Relative Humidity: What is it and What’s an Ideal Level?

Relative Humidity: What is it and What’s an Ideal Level?

Relative Humidity: What is it and What’s an Ideal Level?

What is relative humidity, where does it come from, are there side effects of too low or too high humidity and how to maintain ideal indoor humidity in your home?

If you suffer from allergies or asthma, you already know that the humidity level inside your home can really make a difference. Excessive levels of humidity may cause mold growth and other allergens in the air, and be the root of a number of health concerns. While prolonged exposure to low humidity can cause health issues, especially throat, nose and eye irritations.

In addition to health problems, too high or low levels of humidity can lead to structural damage in your home. With the aid of a humidifier or a dehumidifier, reaching ideal indoor humidity has become quite 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.

So, what is relative humidity?

Relative humidity refers to the amount of moisture present in the air at a certain temperature, compared to what the air can “hold” at that temperature. 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 condenses as moisture, causing the air to feel damp. If relative humidity gets too high, mold and mildew growth and trapped allergens become more likely to proliferate in the air, causing allergy and asthma symptoms to appear.

If you’re trying to make the air in your home healthy, relative humidity is an extremely 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 device is best for you. How much relative humidity is present in your home will obviously depend on the region and climate you live in.

Relative Humidity

Absolute humidity versus relative humidity

Absolute humidity refers to 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 temperature. The more water vapor is concentrated in the air, the higher the absolute humidity will be. 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 in relation to (that’s why is called 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.

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. Some of the main indoor air pollutants and their sources include:

  • Asbestos
  • Cleaning supplies and household chemicals
  • Mold and dampness
  • Formaldehyde
  • Volatile Organic Compounds
  • Bacteria and viruses
  • Carbon monoxide
  • Dust mites and dust
  • Lead

Relative Humidity mold

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

  • 50% or lower relative humidity
  • Free of toxins
  • Well-ventilated and fresh
  • Rich of oxygen

Where does household humidity generate?

There are several ways in which moisture can enter 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:

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

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. Thus the ideal range of relative humidity for a home is between 30% and 50%, according to the EPA.

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

As temperatures fluctuate, 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 climate, you may not have to deal with problems of relative humidity. However, other climates can leave your home in need of maintenance due to too high levels of humidity. Some home damage to watch out for when there is excess humidity include:

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

Health issues caused by excess relative humidity

Bad air quality in your home due to high levels of relative humidity can do more than cause mold growth and a bit of damage to your home, it can have a significant impact on your health as well. People with asthma, allergies, or otherwise compromised immune systems are likely to experience more problems.

Relative Humidity coughing

Still, even people who have no underlying health issues can find that high humidity may cause physical issues, too. Since mold, mildew, and mites thrive in humid environments, they can trigger several of these symptoms:

What happens if 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 of low levels of relative humidity are less obvious, so you may not notice them as easily.

Issues a low relative humidity can cause to homes

When the air is colder outside, you probably use your heater or fireplace more often. 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:

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

Health issues caused by low relative humidity

Apart from your home, low relative humidity levels can also impact your health. Some of the observed symptoms of dry air exposure include:

  • Dry, itchy, or chapped skin, lips, and hair
  • Bloody nose
  • Itchy, chapped skin
  • Body chills
  • Increased susceptibility to colds and respiratory illness
  • Static electricity
  • Dry, scratchy throat and nose
  • 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 sway too high or too low, as well as many health and home problems caused by an inappropriate humidity level, there are also very 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.

optimal level humidity

How do dehumidifiers work?

Dehumidifiers work by removing excess humidity from your home’s air, making it a worse environment for allergens like mold, dust, and mildew to proliferate. You can get either 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, while a whole-house dehumidifier usually requires professional installation and can be very expensive.

How do humidifiers work?

On the other hand, 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 can help resolve 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 not a hard thing to achieve.

Can an air purifier be used in conjunction?

Keeping relative humidity at an acceptable level is an important step for air quality. As we discussed, excess humidity may cause mold to grow, and both high or low moisture levels may impact levels of indoor air pollutants.

EPA describes using an air purifier as an essential additional step for improving indoor air quality.

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.

koru air purifier

A conclusive note

Controlling the level of humidity in your home is an essential step towards controlling the air quality, leading to a healthier life.

While humidity levels can vary depending on a multitude of factors including your location, it is also quite simple to monitor relative humidity and take action whenever it’s either too low or too high.

This will ensure the most optimal conditions for you and your family to enjoy fresh, healthy air all year round.

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