The Importance of Humidity on Indoor Air Quality

5 min readMar 26, 2021


Control the humidity in your home to help control seasonal allergy symptoms.

The days have begun to feel longer again, a sure sign winter’s seemingly endless slog is finally complete. With spring upon us, now’s a great time for seasonal allergy and asthma sufferers to learn about how humidity impacts indoor air quality and how best to regulate it to help control symptoms of seasonal allergies.

What is humidity?

If you were paying attention in high school science class, you’ll recall that humidity is a measure of the water vapor present in the air. For our discussion here, we’ll focus on relative humidity -the concentration of water in the air relative to the maximum amount of water the air can hold at a certain temperature.[1] Relative humidity is expressed as a percentage: the closer to 0%, the less saturated the air is with water vapor and the closer to 100%, the more the air is saturated. As a means to visualize these percentages, consider that at 100% relative humidity air can hold no additional water and condensation occurs -rain!

What’s more, the higher the temperature, the more water vapor air can hold. This explains why during spring and summer months in warmer climates we can experience the surrounding air as feeling wet and heavy, if relative humidity is high. Conversely, cooler temperatures can hold less water vapor, so during winter months in cooler climes we may experience low relative humidity and dry, brittle air.

With the exception of a few areas that perennially lean towards either high or low relative humidity — the muggy swamps of the Florida everglades come to mind — in much of the country humidity levels can shift considerably with the seasons. Generally speaking, fall and winter tend towards cooler, dryer air with spring and summer months usually hotter and wetter.

Climate change, it bears mentioning, is already having an impact on these tendencies. Warmer land and ocean temperatures are leading to wetter, more humid environs throughout North America. Without wholesale climate mitigation, higher humidity levels are likely to continue, with the trend accelerating in coming years.[2]

Maintaining healthy humidity levels

Our efforts towards slowing the spread of COVID-19 have led many of us to spend more time indoors than would otherwise be typical. With adults working from home and distance learning for children the norm rather than the exception these days, maintaining high air quality in the home is imperative. Being well equipped to address changes in relative humidity can mean the difference between a healthy indoor environment and one rife with dust mites, mold, bacteria, fungi and other common airborne allergens. Proper indoor humidity also encourages plant growth and good health in pets.

As a rule of thumb, we should strive to maintain indoor relative humidity levels between 30–60% year round. When indoor air is more humid or wetter than this, dehumidifying — pulling moisture from the air — will be required. Dryer, less humid indoor air will require the opposite; in that case we’ll need to add moisture to the air, to humidify.

Where to start

First off, in order to best understand what adjustments may be necessary, you’ll need to monitor the relative humidity in your home. A hygrometer is a device that can measure water vapor concentration for just this purpose. Your local hardware store or online retailer is likely to stock a number of inexpensive hygrometer/thermometer combo home appliances that can help you keep track of the humidity levels and temperature of the air in a room within your home. Depending upon the overall size of the home, you may need to strategically deploy one or more throughout living spaces to ensure a fully accurate overview. Indoor air quality monitors which in addition to humidity and temperature can also track contaminants such as CO2, VOCs and particulate matter, may also be worth considering for allergy sufferers and those sensitive to subtle changes in air quality.

Winter Months

Dry, stale air in homes buttoned up tight to keep out winter cold can cause symptoms such as headaches, fatigue, dizziness and irritation of nasal and/or bronchial passageways that can mimic seasonal maladies such as flu, the common cold or even COVID-19. Static electricity charges also build up in dry air and can be annoying.

As many parents with babies and toddlers know well, utilizing a humidifier to keep bedroom air from getting overly dry helps nasal mucus to flow freely, thereby leading to healthier lung function. Proper moisture also promotes deeper, more restful sleep. Invest in a humidifier with an output appropriate for the size of your room. Be sure to thoroughly clean and descale it regularly to inhibit the growth of allergens and pathogens.

In a pinch, a large bowl or pot filled with water can also serve to add moisture to dry air. This technique works especially well in smaller rooms and spaces.

Spring and Summer Months

As mentioned, excessively moist indoor air is an ideal breeding ground for dust mites, mold and other allergen and asthma-inducing irritants. Also, some asthmatics can find it difficult to breath in overly humid environments.

For these reasons, during warmer months and in warmer, wetter climates, dehumidifying may be necessary. Luckily, the central and/or window-based air conditioning installed in many American houses and apartments — air conditioning is still relatively rare in Europe and elsewhere in the world — is often able to provide considerable dehumidification. The refrigerant utilized in air conditioner units achieves this by drawing both excess heat as well as moisture from the indoor air, and recirculating the dryer, cooler air back into a room.

Yet while air conditioning alone can often be quite effective at dehumidifying when outside air is hot and humid, a stand alone dehumidifier may be better suited to the task when temperatures aren’t that high but humidity is. Dehumidifiers are also generally more energy efficient than air conditioners, costing significantly less to run. Stand alone dehumidifiers can also be an important year round tool for humidity control in homes with damp, musty basements.

As with humidifiers, make sure to keep the interior components of dehumidifiers clean and descaled.

Other important tips for controlling humidity during spring and summer months: ventilate kitchens and bathrooms, often the most humid areas of many homes; limit dryer use, if possible hang clothes outside during spring and summer for fresher smelling laundry as well as less humidity wafting from laundry rooms into other areas of the home. And don’t forget the power of the humble ceiling fan for keeping indoor air moving to avoid humidity from building up and air from getting stagnant.

Monitoring and regulating the humidity in your home is among the most readily available pathways for generating more comfortable, less irritating indoor air for you and your family. As always, Cleared’s telemedicine and our team of allergists are here to help if you have further questions or if you would like to learn more.

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  2. Ethan D Coffel et al 2018 Environ. Res. Lett. 13 014001, (“Temperature and humidity based projections of a rapid rise in global heat stress exposure during the 21st century”)