People rarely talk about low humidity as a contributing factor in the spread of influenza, but the low humidity levels in the U.S. this year may be one of the reasons for the tough flu season.
Research over the past decade shows increased humidity can help prevent the spread of influenza. Assistant Professor of Biology Silas Johnson said he thinks 40 percent relative humidity is generally accepted as the level above which one can greatly inhibit the spread of the flu.
Aware of this phenomenon, physics department chairman Ken Hayes has been checking the humidity levels on the environmental monitors in a couple of physics lecture rooms.
He said the relative humidity levels, the percentage of water vapor in the air out of the maximum amount needed for saturation at that temperature, has remained at or below 10 percent since the beginning of the semester — significantly lower than the minimum 40 percent researchers would recommend.
While the humidity levels in the rooms have been somewhat consistent this semester, Hayes noted a variation which occured on Feb. 15, a particularly rainy day.
“I could feel the much higher humidity in the Physics department hallway this morning, and the humidity today in the two lecture rooms was measuring 32 percent,” Hayes said in an email that day.
According to Johnson, when a person with influenza coughs or sneezes, he or she generates respiratory aerosol droplets that carry the flu. With lower humidity, some of the water in the droplets will evaporate, making the droplets smaller and allowing them to hang longer in the air.
“They can be there for a number of hours,” Johnson said. “If somebody sneezes in an 8 a.m. class and then somebody in a 9 a.m. class comes in, they may get sick because there is still influenza in the tiny little aerosol droplets hanging out in the air.”
With increased humidity, the aerosol droplets are bigger, and it is easier for them to aggregate and settle onto surfaces. Humidity also causes the virus itself to change; higher humidity can destabilize a virus, while lower humidity increases its stability, Johnson said.
Visiting Lecturer of Biology Angie Pytel said she uses a small humidifier in her office in the Dow Science Building due to the “chronically dry” air. She said this has helped reduce her frequency of nosebleeds, and that this problem is not new.
The study of atmospheric moisture in relation to the flu is particularly relevant this year. According to an article written for Weather Underground by Bob Henson, this is because the U.S. is currently experiencing “one of its worst winters of flu in years” and also its driest winter in more than 30 years.
Even if the relative humidity outside is rather high, it takes less moisture to saturate cold air than warm air, according to the article. So on an extremely cold day, it is not difficult to have a high relative humidity and then have this relative humidity drop once the air is heated inside.
Johnson agreed the humidity might be a factor in the particularly bad flu season this year, but he also noted that the strain of flu this year was particularly bad, and the flu vaccine was also not as effective as it has been in past years.
Recent studies show, according to the Center for Disease Control and Prevention, that when flu viruses are well-matched to a flu vaccine, the vaccine can reduce the risk of illness by 40 to 60 percent. This winter, in contrast, the CDC estimated that the total vaccine effectiveness between Nov. 2 and Feb. 3 was about 36 percent.
“On the humidity question, most of that research has come across maybe in the last 10 years or so,” Johnson said. “There is not a ton of research out there. Maybe a handful of published scientific papers. There is an association that people see, but the actual mechanism of how that works is still kind of unclear.”
Nevertheless, Johnson says the easiest way to inhibit the spread of the flu, even when dealing with low humidity levels, is for people to wash their hands and cover their mouths while sneezing.
“If they are sick, don’t come to class,” Johnson said. “These things will probably do more than raising the humidity levels will.”
Hayes said it would be helpful to start raising awareness on campus about the importance of healthy humidity levels.
“Students should be aware of this issue and should pay attention to the relative humidity in their living quarters,” he said. “Inexpensive relative humidity meters are available at stores in town.”