What Is Relative Humidity, and What’s an Ideal Level for Your Home?

If you have allergies or asthma, you probably know that the humidity level inside your home matters. Excessive levels of humidity can cause mold growth, trap dirt and other allergens in the air and cause a number of health problems. Consistent exposure to low humidity can also cause health issues, especially throat, nose and eye irritation. In addition to health problems, too high or low levels of humidity may cause structural damage to your home. With the aid of a humidifier or a dehumidifier, controlling humidity is fairly easy to do and can help prevent poor air quality, exacerbated allergies and asthma and home damage. Generally, humidity most often refers to relative humidity, not absolute humidity.

Below, learn more about what relative humidity is, where it comes from, side effects of too low or too high humidity and how to maintain ideal levels of humidity in your home.

What is relative humidity?

Relative humidity is the amount of moisture in the air at a certain temperature compared to what the air can “hold” at that temperature. To be clear, air is not actually capable of physically holding onto water vapor because it moves too quickly. When the air in your home reaches a specific capacity of water vapor, it will condense as moisture, causing the air to feel damp. When relative humidity gets too high, usually above 50%, mold and mildew growth and trapped allergens become more likely which, in turn, may cause allergy and asthma symptoms to appear.

When you’re trying to figure out how to make the air in your home healthy, relative humidity is an important factor to consider. It allows you to assess the amount of moisture present, so you can understand how much humidity you have and decide which type of dehumidifier or humidifier is best for you. How much relative humidity is present in your home will depend on the region and climate you live in.

Mathematically, relative humidity is the gram-per-cubic-meter (g/m3) measure of the water vapor in the air divided by the gram-per-cubic-meter (g/m3) measure of the maximum amount of moisture the air can hold at a given temperature. The resulting number is multiplied by 100 to get a percentage, which is the relative humidity. Hygrometers and dehumidifiers can help you calculate the current relative humidity in your home.

What is the difference between absolute humidity and relative humidity?

Absolute humidity is the actual amount of moisture in the air. The National Weather Service describes it specifically as a measure of the amount (g/m3) of water vapor in the air or atmosphere, regardless of the temperature. The more water vapor in the air, the higher the absolute humidity. What we “feel” as humidity in the air is the absolute humidity.

Relative humidity is measured as a percentage or ratio of the amount of water vapor in a volume of air RELATIVE to a given temperature and the amount it can hold at that given temperature. Warm air can hold more moisture than cold air. This means that at the same absolute humidity, the relative humidity can be lower in warm air and higher in cold air.

Though they may seem similar, the terms absolute humidity and relative humidity cannot be used interchangeably.

What makes air healthy?

Most people spend a majority of their time indoors. You may be surprised to find out that levels of indoor air pollutants may be 2 to 5 times higher than the levels of outdoor air pollutants. While you may be able to see dust on a bookshelf or dirt on the floor and know that it is time to clean, you cannot always see what is polluting your air. Some of the main indoor air pollutants and their sources include:

When any of these pollutants are present, it is safe to say your air needs to be cleaned. Conversely, healthy air is:

  • Made up of less than 50% relative humidity
  • Free of toxins
  • Well-ventilated and fresh

Where does household humidity come from?

There are several ways moisture can enter—and stay—in your home. Rooms where water is used frequently, such as the bathroom and kitchen, tend to be more humid than other areas of your home. Humidity can occur in your home due to:

  • Long, hot showers where bathrooms are not properly ventilated
  • Roof leaks
  • Leaking toilets, sinks, and pipes
  • Rainwater
  • Steam from cooking that is not properly ventilated
  • Clothes dryers that are used extensively
  • Houseplants
  • Lack of ventilation

What is an ideal level of relative humidity for a home?

Humidity above 50% is typically considered too high, while humidity below 30% is usually too low. That means that the ideal range of relative humidity for a home is between 30% and 50%, according to the EPA. This, of course, depends on the climate you live in, as well as your personal preference.

What happens when there is excess relative humidity in your home?

As temperatures rise and fall, the ability of air to hold water changes. When air is warmer, it can hold more water. When air is cooler, it cannot contain as much moisture. Excess relative humidity can cause poor indoor air quality which can harm your physical health and the health of your home, alike.

