Controlling the humidity level in your home, or even simply in your bedroom while you sleep, may lower your risk of contracting infectious diseases like influenza, colds or possibly even COVID-19 during the winter months.1 Humidity is an often overlooked factor in the spread of viruses, which become more transmissible in cold, dry winter climates.

This is why many viral diseases are seasonal in nature, peaking during the colder, less humid winter. Dr. Stephanie Taylor, a graduate of Harvard Medical School who also has a master’s in architecture, believes so strongly in the role of humidity in infection control that she’s petitioning the World Health Organization to make relative humidity part of standard recommendations for indoor air, along with other air quality measures like pollution and mold.

Taylor, along with researchers from the Massachusetts Institute of Technology, collected data from 125 countries regarding pandemic responses, COVID-19 cases and environmental data, including estimates of indoor relative humidity.

They analyzed the data for a period of three months, revealing that indoor relative humidity had the most significant correlation with daily new coronavirus cases and daily COVID-19 deaths.

In the northern hemisphere, as indoor humidity levels rose in the summer, COVID-19 deaths had a sharp decline. Likewise, in the southern hemisphere, COVID-19 deaths rose as humidity levels declined during the winter months. “It’s so powerful, it’s crazy,” Taylor told Wired.2 Though the research hasn’t been published yet, years of research support the importance of humidity levels when it comes to warding off infectious disease.

Dry Air Impairs Respiratory Tract Defenses

Back in 2011, researchers found that SARS, another type of coronavirus, was more stable in low temperature, low humidity environments compared to those in higher temperatures and relative humidity.3 It’s also been found in animal transmission studies that when relative humidity is kept in the “Goldilocks” zone of 40% to 60%, viruses become inactivated.4

“It is assumed that temperature and humidity modulate the viability of viruses by affecting the properties of viral surface proteins and lipid membrane,” researchers wrote in the Annual Review of Virology. “… An ideal humidity for preventing aerosol respiratory viral transmission at room temperature appears to be between 40% and 60% RH (relative humidity).”5

The mucosal surface of your respiratory tract is involved in part of a multi-tiered defense system against inhaled pathogens. Your mucus can catch bacteria and viruses, allowing you to expel them via a cough or swallow them before they’re able to enter your cells.

However, proper mucus hydration is required for this to work efficiently, and when you breathe dry, low humidity air it dries out the mucus layer and immobilizes cilia, hair-like structures that help move pathogens out of the body with their wave-like motions.6

Airway epithelial cells act as the second line of defense after the mucus layer, acting as a physical barrier within your respiratory tract. Inhaling dry air has been found to lead to “epithelial cilia loss, detachment of epithelial cells, and inflammation of the trachea” in animal studies, and may also impair epithelial cell repair in the lung after infection with influenza.7

Mucociliary clearance (MCC) is another one of your lungs’ defense mechanisms, which helps eliminate inhaled pathogens and irritants from the epithelial surface in your respiratory tract. Inhaling cold, dry air also impairs MCC, leading to impaired viral clearance following infection with influenza, for example. As noted in the Annual Review of Virology:8

“Given that the MCC depends on the maintenance of double mucus layers with two different viscosities and a delicate osmotic balance, proper mucus hydration is required for an efficient mucus transport.

A review on the relationship between temperature and humidity of inhaled air and properties of airway mucosa found that 100% RH at core temperature is the optimal condition for the efficient mucosal functions and airway defense in humans. Mucus dehydration caused by breathing air of low humidity leads to decreased MCC.”

Exposure to low humidity may even affect your antiviral innate immunity, including the expression of interferon-stimulated genes that help induce an antiviral state.9

Low Humidity in Hospitals, Schools Increases Infections

Considering the strong seasonality of influenza, and the fact that flu outbreaks have been associated with reductions in absolute humidity, researchers decided to raise humidity levels in a preschool to see if it would affect influenza infections. Humidifying classrooms from January to March to approximately 45% RH led to a significant reduction in influenza A virus, both in the air and on objects.10

The control rooms, which were not humidified, had 2.3 times more cases of influenza-like illness than the humidified rooms.11 Taylor, in an interview with the editor-in-chief of Engineered Systems, also described research showing that changing humidity levels in hospital rooms altered the rate of infections:12

“Starting in 2012-2013, I was involved in some research that was initially done in hospitals that clearly pointed to the correlation between low relative humidity in patient rooms and an increase in bacterial and viral infections.

I was startled by this. Subsequent studies in nursing homes, schools, and in offices have shown that people are much healthier, obtain fewer infections, have increased productivity, and sleep better at night with this range of humidity.

