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

The positions of air inlets and outlets in confined spaces, such as elevators, greatly affect the transmission of the virus through the air. Inside Fluid Physics, show researchers at the University of Nicosia in Cyprus, although air purifiers are expected to help, they may actually increase spread.

Air quality in small spaces can degrade quickly without ventilation. However, adding ventilation will increase the rate at which air, possibly laden with viruses, can circulate in the small space. Elevator manufacturers have added air purifiers to address this problem, but the systems were not designed to take into account their effect on general air circulation.

Air purifiers use ultraviolet radiation to kill viruses and other microbes, but they also circulate air, sucking it in and exhausting clean air. This adds to the general circulation, an aspect that has not been considered in previous research.

Previous work by scientists has indicated that drops of saliva can travel 18 feet in five seconds when a person without a mask coughs. The authors extended the same model to examine the effects of facial masks and weather conditions.

The researchers performed calculations for a 3-D space equivalent to a five-person elevator. A mild cough was simulated in one position in space, and air inlets and outlets were added at various locations to study its influence on circulation. An air purifier was also included in the simulation.

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

The researchers found that the risk of transmitting the virus through the air is lower for low ventilation rates.

“This is due to the reduction of the mixing flow inside the elevator,” said author Talib Dbouk. “Regulatory authorities should define the minimum ventilation required, depending on the type of building.”

The study looked at the role of an air purifier, considering only the air intake and exhaust associated with the purifier, but not the purifier’s internal mechanism that kills the virus. Even with an air purifier installed, the transmission of the virus through the air is still significant.

“Our results show that installing an air purifier can increase the spread of droplets,” said Drikakis. “The integrated air intake inside the purifying equipment induces a flow circulation that can contribute to the transport of contaminated saliva droplets in the cabin”.

The observed effect increases with the number of people infected in the elevator. Restricting the number of people allowed in an elevator would minimize the spread of the virus, as well as improve the design of the air purifier and ventilation systems.