Can Indoor Humidity Impact Covid-19 Transmission? Let's Take a Look

The Covid-19 pandemic has taught us all that indoor air quality matters, and that indoor spaces need clean air in order to protect the health and well-being of the people inside them. One of the key factors in helping improve overall air quality is humidity: a measurement of the amount of water vapor in the air.

Improper indoor humidity levels (humidity levels lower than 40% or higher than 60%) can not only impact our comfort but also raise potential health risks. This article illustrates the role humidity can play in the transmission of Covid-19 and other respiratory viruses such as the common cold or flu, and highlights the benefits associated with maintaining optimal indoor humidity.

Transmission of Viruses in the Air

Currently, there are no comprehensive studies focusing on the impact of humidity on the transmission of SARS-CoV-2 (the virus that causes Covid-19). Fortunately, we may be able to gain some insight into this issue by examining previous studies on other types of human coronaviruses (e.g., Middle East respiratory syndrome and severe acute respiratory syndrome associated with coronaviruses).

1. Dry Air Can Promote The Movement of Viruses in the Air

The main transmission route of SARS-CoV-2 is widely believed to be person-to-person through small respiratory droplets produced when an infected person coughs, sneezes, breathes or talks. Large droplets settle on indoor surfaces relatively quickly due to the force of gravity.

Dry air, however, can facilitate the evaporation of moisture within the droplets, which may shrink down their size to what is called “droplet nuclei”. These tiny particles — still containing the virus inside them — can remain suspended in the air and travel within a space along with the air movement, leading to increased spatial spread of viral emissions from an infected individual.

2. Humidity Can Impact Your Immune System

Our immune systems are our primary defense against various pathogens, including SARS-CoV-2. The performance of our immune system is affected by many factors, including humidity. When the air is dry, the effectiveness of our upper respiratory defense mechanisms in protecting our bodies from pathogens, including viruses, can be reduced. Dry air can also dry out nasal passages, which can slow the rate at which viruses and other pathogens are cleared out from the respiratory tract, thereby increasing our susceptibility of infection.

Our immune systems are also impacted by sleep, and sleep quality can be impacted by humidity levels. Dry air can increase the risk of respiratory system irritation, leading to coughing and sneezing during sleep. Very humid air can also make it difficult to fall asleep, which in turn reduces the amount of time we spend in deep sleep stages and ultimately decreases our overall sleep quality. Thus, maintaining moderate indoor humidity levels can promote better sleep quality and help ensure the robustness of our immune system.

The Benefits of Indoor Humidity Control

Research indicates that surface survival of the gastroenteritis virus, a type of coronavirus, is higher under dry (20%) and humid air (80%) conditions compared to moderate humidity levels (50%). Another study has shown similar results — that moderate humidity levels (43-55%) help reduce the viability of influenza viruses, compared to lower and higher humidity levels.

Since both very dry and very humid environments can increase transmission of viruses, such as other coronaviruses and influenza viruses , it is important to maintain moderate indoor humidity levels — between 40% and 60%. This can help reduce the probability of viral transmission and provide additional benefits, summarized below.

Better Thermal Comfort

Many people experience discomfort when humidity is high. However, extremely low humidity can also lead to thermal discomfort by drying the skin and mucous surfaces, resulting in complaints of dry nose, throat, eyes and skin.

To prevent discomfort caused by humidity, experts recommend maintaining indoor relative humidity levels at or below 65%. Currently, there is no generally-accepted lower limit for indoor relative humidity.

Reduce Mold, Fungi & Bacteria

When indoor spaces are subjected to high humidity conditions for long periods of time, microorganisms such as mold, fungi and bacteria can grow on building materials and furnishings. As a result, indoor concentrations of airborne mold spores can increase significantly. Inhaling or touching mold or mold spores may cause allergic reactions such as stuffy nose, wheezing and red or itchy eyes, while people with chronic respiratory diseases (e.g., a chronic obstructive pulmonary disorder or asthma) may experience difficulty breathing).

To reduce the growth of microbial organisms, 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%.

Reduce Condensation

Very humid environments may also lead to condensation on indoor surfaces. Such condensation can corrode critical structural fasteners inside the walls, foundation and roof of a building. Severe damage caused by condensation may result in safety issues, which would require extensive (and expensive) mold remediation work.

Slow the Spread of Covid-19 with Proper Indoor Humidity

While indoor humidity may not immediately spring to mind when thinking about indoor transmission of respiratory viruses and the overall health of your home, it is an important factor that we must take into consideration — particularly in the colder winter months when the air tends to be drier.

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For more information about humidity in indoor environments and the impact of humidity on viral transmission, please see the following websites:

U.S. Environmental Protection Agency (EPA) - Mold: https://www.epa.gov/mold
Centers for Disease Control and Prevention (CDC) - Indoor Environment Quality: https://www.cdc.gov/niosh/topics/indoorenv/temperature.html
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Position Document on Limiting Indoor Mold and Dampness in Buildings: https://www.ashrae.org/file%20library/technical%20resources/bookstore/dampbldgs-humanhealth-hvacdesign.pdf
ASHRAE Position Document on Airborne Infectious Diseases: https://www.ashrae.org/file%20library/about/position%20documents/airborne-infectious-diseases.pdf