Humidity Defined. What is Humidity?

David J. Allen

I spent the better part of 2 ½ years learning about mold and indoor air quality because my child's life depended on it. Now I help others avoid ever having to experience the same thing. The mold industry suffers from conflicting opinions and I do my best to distill loads of data into something practical and useful. I love hearing from and helping readers. • "A house desecrated by mildew, mold, or fungus would be a defiled place to live in, so drastic measures had to be taken." — Leviticus 14:45

Humidity is defined as the amount of water vapor present in the air, as measured on a scale of 0 to 100.

Humidity measures water in its gaseous or vapor state, as opposed to its liquid state or its solid state.

The higher the humidity, the more water there is present in the air; the lower the humidity is, the less water there is present in the air

Zero percent humidity is neither ideal nor practically attainable in a non-controlled environment or laboratory, and some humidity is desirable to aid in our breathing, hydration, and health.

But at levels above 50%, humidity begins to present a mold and mildew concern because the air itself holds enough moisture to support the growth of microscopic mold, Size of Mold Spores

To help prevent mold and mildew growth, indoor humidity should be kept between 30% and 50% at all times throughout the day.2CDC, Basic Facts About Mold and Dampness, reviewed November 14, 2022. Accessed August 28, 2023.

The higher humidity goes above 50 percent, the more likely mold is to grow indoors, as the air itself would contain enough moisture to support mold growth.

Measuring and reducing humidity

An inexpensive home hygrometer will measure and display the relative humidity in a, Hygrometer for Mold

A dehumidifier will help you reduce the relative humidity in a room.

A properly-sized and working HVAC system should do most of the work involved in maintaining healthy indoor humidity levels of between 30-50% RH, and it should not typically be necessary to run a whole-home dehumidifier if a home’s HVAC system is working properly.

Running a humidifier to reduce indoor humidity levels is better than not doing so, if conditions call for it, and running one may be necessary in basements, particularly if they are furnished or being lived in.

Opening windows to increase airflow can help reduce humidity, provided outdoor humidity levels aren’t also high.

Running ceiling fans will also reduce humidity.

Absolute vs Relative Humidity

Humidity can be measured as absolute humidity, or as relative humidity.

Absolute humidity refers to the total mass of water vapor in a specified volume or mass of air—not taking air temperature into consideration.

Because air temperature is a factor in the dew point, or the point at which water in the form of vapor in the air becomes droplets of liquid water, the relative humidity (RH) measure is better for mold professionals and most other practical uses of the scale.

The amount of water that air can hold varies with the temperature of the air: Warm air holds more moisture than cool air can hold.

That doesn’t mean warm air is always more humid, but that warm air has the potential or capability of holding more gaseous water than cool air has the potential to hold.

When water droplets form on the outside of a glass of ice water sitting at room temperature, that difference is on display.

Relative humidity is the ratio of how much invisible water vapor is in the air now, versus how much water vapor the air could potentially hold at a given temperature.

This ratio is expressed as a percentage of 0-100, with 0% being neither ideal or practical, 50% being a safe level for mold prevention, and 100% being the dew point, or the point at which the present air at its current temperature can hold no more water as vapor or gas, and thus converts it into liquid water.4JPL, NASA, “Understanding Climate; Air & Water,” accessed Aug 27, 2023

Homeowner summary

As far as humidity, homeowners or tenants concerned about mold or mildew should know simply that:

  • Indoor humidity levels should be kept between 30 and 50% to prevent mold.
  • That humidity levels can be measured with an inexpensive hygrometer and controlled through the use of a proper HVAC system, open windows, ceiling fans, or dehumidifiers.
  • That if humidity levels rise above 55% for 24-48 hours, mold can grow.
  • That turning off air conditioners for a prolonged period, such as during a vacation or business trip, will very often result in mildew or mold growth.
  • And, lastly, that a home hygrometer reading of below 50% does not mean that hidden pockets of water or moisture don’t exist anywhere in the home. Thus, leaks, slowly dripping pipes, and faulty roofing or basement sealants should also be looked for and corrected to avoid mold growth.


Leave a Reply

Your email address will not be published. Required fields are marked *