The Experiment

For his research, Koep, a native of Brainerd, Minn., partnered with two schools in Rochester, Minn., Lincoln K-8 Choice School and Kellogg Middle School. Over two winter seasons, he measured humidity in the classrooms of both schools using 70 HOBO data loggers. HOBO loggers, a product of Onset Computer Corp., provide data on a wide variety of energy and environmental measurements including temperature, relative humidity, ac/dc current and voltage, differential pressure, time-of-use (lights and motors), light intensity, water level, soil moisture, rainfall, wind speed and direction, pulse signals, and more. Koep’s loggers retained and reported information every five minutes.

Within Lincoln K-8 Choice School, which has an area of 50,000 square feet, Koep placed 30 sensors. At the 160,000-square-foot Kellogg Middle School, he installed 40 sensors. Koep also used seven carbon dioxide sensors to track levels in the schools, which indicated the presence of students in the classrooms.

“If it’s 60 percent humidity, we’d expect viruses not to survive nearly as well as they would at 20 percent,” Koep said. “Right in that 40-60 percent range is what we target in the schools for safety concerns and to achieve maximum inactivity of viruses.”

The Results

“The first thing we did was look at the differences in humidity between rooms, and there was basically no difference within the classrooms,” Koep said. “There was a little variability between the classrooms, but they were mostly the same.”

In the Rochester Public School district, where humidification is not a part of the HVAC system in the majority of the district’s 32 buildings, Koep found that humidity could vary widely depending on the outside weather, and whether students were present in the building.

“Basically, whatever is happening outdoors at a given time determines the humidity inside the school,” Koep said, adding, “The only humidification that happened during the day was due to people being in the building and had nothing to do with the HVAC system.”

Dale Krageschmidt, an industrial hygienist at Mayo Clinic, contributed to the project. He said part of the problem was a lack of fresh air in schools. “When buildings are being made — especially schools — the problem is making sure that enough fresh air is getting into the systems,” Krageschmidt said. “That’s expensive, but with heat exchangers, some of those energy costs work out. One of the things the study shows is the importance of make-up air as part of an HVAC system in schools.”

Tod Rogers, coordinator of facilities and grounds for Rochester Public Schools, said trying to humidify the schools would not be easy — or cheap.

“Humidification is kind of its own monster. It’s expensive, it’s huge for maintenance, and it’s a challenge,” he said. “We don’t focus on humidification in the schools, except in our data centers.”

Modifying the HVAC systems at the schools would be problematic, Rogers said. At Lincoln K-8 Choice School, half of the building is heated with hot water while the other half is heated with steam.

“We have an air handler that is located in the building, and then we have three other rooftop units,” Rogers said. “All four of them would have to be fitted for some mechanism for putting water into the air. For the ones outside, I’m not sure how you would do that.”

The Next Step

“This study is just a preliminary assessment of the potential impact humidity can have on virus survival,” Koep said.

To see how difficult it is to maintain a consistent humidity level, Koep ran an additional small experiment.

“We had some additional grant money and purchased household humidifiers and tried to see if we could change the humidity on the weekend, when nobody was there and the HVAC systems were off,” he said. “We specifically changed them from about 20 percent to 60 percent relative humidity, which is the highest we can go before we start getting concerned about mold.”

However, Rogers noted that using individual humidifiers in each classroom to maintain an ideal humidity level is not practical. “I suppose that’s a possibility, to put individual humidifiers in each classroom, but you’re looking at 20-plus rooms with humidifiers. Who’s going to fill them, do the water treatments, and keep them clean? I’m sure it’s possible to humidify the space, but whether it’s likely, I doubt it.”

Koep is planning to continue his research by testing the air for viruses at varying levels of humidity, which he hopes to do within the next year. He also stressed that, while keeping humidity between 40 and 60 percent decreases virus survivability, it is not a substitute for other cold and flu prevention measures.

“We predict it would decrease virus survival, but it would be another step to say what that means for human health, and it needs to be studied,” Koep said. “The No. 1 preventative measure is still getting vaccinated, washing your hands, staying home when you’re sick, covering your cough, and taking all the other necessary precautions.”

Publication date: 3/11/2013