So, in doing more and more research on the relationship between 40%-60% indoor relative humidity and human health and decreased infections, it’s absolutely a rapid, holistic, and effective disease infection control strategy.

And, now, here comes COVID-19 and it’s more important than ever that we decrease transmission of respiratory viruses. Relative humidity in that range is so effective and, in my opinion, it should be mandated.”

Humidity Between 40% and 60% May Be Ideal

Many studies point to humidity levels between 40% and 60% as a key range for lowering infection risk. In a study on mice, those housed in a low-humidity environment were more susceptible to influenza and had more severe disease.13

Mice exposed to an aerosolized influenza virus and housed at 20% relative humidity, for instance, had more rapid weight loss, drop in body temperature and shortened survival compared to mice housed at 50% relative humidity.

The dry air compromised the mice’s resistance to infection, and those housed at lower humidity levels had impaired mucociliary clearance, innate antiviral defense and tissue repair function, the study found.

The results from another animal study demonstrated that raising relative humidity to 50% decreased mortality from flu infections,14 while yet another study found maintaining indoor relative humidity greater than 40% could significantly reduce the infectivity of influenza virus in the air.15

Studies on the survival of influenza virus also show a humidity connection, with one suggesting that aerosolized influenza survived the longest when the relative humidity was below 36%.16

In an opinion piece published in the Journal of Global Health, it’s again highlighted that indoor relative humidity greater than 40% will significantly reduce the infectivity of aerosolized influenza virus particles. Unfortunately, humidity in residential and commercial spaces in the U.S. is often below 25%,17 which enhances viral transmission. Even in the summer, when humidity levels are naturally higher outdoors, air conditioning limits humidity indoors.

The article, which was written by a collaboration of Croatian, U.S. and German researchers, also suggested that humidified air could be a solution to protecting hospital patients and fighting COVID-19:18

“In addition to being a protection against initial infection, functional mucosal barrier is also important in suppression of viral progression in already infected patients. Since many hospitals have very dry air, providing humidified air to patients in early stages of the disease may be beneficial.

… Considering the evident detrimental effect of dry air on our mucosal barrier and its role of the first line of defense against infection, in a situation of rapidly progressing COVID-19 pandemics it would be essential to aggressively promote active re-humidification of dry air in all public and private heated spaces.

Furthermore, wherever possible patients on ventilators should be ventilated with humidified air.”

Put a Humidifier in Your Bedroom

Using a portable humidifier in your bedroom during the winter months could reduce the survival of influenza virus in the air, according to a study published in Environmental Health.19 A model of a two-story residential residence was used under two ventilation conditions: forced hot air and radiant heating.

Portable humidifiers were used to control moisture content in the air, which was monitored for absolute humidity and concentrations of influenza virus. The addition of a portable humidifier with an output of 0.16 kilograms of water per hour in the bedroom increased absolute humidity 11% and relative humidity 19% during sleeping hours compared to having no humidifier present.

Along with the increases in humidity came a decrease in the survival of influenza virus, by 17.5% to 31.6%. The distribution of water vapor through the whole home was also beneficial, with increases of 3% to 12% AH/RH associated with reductions in influenza virus survival of 7.8% to 13.9%.20

The results suggest that not only could adding a humidifier to your bedroom prove to be an easy way to protect against the flu and other infections, but increasing humidification in public settings could also be beneficial for public health.

In fact, when Japanese researchers used the Fugaku supercomputer to model the transmission of virus particles in indoor environments, they found air humidity of lower than 30% led to more than double the number of aerosolized particles than occurred at humidity levels of 60% or higher.21

How to Monitor Humidity Levels

It should be noted that higher isn’t always better in the case of humidity. If your home’s humidity is higher than 60%, it increases the risk of mold and fungal growth.22 So, you’ll want to keep the level within the 40% to 60% range for ideal health benefits. The best way to test levels in your home is with a hygrometer. This device looks like a thermometer and measures the amount of moisture in the air.

Some humidifiers come with a built-in hygrometer, or humidistat, to help the humidifier maintain relative humidity in your home at a healthy level. If not, you can purchase a hygrometer at most hardware stores.

In one study, adding a humidifier to the bedroom occasionally resulted in relative humidity levels that exceeded 60%, especially when radiant heat was used,23 so you may need to adjust accordingly to keep levels in the optimal range. A dirty humidifier can also lead to the growth of mold and bacteria, so keeping it clean is important.

A hydrogen peroxide solution and soft bristle brush can be used to clean your humidifier, which should be done every three days. If your humidifier has a filter, be sure to change it at least as often as the manufacturer recommends and more if it’s dirty.24

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