What is “Relative” Humidity?

We have all heard the term relative humidity before. We know that it means something about how much water vapor is in the air.

And we know that relative humidity is important especially inside a building where we want to keep mold and other contaminants to a minimum.

But what is “relative” humidity, exactly? What is it relative to? And what does it mean to us individually?

The air we breathe is more than just air

We know that the air is far more than just oxygen. The atmosphere and air we breathe holds much more than just one gas. It holds a mix of Oxygen, Nitrogen, Carbon Monoxide, Carbon Dioxide, and lots of others, in a mix that our lungs are used to sorting through. Our sensitive lungs can filter out their favorite candy from this Halloween grab bag of ingredients.

But also suspended in the air is a host of minor and major pollutants, including pollen, dust, and mold. And also, there is water vapor in the air, and that causes a problem.

The problem with humidity

Humidity over 70% supports and carries a whole boatload of allergens, irritants, and contaminants. It carries more pollen, more dust, more smoke, and of course, more mold. All of these wreak havoc on your lungs, making them work harder in an effort to reject them.

Individuals with asthma know that humidity means asthma attacks because of these pollutants and irritants.

Humidity makes these problems worse, so we work to keep it at an acceptable level, usually no higher than 60%

Relative humidity, defined

But what is relative humidity?

Well it turns out that air can hold different amounts of water at different temperatures. The warmer the air is the more water that it can hold. Colder air can hold less water vapor.

So the word “relative”  doesn’t mean it is compared to other times, it means that the humidity is compared to other temperatures.

So a “relative humidity” of 60% can feel quite different, and represent different amounts of water vapor in the air, based on the actual temperature.

A relative humidity of 60% when it is 60 degrees is very different from a relative humidity of 60% when it is 90 degrees. It represents far more actual vapor in the air at the warmer temperatures.

Of course, since indoor temperatures are usually highly regulated, this doesn’t mean too much of a difference inside, but whenever the inside air feels humid, you should grow concerned about the presence of mold and other contaminants.

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