Relative Humidity & the Health of Your Home

Relative Humidity & the Health of Your Home

Humidifiers can help ease health concerns caused by dry air and promote overall comfort.

Humidifiers are an effective tool for controlling the amount of water vapor in the air. To fully understand the importance of keeping humidifiers in our homes, we must first understand the impact of water vapor — also known as relative humidity level — on our daily lives and our health.

Water vapor is an important component of the atmosphere, as well as our indoor air. Relative humidity (or simply ‘humidity’) is a common metric to measure the amount of water vapor in the air and is expressed on a scale of 0-100%. When the air is saturated, such as on a hot, rainy day, the humidity will be close to 100%. Humidity decreases when the air in the environment becomes drier.

Below we discuss some of the ways in which the humidity in your home can impact your satisfaction with the thermal environment (i.e., your thermal comfort), as well as your overall health.


Humidity and Transmission of Viruses in the Air
Joseph G. Allen, an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health, recently released an op-ed emphasizing the importance of maintaining relative humidity in the 40 to 60% range as a key defense strategy against Covid-19.

Based on scientific studies on viral transmission, we know that dry air can promote the movement of viruses in the air. When an infected person coughs, sneezes, breathes, or talks, they emit small respiratory droplets into the air. Dry air causes these droplets to evaporate faster, forming smaller virus-carrying particles that can stay in the air for a longer time and travel through larger spaces. Their smaller size may also allow them to penetrate mask materials more easily.

Dry air can also impact our immune defenses, thereby increasing our susceptibility to airborne infections such as the flu, common cold, and Covid-19. Dry air can dry out nasal passages and reduce mucous, which can slow the rate at which viruses and other pathogens are cleared out from the respiratory tract, which may allow viruses to penetrate the deepest part of our lungs. For more details about the impact of humidity on viral transmission, see our previous article: Can Indoor Humidity Impact Covid-19 Transmission?


Thermal Comfort
Humidity is also related to temperature and thus affects our thermal comfort. Warmer air can hold more water vapor. If the water vapor content stays the same and the temperature drops, the relative humidity increases. Conversely, if the water vapor content stays the same and the temperature rises, the relative humidity decreases. In the summertime, when the air is more humid, the heat, moisture and friction create an uncomfortable ‘sticky’ feeling on our skin and clothing. Meanwhile, during the winter months, very low humidity can cause discomfort by drying out the skin and mucous surfaces, such as eyes and nasal passages. A humidifier helps maintain comfortable humidity levels, thereby reducing the chance of thermal discomfort.


Sleep Quality
Humidity – in both low (<30%) and high (>60%) levels – can negatively impact sleep quality. Dry air can increase the chance of respiratory system irritation, which may cause coughing or sneezing during sleep. Conversely, very humid air can increase the temperature in a room. This can make it difficult to fall asleep, reduce the time spent in deep sleep stages, and decrease sleep quality overall.


Mold, Fungi & Bacteria
Microorganisms such as mold, fungi, and bacteria can grow rapidly on indoor surfaces when humidity levels are high over long periods of time. Inhaling or touching these microorganisms may cause allergic reactions for people with asthma or allergies to mold. For individuals with chronic respiratory diseases (e.g., a chronic obstructive pulmonary disorder), the presence of microorganisms in the air may lead to difficulty breathing and can exacerbate the symptoms of the disease. To reduce the growth of microorganisms, the EPA recommends maintaining indoor relative humidity between 30% and 60%. However, to discourage mold growth, the optimal range for humidity is between 40% and 50%.


Condensation can be a common issue in the home, due to the water vapor generated from human activities (such as bathing), infiltration from outdoor air in hot and humid climates, and/or improper use of humidifiers or dehumidifiers. As discussed in our previous article on humidity, condensation can result in severe structural damage and pose a safety risk to your home. Condensation can also result in mold growth, which is not only associated with the adverse health outcomes discussed above but may also further degrade building materials.


Common Types of Humidifiers

Humidifiers are commonly used during the heating season to help ensure optimal humidity levels, since humidity is naturally lower during the cold season and is further reduced by indoor heating. Depending on the method of installation, residential humidifiers can be categorized into two groups:

  • Standalone/portable humidifiers
  • In-duct humidification for central air systems

The mechanism for these two groups is similar; the major difference is in the installation and operation.


Standalone Humidifiers
Standalone humidifiers can be used in residential buildings that do not have a central humidification system. Typical standalone humidifiers come in three types:

  • Ultrasonic humidifier: Uses high-frequency oscillation to transform water into vapor at low pressure. The oscillation drives the water particles from the surface so they can be quickly absorbed into the airstream.
  • Evaporative humidifier: Circulates water over a porous media and generates airflow to aid the evaporation of water into the air.
  • Vaporizer: Creates steam by heating water with an electrical heating element or electrodes.


In-duct Humidification System
For homes with a central air conditioning system, an in-duct humidification system can be a good option. There are several common types of in-duct humidification systems:

  • Pan Humidifiers: Water is supplied to a shallow pan in the HVAC system and is heated to increase evaporation. This type of in-duct humidification works similarly to the vaporizer mentioned above.
  • Wetted-Element Humidifiers: Air circulates over or through an open-textured, wetted medium and aids evaporation in a similar way to the evaporative humidifier mentioned above.
  • Atomizing Humidifiers: Ultrasonic technology atomizes the water particles and disperses them into the air in the form of a fine mist.

Choosing a proper humidification technology is important for creating a healthier and more comfortable environment. Stay tuned for an upcoming article in which we explore the advantages and disadvantages of different types of standalone humidifiers and in-duct humidification systems, so that you can be better equipped to select the best humidifier for your home.


Learn More
US EPA – Use and Care of Home Humidifiers
CDC – Indoor Environment Quality
ASHRAE Position Document on Airborne Infectious Diseases


Edited by: Bing Bing Guo, MPH, Regina Vaicekonyte, MS and Radhika Singh