Humidifiers Decrease Levels of Flu Virus in Homes

NOVEMBER 22, 2011
Flu season is fast approaching, and as the weather outside gets frightful, more households will begin to utilize humidifiers as a temporary method to mitigate parched, stuffy air. According to a newly published study, however, this action could also help decrease the incident rates of influenza, as the flu virus floating around in the air can be killed with the aid of humid air.

Co-author of the article Ted Myatt, ScD, reported, "A review of past peer-reviewed studies conducted over the last 70 years also showed that homes kept at 40-60% relative humidity-the optimal range-are likely to have fewer flu viruses lingering in the air and on surfaces like sink faucets, door handles, and countertops compared to lower relative humidity levels.”

“The typical flu virus,” continued Myatt, who is the senior scientist at consulting firm Environmental Health and Engineering, Inc, “can survive in the environment at least 24 hours. In more humid environments, the virus survival time is markedly decreased."

The researchers evaluated the efficiency of a humidifier to decrease the survival of the influenza virus by utilizing a multi-zone indoor air quality model. “We modeled indoor [absolute humidity] and influenza virus concentrations during winter months (Northeast US) using the CONTAM multi-zone indoor air quality model,” wrote the authors in the article.

The researchers also assessed the affects different ventilation conditions had on the air in a two-story residential template. “Simulations were conducted for two types of heating systems: a forced hot air system and a radiant heating system,” they wrote in their article. “The primary difference between the two systems for purposes of this analysis is that forced air systems provide more rapid mixing of indoor air throughout a residence than radiant heating.”

They evaluated the levels of humidity on a room-specific as well as a whole house basis; estimations of influenza emission rates were particle-size specific and based on previous studies including emission during “tidal breathing and coughing events,” they wrote. The determination of survival of the influenza virus was based on the already established relationship between absolute humidity and virus survival.

The researchers found that the presence of a portable humidifier with an output of 0.16 kg water per hour in the bedroom resulted in decrease in influenza virus survival of 17.5%—31.6%. Further, allocation of water vapor throughout the house was projected to increase the absolute and relative humidity of the residence by 3% to 12%, resulting in a reduction of the virus survival by 7.8%—13.9%.

Many humidifier manufacturers are cashing in on this fact by patenting germ-free technology, which includes top-fill humidifiers which are easier to clean as well as easier to load up with liquid.  One such manufacturer, Kaz, is advertising their Honeywell humidifiers as “Germ-Free Cool Moisture Humidification (HCM-350).” The product has patented ultraviolet technology designed to eradicate up to 99.9 percent of bacteria, mold, fungus and viruses in the water poured into the product.  More information on Kaz line of Honeywell humidifiers is available at

Myatt concluded with advice to all potential consumers who wish to raise the humidity in their homes and thereby decrease the chances of the flu virus surviving. “In order to reach an appropriate humidity level in your home,” he said, “you need to consider the size of each room and your humidifier's maximum moisture output.”

The authors also noted that a possible future method for decreasing public health risks of influenza could be to keep public spheres humid to reduce the survival rate of the virus in the air as well as on surfaces.

The study, entitled "Modeling the airborne survival of influenza virus in a residential setting: the impacts of home humidification," was published in the September 3, 2010 issue of Environmental Health.

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