Ultraviolet light systems have started to gain popularity as a way to clean large swathes of schools quickly and frequently.
For example, according to a February EdWeek Reseach Center survey, 13 percent of district leaders and principals reported using UV light systems for sanitation. And other public institutions, like New York City‘s transportation system, have been testing the effectiveness of handheld UVC lamps to disinfect subways, trains, buses, and transit stations.
But evidence is still mixed on how to use such systems effectively for surface as well as airborne contamination of COVID-19.
“Dubious and nonscientific performance claims by some of the UV system designers and manufacturers are unfortunately widespread,” concluded Canadian researchers Milad Raeiszadeh and Babak Adeli in a 2020 review of evidence on UV systems’ use since the pandemic.
The Centers for Disease Control and Prevention does, however, recommend schools “consider using ultraviolet germicidal irradiation (UVGI) as a supplement to help inactivate the virus that causes COVID-19, especially if options for increasing room ventilation are limited.”
But Claire Barnett, the executive director of the Healthy Schools network, a national clearinghouse on school infrastructure safety and maintenance issues, said the network does not recommend schools opt for UV sanitizing systems, noting that they are “too high tech” and might have “devastating side effects on human health if not properly installed and operated.”
In the past year, a confusing mix of technologies and uses has made for expensive missteps for districts looking to use the systems. Cobb County, Ga., schools, for example, this month canceled a $12 million contract after a UV disinfecting system malfunctioned at an elementary school, which did not cause any injuries, according to local reports. So what, if any, role can UV systems play in controlling COVID-19 in schools? Here’s what district leaders need to know.
What does ultraviolet light do?
Ultraviolet light is categorized based on its wavelength, measured in nanometers. UVA (315-399 nm) and UVB (280-314 nm) penetrate more deeply into human tissue and are associated with sunburn, skin cancer, etc. UVC (100-279 nm) can still cause eye damage or skin irritation with exposure, but around 254 nm germicidal ultraviolet light has long been shown to destroy the outer protein coating of viruses that cause measles and the SARS-coronavirus—a close relative of the coronaviruses that cause COVID-19.
UVA and UVB light have been found less effective at deactivating virus particles and potentially more dangerous to people if they are exposed to the radiation.
Researchers have also been experimenting with so-called “far UVC,” or light at the shorter 222 nm wavelength, which has been shown to be effective at destroying related coronaviruses in a lab and is now being tested to determine whether it is safe to be used in occupied areas. Systems using far UVC are so far less available and significantly more expensive than those using standard UVC.
What’s the difference in UV systems?
There are three basic types of UV systems—handheld UV lamps or wands used by shining over surfaces; UV sanitizers installed inside of ventilation systems; and UV air exchange systems placed either at the floor or ceiling level—and the evidence on each varies significantly when it comes to the virus that causes COVID-19.
Hospitals do use HVAC-based UV systems to eliminate viruses known to spread through ventilation systems, such as the highly infectious measles. However, the Food and Drug Administration noted there is little published research on how long, how much, and how intense a UV treatment must be to destroy SARS-COV-2, the virus that causes COVID-19. While SARS-COV-2 is transmitted through airborne droplets and aerosols, research has suggested transmission has been more common within rooms or in adjoining spaces rather than through ventilation systems within a building.
In addition, “there really isn’t good evidence for schools using the handheld surface cleaners or surface lamps in the HVAC systems,” said Edward Nardell, a professor of medicine and an expert in airborne disease transmission at Harvard Medical School.
That’s because UV radiation only eliminates viruses that are directly exposed to it; anything from dust and dirt to human sweat on a surface can make the radiation less effective. Nardell noted that even “microshadows” can protect virus particles in crevices between objects. Raeiszadeh and Adeli note in a new study that on surfaces the exposure time needed to break down the coronavirus that causes COVID-19 can vary dramatically based on the different surface materials as well as the temperature and humidity in a room.
Germicidal ultraviolet light is more frequently used to destroy airborne virus particles in vents or at the ceiling level.
“[HVAC] systems can clean things, but with regard to the COVID in particular, there doesn’t seem to be research suggesting that it actually spreads in vents,” he said. “So the HVAC system wouldn’t be the best way of actually eliminating particles.”
Nardell, who has studied floor and ceiling air exchange systems used to prevent the spread of tuberculosis, argued school leaders should focus on sanitizing the air within classrooms, which in most schools is exchanged only two to three times per hour, compared with the six to 12 times an hour recommended to prevent the spread of COVID-19. That could mean floor-level UV tools, though Nardell said ceiling-level systems may be more efficient for active classrooms.
“You either need several [floor-based UV fans], or they have to be run at high speed, and in that case, the noise and air speed become an issue,” he said, noting his own campus gym runs four very large air cleaners. “I wouldn’t want to be teaching in that room.”
