathroom also needs
an exhaust fan that leads to the outside. This fan should be used when anyone is taking a shower and for a short
time afterwards, until moisture levels have decreased to the point that there is no condensation on windows and
mirrors. In the laundry room, a clothes dryer should be vented to the outdoors. Having good quality windows is also
important. For example, when the temperature inside is 700F and 1000F outside, condensation
can occur on the interior surface of a single-pane window when the indoor relative humidity level is at only 15
percent RH. Condensation can occur on a double-pane window at 42 percent RH. By increasing the energy efficiency of
your windows, therefore, you can lessen the incidence of condensation because the inside glass surface does not get
as cold. For more information, please refer to the Indoor Air
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