You probably don’t think much about how dry or how moist your air is until it’s at one extreme or the other.

Is your skin cracking and bleeding for no reason? It could be dry air. Just hopped out of the shower and can’t seem to get the moisture off your body? It’s probably really humid in the house.

The moisture in the air is measured by relative humidity (RH). This refers to how much water vapor is in the air in relation to how hot or cool it is. The most comfortable RH level is thought to be between 30-50%. Any more than 50% can cause bacterial growth.

Normally, our climate does most of the work of controlling RH.

But in extreme cases, air that’s too humid or too dry can result in symptoms like coughing, shortness of breath, and fatigue. It can even make conditions like allergies and asthma worse by drying out your airways.

So here are two possible answers to your problems with air that’s too dry or too humid:

  • Humidifier: Adds extra moisture to the air.
  • Dehumidifier: Removes excess moisture from the air.

Let’s get into how these two devices work, how they can benefit your health when used indoors, and how they compare to air purifiers.

Humidifiers basically add moisture to the air to increase the humidity of an indoor space. Here’s how humidifiers work, as well as the different types of humidifiers you can buy.


There are two main types of humidifiers: cool-mist humidifiers and warm-mist humidifiers, also sometimes called steam vaporizers.

They both add moisture to the air equally well. There’s not necessarily an advantage to one or the other besides your personal preference.

That said, how they create moisture might make a difference in your choice between the two.

Here’s how some common types of cool-mist humidifiers work:

  • Impeller: A plate made of ceramic or metal called a diaphragm vibrates at extremely high speeds to stir up water into tiny droplets that exit the humidifier through an opening to go out into your air.
  • Ultrasonic: A disc immersed in water spins at high speeds and generates tiny water droplets in the form of steam that exit the humidifier into the air.
  • Evaporators: Cool air is blown by a fan through a wet object that moisturizes the air, such as a belt or filter.

In short, cool-mist humidifiers break up water into tiny particles that then enter your air as water vapor. The water starts cold and stays cold, so they can help cool your air, too.

Here’s a general idea of how most steam vaporizers work:

  1. The humidifier uses a power source, like a plug into an electrical outlet.
  2. The energy from the electricity heats a container filled with water until it’s boiling.
  3. The boiling water produces steam that’s cooled down before it exits the humidifier through an opening or a tube through which you can breathe the steam.

Steam vaporizers can also be used with inhalants or essential oils that may be helpful for people with allergies or asthma.


Dehumidifiers do the opposite of humidifiers. These devices take moisture out of the air and help keep your indoor air dry. A typical dehumidifier can reduce your indoor air’s RH to a more comfortable 30 to 50 percent.

Here’s how the typical dehumidifier works:

  1. The dehumidifier takes in warm air through a fan that sucks air into metal coils that are cooled down by a refrigerant that uses chemicals to reduce the temperature of the coils.
  2. Warm air essentially “shrinks.” Remember thermal expansion from science class? Hot things are bigger than cold things — and tiny water droplets fall out of the air as it shrinks.
  3. Water droplets leave the warm air as condensation that’s stored in a tank on the outside of the dehumidifier.
  4. The cooler air that’s had its moisture removed is blown back out into your indoor environment through a tube powered by a fan.

Humidifiers can help add moisture to the air when your air is too dry. Dry air can exacerbate allergy and asthma symptoms and make you more susceptible to airway infections and other issues, including:

So which humidifier should you use?

Cool-mist humidifiers are safer because they don’t use hot water that can burn you if you spill it. This makes them safer to use around children.

But cool mist can also carry airborne bacteria or viral material if the humidifier isn’t cleaned regularly or properly. Steam vaporizers boil away bacteria or pathogens in the water before releasing steam.

If you have children or pets, you may want to stick with a cool-mist humidifier. The hot water inside steam vaporizers may burn if it’s spilled.

Now what about dehumidifiers?

Dehumidifiers help remove excess moisture from the air. This is especially helpful if you live in a humid climate or if your home has recently had a leak or flooding.

Excessively moist air can cause mold and dust mites to thrive in your home, causing allergic reactions and triggering severe asthma symptoms. Hot humid air can cause airway constriction as the result of nerve responses.

Dehumidifiers can also help when:

  • you have chronic or seasonal allergies
  • you’ve just moved somewhere new where your allergy symptoms are worse
  • your home smells wet or there are excessively moist areas of your home
  • your home leaks after rain
  • you see water vapor in your air at home or notice that the air feels heavy and wet
  • you’re allergic to dust mites
  • you have too many pests like spiders or silverfish
  • your clothes take a long time to dry when you line-dry them
  • you find yourself coughing or having runny noses frequently

Here’s how a humidifier or a dehumidifier can help you with a few common respiratory conditions:

  • Asthma: A humidifier can help moisten dry air that can dry out your airways and increase your risk of infections. A dehumidifier can reduce excess moisture and make it easier to breathe as well as keep airborne pollutants from remaining suspended in the air.
  • Allergies: Humidifiers can add moisture to dry air that contains allergens and help you breathe easier by reducing sinus inflammation. They can even help with non-allergy sinus problems. Dehumidifiers can help dry excessively moist environments where allergens like dust mites and mold can thrive.
  • Bronchitis: A humidifier can help reduce symptoms and occurrences of bronchitis. A dehumidifier can also reduce mold and bacteria that may trigger bronchitis.
  • Cold: A humidifier can help relieve cold symptoms like coughing and wheezing.

Humidifiers and dehumidifiers can also help keep your indoor air at a comfortable RH level for babies and young children, especially if they have allergies or asthma. Everyone can breathe a little easier when humidity is controlled.

Air purifiers simply draw air in from your indoor space, filter particles and other air pollutants from that air, and then circulate purified air back into the room.

There’s no humidifying or dehumidifying mechanism in most air purifiers.

In some cases, air purifiers can help reduce humidity by removing moisture from air as it passes through the filters. But they’re not as effective as dehumidifiers for this, and too much moisture, over time, can damage filters.

That said, air purifiers are also good tools for allergy and asthma treatment because they can remove airborne pollutants that can trigger respiratory symptoms.

Some air purifiers can also remove extremely tiny particles from the air called fine particles — often called PM2.5 because they’re smaller than 2.5 micrometers.

PM2.5 is small enough to pass from the air sacs in your lungs, called bronchioles, into your bloodstream and cause heart, lung, and brain conditions.

Some conditions that an air purifier can help include:

Humidifiers and dehumidifiers both have their uses when the humidity of your indoor air is too high or too low. You may want to have both ready for different times of the year when weather changes affect your RH.

Air purifiers can also work as dehumidifiers while also removing irritating pollutants from your air that can exacerbate allergy and asthma symptoms.

Consider using a combination of all three to keep your home at a comfortable, safe level of humidity and purified air. Together, they can help protect you from health issues related to your air quality.