Outbreaks of influenza are seasonal in nature, with transmission and death rates increasing in winter months. Dry air and cold temperatures are particularly known to promote the spread of influenza, and, in temperate regions, flu outbreaks have been closely correlated with decreased humidity.1
While it’s previously been shown that low ambient humidity encourages the transmission of viruses, what this means for infection in humans, as well as outcomes among those infected, is less clear, leading researchers to look into whether changes in humidity could play a role in influenza infection.
It turns out that altering humidity levels could be one way to not only reduce the transmission of flu, but also to bolster humans’ resistance against infection.
Influenza’s Link to Humidity
Environmental factors play a prominent role in influenza’s tendency to flare up seasonally. Along with seasonal factors like changes in exposure to sunlight, vitamin D levels and time spent in crowded indoor areas, fluctuations in temperature and humidity certainly play a role.
Specifically, it’s known that the combination of cold temperatures and low humidity is ideal for influenza to spread. According to a study in PNAS:2
“IAV [influenza A] outbreaks occur during the winter months in temperate regions, peaking between November and March in the Northern Hemisphere and between May and September in the Southern Hemisphere …
A key epidemiological study analyzing data collected over 30 y[ears] across the continental United States showed that a drop in absolute humidity, which is dependent on relative humidity and temperature, correlates most closely with the rise in influenza-related deaths.
Experimental studies in guinea pigs demonstrate that low temperature and low humidity enable aerosol transmission of influenza virus, providing one explanation for the seasonality of viral transmission.”
Studies on the survival of influenza virus also show a humidity connection, with one suggesting that “aerosolized influenza survived best when the relative humidity was below 36%.”3 There are fluctuations, too, between relative humidity (RH) and absolute humidity (AH) in the air.
AH, defined as “the mass of water per volume of air,” may have an even stronger relationship with influenza than RH, which is defined as “the ratio of the vapor pressure of water to the saturation vapor pressure at a prescribed temperature and pressure.” In fact, it’s been suggested that “AH, as a modulator of influenza transmission, drives seasonal variations of influenza transmission in temperate regions.”4
This is noteworthy since indoor moisture levels can be influenced by a number of factors, including outdoor moisture loads, sources of indoor moisture and levels of ventilation, and it’s believed that targeting indoor humidity levels during flu season could lessen the spread of influenza viruses in the air and on surfaces.
Raising Indoor Humidity Levels Reduces Survival of Flu Virus
Using a portable humidifier in your bedroom during flu season could reduce the survival of influenza virus in the air, according to a study published in Environmental Health.5 A model of a two-story residential structure 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%.6
The results suggest that not only could adding a humidifier to your bedroom prove to be an easy way to protect against the flu, but increasing humidification in public settings could also be beneficial for public health. According to researchers:7
“Results from this modeling analysis demonstrate that the use of portable residential humidifiers increases RH and AH to levels that can potentially decrease the survival of airborne influenza virus in a residential setting. This effect is more pronounced in rooms where the humidification is located.
While this study evaluated the impacts in a residential setting, the expected benefits of humidification are likely to be larger in places where larger populations of people with the flu and people susceptible to the flu congregate.”
Dry Air May Make It Easier To Be Infected by Influenza
Not only does increased humidification lower the survival of influenza virus in the air, but it also influences the host’s response to infection. In a study on mice, those housed in a low-humidity environment were more susceptible to influenza and had more severe disease.8
Mice exposed to an aerolsized 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 researchers further noted, “Moreover, mice exposed to dry air were more susceptible to disease mediated by inflammasome caspases.”
It seems that humidity levels affect physical and innate immune defenses against viral infections, with researchers concluding:9
“These mechanisms … may in part underlie the epidemiological correlation of a drop in absolute humidity preceding death from seasonal influenza infection in temperate regions … Our study suggests that increasing ambient humidity may be a viable strategy to reduce disease symptoms and to promote more rapid recovery in influenza-infected individuals.”
There is one caveat, however, that needs further exploration, which is the fact that tropical climates with wet, warm conditions also have influenza outbreaks, and different areas of the world appear to be influenced by humidity in different ways.
What Else Are Humidifiers Good For?
Humidity, which in simple terms is the amount of water droplets suspended in the air, should be optimized in your home for optimal health. Doing so can affect a number of conditions, including respiratory infections, as the featured study suggested.
In another study, a meta-analysis of nine epidemiological studies, researchers found the incidence of absenteeism and respiratory infections was lower in people who worked or lived in environments with a midrange relative humidity.10
Another study demonstrated “prolonged exposure to high (greater than 50%) or low (lower than 30%) relative humidity was associated with an increased risk of respiratory symptoms among teachers.”11
Yet another study found maintaining indoor relative humidity greater than 40% could significantly reduce the infectivity of influenza virus in the air.12 Dry skin and throat, allergy symptoms and dry eyes can also be affected by humidity levels. The Environmental Health study noted:13
“Increasing low indoor moisture levels may have benefits beyond reducing survival of the influenza virus.
Low RH has been associated with a number of symptoms including dry skin, throat and mucous membranes and eye irritation in office and hospital workers. In a home humidification intervention, the authors reported a decrease in dryness of the nose and throat and improved breathing in patients with allergies.”
In terms of allergies, relative humidity below 50% will minimize mite populations, while keeping levels below 60% will also make it difficult for most species of fungi to grow.14 The humidity level in your home even influences the offgassing of formaldehyde from indoor building materials and the formation of ozone, with experts suggesting the ideal level of relative humidity to avoid adverse health effects is between 40% and 60%.
“This would require humidification during winter in areas with cold winter climates. Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens,” researchers explained in Environmental Health Perspectives.15
Be Careful About Mold Growth
There’s a definite sweet spot when it comes to humidity and it’s important to keep levels neither too high nor too low. While low levels may allow infections to flourish, high levels can lead to mold and mildew growth. In the featured Environmental Health study, adding a humidifier to the bedroom occasionally resulted in relative humidity levels that exceeded 60%, especially when radiant heat was used.16
If your home’s humidity is higher than 60%, it increases the risk of mold and fungal growth and may make your home feel stuffy. You might notice condensation on the walls, floors and windows, increasing the risk of triggering growth of bacteria and dust mites. These allergens may increase your respiratory problems and provoke flare-ups of allergies and asthma.17
A dirty humidifier can also lead to the growth of mold and bacteria, so keeping it clean is important. Unless otherwise recommended by the manufacturer of your device, you can keep your machine running clean using hydrogen peroxide and white vinegar. Change the water often so mineral deposits and film do not develop, being sure to unplug it first. Distilled or demineralized water is typically recommended.
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.18
How to Measure Humidity Levels in Your Home
To keep humidity levels between the recommended 40% and 60%, you’ll need to know what the level is in your home. 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.
Depending on the humidity level in your home, you may need to add moisture with a humidifier or remove moisture with a dehumidifier. During flu season, making sure your home has enough moisture in the air is a simple way to lower the spread of influenza virus and increase your resistance to infection.