Air purifiers can do more harm than good in confined spaces with viruses in the air

The positions of air inlets and outlets in confined spaces, such as elevators, have a major influence on the transmission of viruses into the air. In Physics of liquids, shows researchers from the University of Nicosia in Cyprus, although air purifiers are expected to help, they can actually increase the spread.

Air quality in small spaces can decline rapidly without ventilation. Adding ventilation, however, increases the rate at which air, possibly laden with viruses, can circulate in the small space. Elevator manufacturers have added air purifiers to solve this problem, but the systems are not designed to offset its effect on the overall air circulation.

Air purifiers use ultraviolet radiation to kill viruses and other microbes, but they also circulate air, suck it in and clean up air. This contributes to the general circulation, an aspect that has not been taken into account in previous research.

Previous work by scientists has indicated that saliva droplets can move 18 feet within five seconds as an unmasked cough. The authors expanded the same model to investigate the effects of face masks and weather conditions.

Investigators conducted calculations for a 3-D space equivalent to an elevator that can accommodate five people. A mild cough was simulated in one place in space, and air inlets and outlets were added in different places to study their effect on circulation. An air purifier was also included in the simulation.

“We quantified the effect of air circulation on the transmission of air viruses and showed that installing an air purifier inside an elevator significantly changes the air circulation, but does not eliminate the transmission of air,” said author Dimitris Drikakis.

The investigators found that the risk of airborne transmission through the virus is lowest for low ventilation speeds.

“This is due to reduced mixing of flow in the elevator,” said author Talib Dbouk. “Regulatory authorities must therefore determine the minimum ventilation, depending on the type of building.”

The study looked at the role of an air purifier, considering only the air inlet and exhaust fumes associated with the purifier, but not the mechanism in the purifier that kills the virus. Even with an air purifier, the transmission of viruses into the air is still important.

“Our results show that installing an air purifier can increase the droplet spread,” Drikakis said. “The air inlet integrated in the purification device causes flow of circulation which can contribute to the transport of infected saliva droplets in the cabin.”

The observed effect increases with the number of infected persons in the elevator. Limiting the number of people allowed in an elevator will minimize the spread of the virus, as well as the design of air purifiers and ventilation systems.