The Science of Humidity and how it affects us

Humidity is super-important to our health, and the health of the spaces in which we live (which in turn affect our health).  But do you know how humidity is measured and how it affects us?  

Humidity is the concentration of water vapor in the air.  Necessary to life, we need water in the air we breathe, as well as to drink.  Water vapor hydrates our respiratory pathways and secondarily, enables us to stay warm–or cool.  Humidity is measured in percentage–but a percentage of what?  

Sometimes it’s easier to imagine a liquid, instead of a gas.  If you’ve ever dissolved sugar into water and keep adding sugar, there comes a point where the water will no longer hold any more dissolved sugar–the sugar will lay at the bottom of the container–at that temperature.  In order to dissolve more sugar, you will need to increase the temperature of the water.  This is the saturation point of the water for sugar.  Air works in the same way.  There is a maximum amount of water vapor that the air can hold before–you guessed it–it starts raining.  To increase the amount of water vapor, air temperature must increase.  Warmer air can hold more water vapor than cooler air, the same as warmer water can hold more sugar than cooler water. 

So then, “relative humidity” is the percentage of maximum water vapor the air can hold at a specific temperature.  It’s all “relative” to the temperature.  If you had 2 rooms at the same temperature (say 78 deg F), but one had 60% percent humidity and the other had 40% humidity, the 60% humidity room is holding a lot more moisture in the air, at the same temperature.  It is a noticeable difference.

Humidity is usually higher during summer months.  In the US, we are accustomed to the Heat Index (HI), “a measure of how hot it feels when relative humidity is factored into the air temperature.” (MintLounge, 2020)  But a more accurate picture of the stress heat and humidity have on the body is given by “wet bulb temperature”.  Essentially, a wet "sock" is put over the bulb of a thermometer, and it measures “the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure.... (In contrast) The dry bulb temperature is the ambient temperature. The difference between these two temperatures is a measure of the humidity of the air. The higher the difference in these temperatures, the lower is the humidity.” (, 2007).  It is very important to human life because it governs how evaporation of sweat into the air cools our bodies.  Think of the thermometer in a wet sock as your body covered in sweat.  When wet bulb temperature = ambient temperature, the sweat is not evaporating, because humidity = 100%.  If we dial up the heat, no evaporation allows no heat loss, which causes rapid overheating.  Scientists have marked 95 degrees F (35 degrees Celsius) at the upper limit of survivability: at or above this temperature/humidity combination, the body will overheat, causing organ and brain damage and death.   During this research, I looked at a country that knows heat: India.  India has been suffering with extreme heat for several years now.  2015 saw the country’s fifth deadliest heat wave (2400 people died), and other records were set in 2016 and 2019.  

Compared to the heat index in summer, wind chill is that mystifying element that makes cold northern winter days (usually drier, lower humidity) seem bearable and damp southern winter days seem very chilling!  Higher humidity causes the clothes we wear to retain moisture, which being next to our skin, will wick heat from our bodies, chilling us. 

If then, “feels like” temperature is most important, we can focus on controlling the humidity in our homes along with temperature.  

Humidity is super-important in heating and cooling.  Air kept at lower humidity is less costly to heat and cool.  Why is that?  Going back to the liquid analogy, think of boiling water on a stove or microwave or kettle.  Which takes longer to boil at the same heat level: 1 cup of water, or 3 cups of water?  Three cups, of course!   When trying to cool a humid room, the same principal applies–most of the energy is spent actually changing the temperature of the water vapor in the air.  Therefore lower concentrations of water in the air (lower humidity) takes less energy to cool.  The same thing happens in the winter: overly humid air takes longer (more energy) to heat.  According to, “hot and humid areas use 21.1% of their energy on air conditioning each year, while hot and dry areas only use 9.6% of their energy to air condition.”

Most importantly, the effects of high and low humidity have direct impacts on our health.

Water vapor in the air (humidity) is essential to keeping our nasal passages hydrated.  When the humidity drops low, the air is dry, and nasal passages feel congested because they are dried out and inflamed.  Eyes become irritated because tear production cannot keep up.  Skin becomes dried out, feeling itchy and more susceptible to infection.  In the age of coronavirus, air with low humidity is more hospitable to the spread of the virus in aerosol form (fine droplets that occur when someone sneezes or coughs).  This is because the water entrained with the virus quickly evaporates in dry air, allowing the virus particles to float indefinitely.  Drier virus particles actually survive longer by floating in the air than those encapsulated in heavier water droplets, which fall quickly to the floor. (

On the high side, too much humidity is also bad.  Excess water vapor in the air contributes to mold growth, more pests (insects and mites), and structural damage due to mold and rot. 

For many years, the optimal humidity range of indoor environments was 30-50%.  Due to the current coronavirus pandemic, some doctors are petitioning for the World Health Organization to change the recommended range to 40-60% relative humidity (  What is the top factor on CNN’s list of ways to prevent flu in your home?  Control humidity!

Humidity is a huge factor for optimizing our home environment, then!  How do we measure and control it?

To measure humidity, sensors like these are cheap and easy to place around your home (this particular type have built-in batteries, but you should check the battery requirements of others before purchase). 

Now–what do I do if I’m below 30%?  Typically, low humidity (below 30%) occurs in winter months when we turn on indoor heat, making skin and nasal passages dry and irritated.  Humidifiers help by introducing moisture into the air.  You can place a portable humidifier in the most-used room during the day, and move it into your bedroom at night for more comfortable sleeping.   If you are not sensitive to fragrance, some humidifiers allow you to add a few drops of essential oils for a pleasant scent.  Whole-home humidifiers can also be installed in your HVAC system.  If you are looking to increase humidity in your home as well as clean the air, you may consider an “air washer”.  This type of unit uses a stack of thin discs to capture dust in the air, which is then “washed” off the disc in a water reservoir.  Clean air that is exhausted from the unit is more humid.  It is a low cost air purifier/humidifier combination that only requires regular addition of water and rinsing out the disc stack.

What do I do if my home is above 60%?  Now this is a more common and complex problem.  Sometimes it is the outside climate coming inside through air leaks, and sometimes it’s a source of water inside the home that can be corrected to bring indoor humidity down.  You can check out our Indoor Moisture Inventory which will walk you through the most common ways to reduce indoor humidity.   If it's still too high after mitigation, you can hire a professional to perform calculations and recommend dehumidifier units, or in this age of DIY, you can actually calculate and research (and install!) units on your own. 

Here is a simple table that takes into consideration the square footage of your home and it’s “tightness” (insulation level and leaks).  According to this, I need about an 8.0 gallon-per-day dehumidifier unit for my 2000 square foot “average” home.   Here are how the measurements are factored into the sizing.   However, since my AC system is older (more than 10 years old), it would most likely be most cost-efficient to replace the whole system with a new system that includes a dehumidifier!  

Humidity control should be a top priority in all of our homes and indoor spaces–why not take some steps today?