What Relative Humidity means for indoor air quality

Water is a powerful chemical, capable of digging massive canyons, destroying property, and of course sustaining life. Almost all of the water on Earth is either liquid or ice and at any given time only 0.001% is evaporated in the atmosphere as water vapor. But even that tiny slice is still millions and millions of gallons floating around the air. Depending on the altitude and local temperature, the concentration of water in the air could be anywhere from a tiny fraction of a percent in cold regions to as much as 25% in the tropics.

It helps to know how the water content in our air, also known as the humidity, impacts our lives. Most public services and consumer electronics that detect humidity use the same measure, relative humidity. While higher relative humidity does mean more water is in the air, it’s not quite as simple as it sounds. In this post we’ll cover exactly what relative humidity is and how you can use it to figure out how the water vapor in your air is affecting you and your indoor space.


How we measure water vapor

Relative humidity (RH) is expressed as a percent but does not tell us the actual percentage of water in the atmosphere. This is because air can contain different amounts of water vapor depending on the ambient temperature. The actual physics are a little challenging to understand, but basically hotter air holds more water vapor and cooler air holds less. 

At 68?, a cubic meter of air can hold 18 grams of water. At 77?, it can hold 22 grams, which is 22% more water. If a cubic meter of air at 77? holds 22 grams of water, the RH is 100%. Correspondingly, if that same cubic meter of air held 11 grams of water, the RH would be 50%. As the temperature goes up, the RH goes down with the same amount of water vapor, and the opposite with decreasing temperature.

RH is most useful to us as a scale. At 0% RH there is no water in the air. At 100% RH there is so much water in the air that it will condense into liquid (or ice, if it’s cold enough) on any nearby surfaces that are colder than the ambient temperature. That’s because any cold regions in the room hold less water than the warmer room air. So 90% RH in a cold climate has much less water than 90% RH in a hot climate. This is why you can literally feel the humidity in tropical areas, there is simply a lot more water present in the air.

Hygrometer and thermometer combination

Ideal RH for human living conditions is usually recommended between 30% and 60%. Your local weather website can typically tell you what the RH is outside. If you want to know the RH inside, get a humidity meter, sometimes known as a hygrometer. They typically also contain thermometers, are less than $10, and widely available.


Indoor Relative Humidity and you

You are already familiar with the most obvious way that RH affects you— comfort. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), an organization that creates building standards for health, safety, and efficiency has shown in research that comfort is a combination of RH, temperature, air flow, metabolic rate, and degree of clothing, and have created a standard for comfort. They have a very complex online tool to calculate how these factors interact to make an indoor space comfortable. But the bottom line is that higher temperatures mean more humidity can be in the air so a lower RH is more comfortable at higher temperatures.

If the RH is 100% that means no more water can evaporate into the air. This can be problematic if it’s hot outside and you’re sweating. Sweat cools us off by evaporating and taking heat away with it, but that can’t happen if the air is already saturated with water. Sweating in very high humidity isn’t likely to cool you off, just soak you down. This is particularly true when sleeping, if you sleep in high RH your body can’t regulate temperature as well which can disrupt your sleep.

You may have seen the heat index (also known as the “apparent” temperature or the “feels like” temperature, copied below) on the news during hot weather, this is a measure of perceived temperature when taking into account that sweat isn’t as effective in high RH. If it’s 90? out and the RH is 80%, it feels like 113? at low RH.

The NOAA Weather Service Heat Index

The National Oceanic and Atmospheric Administration Weather Service Heat IndexWhile high RH can be uncomfortable, a large amount of water vapor in the air won’t just affect you, it will affect all the little things that live in your home. Dust mites, for example, cause allergies and thrive at an RH greater than 50%. If you want to restrict dust mite growth it’s best to keep your RH below 50%. Another potentially problematic group of organisms that grow indoors are fungi, which also may cause health problems. Fungi grow at over 80% RH, so if the humidity in your home is regularly high, mold may grow in the spaces where water condenses.

Low humidity may feel less uncomfortable, but comes along with its own issues. At a basic level your skin can lose moisture, which may lead to cracking, wrinkles, or other damage to the barrier that protects your internal organs from drying out or being infected with pathogens. Low RH also dries out your lungs, making you more susceptible to allergies, asthma, and respiratory infections such as influenza or even Covid-19.

Also, static electricity passes through the air better at a RH below 30%, so if you work with electronics or just don’t like being shocked then changing the humidity could be a solution. Finally, the air may seem colder if the RH is low, so modulating the humidity in your home can also save on power bills.


Humidity sources and sinks

So you have a cheap RH meter and you find out that your RH isn’t where it should be. There are a few different things that you can do to solve the issue.

Depending on where you live, ventilation can be a great solution to humidity. If the air outside has better conditions than indoors, opening the windows is always a good plan. Some rooms don’t have windows, so if possible installing an exhaust fan can help. This is particularly true in bathrooms, all of which should ideally have an exhaust fan.

Use a humidifier or a dehumidifier. We’ve covered humidifiers extensively in other blog articles, but basically they just boil water and release it into the air to increase RH. Dehumidifiers do the opposite, they condense humidity into liquid so it can be poured down the drain. With either device, care should be taken to be sure that the RH doesn’t get too high or low, so rely on your trusty RH meter to be sure the humidity is between 40% and 60%.

If you have indoor plants, they may be causing humidity problems. Wet soil from a watered potted plant will evaporate, contribute to the water vapor in the air, and turn your room into a jungle. If RH is too high, consider moving plants outdoors or ventilating after watering.

Keep in mind that the way indoor climate is controlled usually leads to dryer air. If it’s cold outside then the air is probably very dry since cold air can’t hold much water vapor, so heating it up in your home will create warm dry air with a very low RH. Using a humidifier in that dry air will not only prevent you from drying out, but will make the air seem warmer. However, the opposite is not true for chilling air. The principle of refrigeration works by removing water from the air, so the air from your air conditioner is already dry and a dehumidifier won’t make it seem cooler.

We hope this helps you to understand Relative Humidity. It can cause all sorts of problems, so consider the steps above to fix your RH if it’s outside a comfortable range. If you’re having any problems with air quality related or unrelated to humidity, consider an air purifier to help reduce pollutants.


Header image by Brocken Inaglory
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