All About Humidity

One of the conditions people often wish to monitor is humidity. Because of its impact on health, home comfort, energy costs, and sensitive equipment, a clear understanding of humidity will help one maintain optimal conditions. As you learn more about humidity, especially very high and low levels, you will see the importance of a quality hygrometer.
What is Relative Humidity?
Everyone is familiar with the word "humidity," especially as it applies to one's comfort indoors or outdoors. We can feel the humidity on the hot, sticky days of summer, and we know it is low when static electricity shocks us during cold, dry winters. But few people understand the science behind humidity or what is meant by the more precise term "relative humidity."
The National Oceanic and Atmospheric Administration (NOAA) defines relative humidity as:
"A dimensionless ratio, expressed in percent (% RH), of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present."
For example, if the air contains half as much water vapor as is possible, then the relative humidity reading would be 50% RH. Bear in mind that the warmer the air temperature, the more moisture it can hold, and vise versa. This is an important factor to remember when looking at a particular RH. Using the same example, there is less water vapor at 50% RH (65°) than 50% RH (85°). Almost every meteorologist and hygrometer expresses humidity in this fashion.
Humidity and Comfort
Our comfort, whether we feel warm or cold, is determined, among other factors, by the rate at which moisture is evaporated from our bodies. It is this fact which makes the humid summer day so uncomfortable. Why? Because there is already so much moisture in the air that the moisture from our skin evaporates very slowly. Therefore, as we perspire, we feel sticky and are generally uncomfortable. Conversely, if the air is dry, evaporation is much more rapid; the more rapid the evaporation, the cooler we feel.
Humidity and Health
Air has a tremendous need for moisture. So when we heat our buildings in the winter (drawing in cold, dry air), this air is going to take moisture from wherever it can. This dry air in our homes and offices not only dries our skin, but also robs the delicate membranes of the nose and throat of their normal moisture. Low humidity may make us more uncomfortable or even subject to various respiratory problems.
However, there may be more effects of both high and low humidity than just discomfort. Studies show that humidity may affect three groups of factors with respect to health:
    1. Biological contaminants including bacteria, viruses, fungi, and mites.
    2. Pathogens causing respiratory problems including allergic rhinitis and asthma.
    3. Chemical interactions including ozone production. Some of these factors may thrive at low levels of RH while others may prefer high levels of RH.

    The chart shows by the shaded areas the %RH that various organisms may prefer. As the shaded area decreases in size, there is less effect of humidity on a particular factor and vice versa. For example, certain bacteria thrive and grow at very low levels of humidity (0-25% RH) while other bacteria grow and thrive at very high levels of humidity (65%-100% RH). The shaded area labeled "OPTIMUM ZONE," from 40% RH to 60% RH is an estimate of the RH range to minimize the possible effect of most factors. During cold weather this range may be 30% to 60% RH.

    Humidity and Energy Costs

    In winter, heated, non-humidified air may dry out and/or shrink wood framing around doors and window frames. Gaps may occur allowing cold, dry outside air to enter the home. This heat loss causes heating systems to output more dry air. To maintain a certain humidity level, many people compensate with the use of a humidifier. However, since it takes four times as much energy to heat water than to heat dry air, it costs more to maintain a specific humidity level in your home for health and comfort reasons. Monitoring the humidity and careful attention to areas of heat loss will help offset costs.

    The benefit of humidified air is its effect on how we feel in certain temperatures. In the winter, the air in our homes is dry and the increased evaporation of moisture from our skin makes us feel cold. While 70°F is recommended for indoor air temperature, some find that the temperature in our homes (when dry) must be near 80°F or even higher for us to feel warm enough to be comfortable. The proper humidity will make 70°F feel comfortable and may help offset the increased energy it takes to heat humidified air.

    Humidity and Home Furnishings

    Low humidity in winter may cause drying and/or shrinking of furniture, wood floors and interior trim. Doors and drawers may warp or crack and glue joints in fine furniture and veneers may open or split. Low humidity may rob plants of their moisture, and it may contribute to wall and ceiling cracks. All of these problems may be the result of dry air absorbing moisture from whatever source it can find inside your home.

    High humidity can also cause problems. For example, the keys of a piano may stick and strings may rust. Buildup of moisture may also cause rotting of wood, mildew and mold. In areas of your home where humidity may be very high, such as bathrooms, laundry rooms, or kitchens, it is important to prevent damage to your home from excessive moisture. Use of an exhaust fan or additional ventilation may be helpful.


    For the cigar enthusiast, proper humidity as it applies to tobacco storage is very important. The very word "humid" is incorporated into this cigar storage case as its purpose is to maintain a humidity ideal for cigars. By simulating the atmospheric conditions found in tobacco growing regions of 70°F and 70 % RH, a humidor preserves cigars and tobacco.

    Humidity and Electronics

    By touching your computer when humidity is very low, you may discharge static electricity, which in turn may cause damage to your home or office computer. Static mats are available to minimize this problem, but they do not change the environment, which produces static electricity. Adding humidity to the air can reduce or eliminate this problem.

    When humidity is very high, moisture damage to your computer may occur. Please consult your operating manual for specifications regarding humidity and your equipment's operation.

    Musical instruments need to be kept from humidity that is too high or low. While wooden instruments such as violins or guitars can dry out, altering their "tune," brass instruments not only tarnish in high humidity, but can also slow moving parts. Our Caliber Hygrometer is a great way to monitor humidity inside their cases.


    We hope this article helps you understand humidity and some of its possible effects. We sell a number of quality hygrometers to assist you in monitoring humidity and/or temperature.

    Recommended Hygrometers

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    Nothing in this article is to be construed as medical or professional advice. We urge you to seek advice from your physician and Heating/Air-conditioning (HVAC) specialist about particular problems in your home or office.


    Questions? Call us Toll Free 1-800-646-1203 for friendly, expert advice.