Can Indoor Humidity Impact Covid-19 Transmission?

Can Indoor Humidity Impact Covid-19 Transmission?

Maintaining indoor relative humidity levels between 40-60% may help reduce the risk of Covid-19 transmission indoors, and promote better indoor air quality.

During the Covid-19 pandemic, as we spend more time in our homes, the quality of our indoor air is more important than ever. 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 plays in the transmission of Covid-19 and other respiratory viruses such as the common cold or flu, and what precautions we can take to maintain proper humidity levels in our indoor environments.

Impact of Humidity on Viral Transmission

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 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 coronaviruses).

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. However, dry air 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.

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.

Maintain Moderate Humidity Levels

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% RH) help reduce the viability of influenza viruses, compared to lower and higher RH 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.

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

US EPA – Mold
CDC – Indoor Environment Quality
ASHRAE Position Document on Limiting Indoor Mold and Dampness in Buildings
ASHRAE Position Document on Airborne Infectious Diseases