Humidity is not always thought of as an important aspect of air quality. Dry or humid, most people don’t consider dry, itchy skin or tiny electrical shocks a major concern.

Yet, scientific consensus suggests that properly maintained indoor humidity levels are indeed important to good indoor air quality.

Read on to learn what humidity has to do with air quality and 5 tips you can use to optimize your relative humidity (RH) for better air quality.

Humidity and air quality

Dry skin, irritated sinuses, sore throat, itchy eyes, and aging skin are a few common, familiar symptoms often linked to dry air.¹ These symptoms are also commonly associated with allergies – especially seasonal allergies that happen when airborne concentrations of allergy triggers like pollen and mold spores increase dramatically.

And dry air can actually make your allergies worse. Your airways, including the nasal passages, throat, and lungs, are coated with mucous membranes that provide a protective layer against airborne irritants. When you inhale dry air, these membranes can lose moisture, thinning out the mucous layer and increasing your risk of inflammation and infection.²

Dry air may also contribute to higher risk for getting colds and infections because viruses tend to survive longer in drier environments.

This notion was recently investigated and confirmed in a 2020 study on COVID-19 in 50 cities published in Infectious Diseases.³ Researchers found that, like many seasonal respiratory viruses, the risk of infection and mortality from COVID-19 increased with lower humidity levels.

Furthermore, mucous membranes that are dried out and inflamed from dry air are more susceptible to illness and even death.⁴ This is especially true during cold, dry seasons in much of the world.

A 2015 study conducted in Auckland, New Zealand looked at humidity and health effects from 1980 to 2009, finding that both cold and dry weather each contributed to a greater risk of death from the flu or pneumonia.⁵

Excess moisture, on the other hand, can bring about other biological air pollutants, such as dust mites and mold – both of which produce materials that can affect the respiratory system.

Data from the Lawrence Berkeley Laboratory suggests indoor humidity levels beyond 50% can help dust mites thrive, leaving shed skin and droppings that trigger allergies and illustrating a link between humidity and asthma.⁶

The United States Environmental Protection Agency (EPA) further cautions that high humidity levels beyond 60% can promote mold growth.⁷ Indoor mold produces spores for reproduction that are also dangerous triggers of allergies and asthma, especially black mold like Stachybotrus charatrum.

5 Tips to optimize indoor humidity

An indoor relative humidity (RH) of 35-50% is key to helping prevent the health effects of both excessively dry and humid indoor air.

Here are 5 to help optimize your indoor RH for better indoor air quality.

1. Use a Humidifier

Humidifiers are a common household appliance. Some homes even have built-in humidifier systems.

Humidifiers can be a relatively cheap and effective way of adding moisture to the air to alleviate some of the symptoms of allergies, asthma, and airborne infections that can be exacerbated by dry summers and winters.

Not all humidifier systems are equally safe to use, and all require some level of maintenance and care to safely operate.

However, pay special attention to humidifiers built in into home heating and cooling systems. The U.S. Environmental Protection Agency (EPA) and Consumer Product Safety Commission (CPSC) have concluded that Ultrasonic and Impeller units (AKA Cool Mist) are more likely to disperse microorganisms and minerals into the air because they use large internal tanks that often pull from mineral-heavy tap water.⁸

If you have one of these systems, ensure that the water used is both clean and pure.

2. Use clean water

Most tap waters around the world contain varying amounts of minerals. Mineral deposits are often visible as white powdery substances collecting along the sides of tea kettles or pots.

Minerals can provide health-boosting benefits in drinking water. However, when vaporized, these minerals can mix into the air you breathe and potentially cause adverse health effects.

For most humidifiers, use bottled water labeled “distilled,” as distilled water is likely to contain lower mineral content than either tap or commercial water. Distilled water can be costly, so you can also boil and distill your own tap water to help reduce minerals and microorganisms in the water.

Above all, avoid putting dirty, mineral-heavy, or scented water into your humidifier, as this will likely do more harm than good by adding harmful airborne pollutants and substances into the air.

3. Monitor your humidity

Monitoring your indoor RH levels is critical to achieving and maintaining a good level of indoor humidity, indicating when you need to either humidify or dehumidify the air. Be sure to keep indoor RH between 35-50%.

Most low-cost humidity sensors will convey an accurate humidity level. Sensors that display RH are highly recommended – RH is expressed as a percentage calculating the amount of water vapor in the air to the amount needed to saturate the air at a given temperature.

Some air quality monitors include RH sensors that provide highly accurate RH measurements and give specific recommendations for when humidity is too high or too low.

4. Adjust based on humidity

RH less than 35%? Use a humidifier.

RH greater than 50%? Dehumidify using one of these cheap and effective tricks:

• Set your AC to the “dry” setting (if available).
• Make your own dehumidifier with 2 stackable plastic buckets. Make five to eight 0.5 cm holes in the top bucket and fill with 5 pounds of rock salt. Empty the bottom bucket as it fills with water.
• Use moisture absorber (available online or at local home improvement or department stores)
• Fix or replace leaking pipes in your home.

5. Keep your humidifier clean and deposit-free

Change the water in your humidifier daily to reduce the risk of microorganism growth. Try to clean your portable humidifier at least every third day of operation by emptying and scrubbing the tank.

Don’t use any cleaning solutions that contain chemicals, as these may leave traces of chemicals that can pollute your vaporized air.

Once your humidifier is clean, use 3% hydrogen peroxide to disinfect the device and kill any remaining microorganisms that may be growing.