Home issues caused by excess relative humidity

If you live in a dry, desert climate, you may not deal with problems of relative humidity. However, other climates can leave your home in need of maintenance due to high levels of humidity. Some home damage to watch out for when there is excess humidity include:

  • Mold growth
  • Musty smells throughout your home
  • Peeling wallpaper or bubbling paint
  • Warped wooden floors or furniture
  • Malfunctioning electronic equipment
  • Condensation or foggy windows
  • Water stains on walls or ceilings
  • Shrunken window and door frames

Health issues caused by excess relative humidity

Poor air quality in your home due to high levels of relative humidity can do more than cause mold growth and peeling wallpaper—it may have a significant impact on your health. People with asthma, allergies, or otherwise compromised immune systems are likely to experience more problems. Still, people who have no underlying health issues may find that high humidity may cause physical ailments, too. Since mold, mildew, and mites thrive in humid environments, they can trigger many of these symptoms. Some of the health effects from excess humidity may include:

  • Respiratory issues, such as exacerbated asthma or allergy symptoms
  • Skin rashes from blocked pores that cannot release sweat (heat rash) when hot and humid
  • Nose, eye, and throat irritation
  • Increased off-gassing of Volatile Organic Compounds (VOCs) in home, leading to other health risks such as eczema, cancer, and nervous system damage (source: Arundel, et. al, 1986)
  • Fatigue

What happens when the relative humidity in your home is too low?

While excessively high levels of relative humidity are known to be harmful, the same is true when levels are too low. However, the symptoms—both in your home and your body—of low levels of relative humidity are more obscure, so you may not notice them as readily.

Home issues caused by low relative humidity

When the air is colder outside, you probably use your heater or fireplace more. This can cause the humidity in your home to drop below ideal levels and cause the air to become excessively dry. Some of the ways low relative humidity affect your home include:

  • Shrinking wood furniture, door, and window frames
  • Hardwood floor separation and warping
  • Peeling wallpaper
  • Gaps between the walls and ceiling

Health issues caused by low relative humidity

Low relative humidity can also impact your health. You may experience some or all of the following symptoms because of dry air, according to the Cleveland Clinic:

    • Increased susceptibility to colds and respiratory illness
    • Body chills
    • Dry, itchy, or chapped skin, lips, and hair
    • Dry, scratchy throat and nose
    • Itchy, chapped skin
    • Static electricity
  • Dry eyes
  • Bloody nose
  • Worsened colds and flu
  • Worsened asthma and allergy symptoms

How to keep the relative humidity in your home at an optimal level

Though there are many ways in which humidity levels can become too high or too low, as well as many health and home problems caused by an imbalance of humidity, there are also simple ways to keep it at an ideal level. To keep the relative humidity in your home between the ideal levels of 30% and 50%, you may want to consider using either a dehumidifier or a humidifier.

How do dehumidifiers work?

Dehumidifiers work by removing excess humidity from your home’s air, making it a less hospitable environment for allergens like mold, dust, and mildew. You can get a portable dehumidifier or a whole-house dehumidifier that is installed into your home’s HVAC system. Portable dehumidifiers tend to be much more affordable and easy to use. A whole-house dehumidifier usually requires professional installation and can be very expensive.

How do humidifiers work?

Humidifiers are devices that emit water vapor to help keep the air in a room moist. When used correctly, a humidifier may help improve your dry indoor air. You can find both cool and warm-mist humidifiers, each of which has benefits and disadvantages. Using a humidifier when relative humidity is too low may help alleviate many of the health symptoms associated with dry air.

Keeping the relative humidity of your home at an ideal level is important for both your health and the condition of your house. While the concept of relative humidity may be new to you, keeping your home between 30% and 50% humidity is quite doable.  You may need to obtain a dehumidifier or a humidifier, depending on whether you have low or high relative humidity. If you live in a region that experiences all four seasons, you may find that both devices come in handy at different times of the year.

Can an air purifier be used in conjunction?

Keeping relative humidity at an acceptable level is an important step for air quality. As mentioned above, excess humidity may cause mold to grow, and both high or low moisture levels may impact levels of indoor air pollutants. Controlling humidity is an important part of what the EPA defines as “source control,” or preventing problems of indoor air pollution from occurring in the first place.

After source control methods are taken, the EPA describes using an air purifier as an additional step for improving indoor air quality. The technology inside the Molekule air purifier has been shown to destroy organic compounds like mold and certain airborne chemicals (volatile organic compounds, or VOCs). An air purifier can be used in conjunction with a humidifier/dehumidifier as part of a solution for better air quality in your home.

Ultimately, creating the right air quality conditions, whether through controlling humidity levels, increasing ventilation, or using an air purifier, can reduce indoor air pollution in the place your family likely spends the most time: your home.