With many schools reopening across the country, districts are deciding not to run their heating, ventilation and air conditioning, or HVAC, systems out of fear of spreading the novel coronavirus, instead choosing to run large fans throughout their schools.

A study from the Mayo Clinic has shown, however, that’s not the best idea for classrooms to fight infectious diseases. 

The study proved keeping the relative humidity in classrooms between 40% and 60% could reduce the infectious capacity of respiratory diseases to survive on surfaces, or spread between classmates as aerosols.

“If the relative humidity is below 40%, it’s too dry,” said Valerie Bradt, marketing communications manager for DriSteem, a manufacturer of humidification systems for commercial buildings who provided the humidifiers to the Mayo Clinic for the study. “If it’s above 60%, the air holds too much moisture, which could cause mold to form, increasing chances for disease.”

Hand washing and wearing masks, along with frequent cleaning and disinfecting, provide a good start in reducing the spread of viruses that cause outbreaks of respiratory diseases like COVID-19 in schools. However, each of these measures rely on vigilance and effort on the part of teachers and students, and the cost of these disposable items adds up quickly, upwards of $2 million for many school districts.

The Mayo Clinic study was a non-invasive study held in four preschool classrooms at the Aldrich Memorial Nursery School in Rochester, Minnesota in 2016, and was conducted without collecting clinical data from students or staff over two months. 

Instead, while staff and students went about their business teaching and learning, air and surface samples were collected from classrooms for analysis in the Mayo Clinic lab.

From Jan. 25 to Feb. 23, two classrooms were humidified with steam humidifiers, while two identical classrooms were not humidified. From the beginning of humidification through March 11, Dr. Jennifer Reiman and her team collected attendance data and the following samples from all four classrooms:

• Air samples with a cyclone sampler, which collects and sorts airborne particles in separate chambers from smaller than one micron to more than four microns in diameter.

• Surface samples from paper-wrapped markers, blocks, and Play-Doh utensils that had been handled by students during class times.

A total of 650 samples were collected from the classrooms — 360 air samples and 290 surface samples. Half of the samples were from the humidified rooms and half from the unhumidified rooms. 

Research, according to the Mayo Clinic, has established that flu outbreaks can be predicted 14 to 16 days after outdoor humidity bottoms out in the continental United States. 

Therefore, in order for the pilot study to be instructive, the humidifiers were turned off on

February 23 in response to Rochester’s seasonal increase in outdoor humidity.

A subset of 45 influenza-positive samples from the beginning of humidification through March 11 were tested in cultures to see if they were able to infect cells (in a plastic dish). Of these 45 samples, 27 were from unhumidified rooms, and 18 were from humidified rooms.

Testing cultures for infectivity is a secondary test for flu that is more stringent than testing for presence. The infectivity results were even more remarkable

than the presence results. Of the 45 samples tested, 16 resulted in infectious cultures:

• 13 of the 27 cultures (48%) from unhumidified rooms tested infectious.

• 3 of the 18 cultures (17%) from humidified rooms tested infectious.

Brandt said while the Mayo Clinic study backs up several studies dating back to 1986, every building is different, as well as every HVAC system. However, she said a quality humidification system that can last up to 20 years, along with school staff being diligent about washing their hands and wearing masks, the spread of COVID-19 can be lessened.

“It’s an added layer of protection teachers and students don’t have to worry about,” she said.