The relative humidity of the air affects how comfortable we
feel. But what is humidity, and what is "relative humidity"
Humidity is defined as the amount of
moisture in the air. If you are standing in the bathroom after a
hot shower and can see the steam hanging in the air, or if you are
outside after a heavy rain, then you are in an area of high
humidity. If you are standing in the middle of a desert that has
not seen rainfall for two months, or if you are breathing air out
of a SCUBA tank, then you are experiencing low humidity.
Air contains a certain amount of water vapor.
The amount of water vapor any mass of air can contain depends on
the temperature of that air: The warmer the air is, the more water
it can hold. A low relative humidity means that the air is dry and
could hold a lot more moisture at that temperature.
For example, at 20 degrees C (68 degrees F), a
cubic meter of air can hold a maximum of 18 grams of water. At 25
degrees C (77 degrees F), it can hold 22 grams of water. If the
temperature is 25 degrees C and a cubic meter of air contains 22
grams of water, then the relative humidity is 100 percent. If it
contains 11 grams of water, the relative humidity is 50 percent.
If it contains zero grams of water, relative humidity is zero
The relative humidity plays a large role in
determining our comfort level. If the relative humidity is 100
percent, it means that water will not evaporate -- the air is
already saturated with moisture. Our bodies rely on the
evaporation of moisture from our skin for cooling. The lower the
relative humidity, the easier it is for moisture to evaporate from
our skin and the cooler we feel.
You may have heard of the heat index. The chart
below lists how hot a given temperature will feel to us in various
If the relative humidity is 100 percent, we
feel much hotter than the actual temperature indicates because our
sweat does not evaporate at all. If the relative humidity is low,
we feel cooler than the actual temperature because our sweat
evaporates easily; we can also feel extremely dry.
Low humidity has at least three effects
on human beings:
It dries out your skin and mucous
membranes. If your home has low humidity, you will notice things
like chapped lips, dry and itchy skin, and a dry sore throat
when you wake up in the morning.
It increases static electricity, and
most people dislike getting sparked every time they touch
It makes it seem colder than it
actually is. In the summer, high humidity makes it seem warmer
than it is because sweat cannot evaporate from your body. In the
winter, low humidity has the opposite effect. If you take a look
at the chart above, you'll see that if it is 70 degrees F (21
degrees C) inside your home and the humidity is 10 percent, it
feels like it is 65 degrees F (18 degrees C). Simply by
bringing the humidity up to 70 percent, you can make it feel 5
degrees F (3 degrees C) warmer in your home.
Since it costs a lot less to humidify the air
than to heat it, a humidifier can save you a lot of money!
For best indoor comfort and health, a relative
humidity of about 45 percent is ideal. At temperatures typically
found indoors, this humidity level makes the air feels
approximately what the temperature indicates, and your skin and
lungs do not dry out and become irritated.
Most buildings can not maintain this level of
humidity without help. In the winter, relative humidity is often
much lower than 45 percent, and in the summer it is sometimes