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Cashins & Associates Blog

Industrial Hygiene: What is Relative Humidity and Why is it Important?

Posted by Zachary Keefe on Fri, Aug, 15, 2014 @ 15:08 PM

G184We hear the term "Relative Humidity" quite often. It is generally viewed as a simple concept - how much moisture is in the air. It turns out that it's a bit more complicated than this (but not by that much).

Relative Humidity Explained

Oftentimes relative humidity is described as the amount of water (in the form of water vapor) that is in the air versus how much water vapor the "air can hold". This type of thinking reflects a misunderstanding of what constitutes "air". Air is a mixture of various gases - oxygen, nitrogen, carbon dioxide, water vapor, as well as many other chemicals that can be present at any time. Thus, water vapor is a basic constituent of air - not something that is "added" to it. To put it another way, you wouldn't say, given a jar full of red and blue marbles, that the blue marbles are "holding" the red ones. Similarly, the various gases that make up what we call air do not "hold" water per say.

There is, however, a limit to how much water vapor can be in the air at a given time - and this limit is decided by temperature. The more thermal energy present in the air, the more water vapor may be present. The less thermal energy present in the air, the less water vapor can be present. This is generally why warm summery days (with the exception of desert climates) tend to have higher levels of water vapor, and colder days tend to have lower levels of moisture.

However, relative humidity does not refer to the exact amount of moisture in the air. It is the amount of water vapor in the air (partial pressure) divided by the maximum possible amount that could be in the air (saturation water vapor) as allowed by the temperature.

For instance, right now, the partial pressure of water vapor in this room (how much actual water vapor is in the air) is 0.4 inches of mercury (inHg). 

The saturation water pressure (the highest number of water vapor molecules that could possibly be in the room) is 0.8 inHg.

We then put these values into the following equation:


Partial Pressure of Water Vapor         X 100
    Saturation Water Vapor


Thus, the relative humidity in this room is (0.4/0.8) x 100, which is 50%. That means the number of water vapor molecules in the air are half of the maximum number given the current temperature.


The following video does an excellent job of explaining all of this in a simple demonstration.



Relative Humidity in the Indoor Environment

Relative humidity is an important factor in a healthy and comfortable indoor environment. Levels below 30% can lead to eye, nose and throat discomfort, contact lens irritation, dry skin, and static electricity buildup. Relative humidity levels above 60% can lead to physical discomfort, as sweat is not allowed to readily evaporate from the body. Also, elevated relative humidity can ultimately cause mold growth, something that occupants certainly do not want flourishing in the indoor environment.

In most workplace environments, the relative humidity levels are controlled by the central HVAC system (at least during summer months). Warm, humid air is drawn into the units, and passes over cooling coils. As the air is cooled, the relative humidity raises until the air must release some water vapor in the form of liquid water. This water collects on the coils and eventually makes its way to the drain pan. It is then that the cooled, dehumidied air is delivered to the occupied space.

During the winter months, things tend to be different. HVAC systems draw in cool air that already has a low relative humidity. This air is then heated, which lowers the relative humidity even further. Unless the system has a built-in humidification system, this very dry air is then delivered to the interior spaces. These humidification systems do help in maintaining comfortable levels of relative humidity, but are costly and require constant maintenance.

Measurement of Relative Humidity

Thankfully, there are sophisticated devices available for the measurement of relative humidity, Keep in mind, though, these instruments are really only as good as the person using them. This is why it is important to retain the services of a qualified and experienced indoor air quality testing firm. A competent testing professional will sample the air at multiple times during the day. One set of data taken from one round of sampling is insufficient for coming to useful conclusions based on the data gathered. Further, the specialist should view the indoor environment as a whole - in other words, they shouldn't simply be looking at their instruments during the site visit. Knowledge and examination of the HVAC system is crucial to diagnosing potential IAQ problems. Finally, the reports issued should be clear, technically sound, and helpful. A simple readout of various data is not sufficient - it must be analyzed by the professional, explained in a coherent and useful manner, and be followed up by conclusions and recommendations.

So, if you think relative humidity may be a problem in your building, it could be that the HVAC system is malfunctioning. In this case, it will save money, time, and productive work hours to solve the problem as soon as possible.

So contact Cashins & Associates and let us help you building and maintain a healthy, productive, and comfortable indoor environment.

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Topics: industrial hygiene, indoor air quality, ventilation assessment, air testing, indoor air quality Issues, IEQ, Indoor Environmental Quality, IAQ, Relative humidity

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