In especially confined spaces, like an elevator, air purifiers may actually do more harm than good in the fight against airborne viral transmission.

Since the beginning of the global pandemic, health officials have warned of the dangers of viral transmission in confined spaces where air circulation is limited.

In hospitals, powerful ventilation and air purification systems ensure air a regularly cleaned and recycled to prevent the accumulation and spread of viral aerosols. Similarly, in airplanes, air is constantly recycled to minimize the viral transmission.

The air in elevators is mostly stagnant, periodically stirred up by the opening and closing elevator doors. But some elevator manufacturers have attempted to solve this problem by installing air purification systems.

These attempts may be misguided, however, according to a new study published Tuesday in the journal Physics of Fluids.

Air purification systems don't impact the cleanliness of the air in a confined space, they also influence air circulation. Until now, authors of the new study argue, the effects of air purifiers on air circulation in confined spaces weren't well understood.

For the new study, researchers tweaked a model designed to simulate the spread of saliva aerosol droplets produced by a mild, unmasked cough. They then adapted the model to the 3D equivalent of a five-person elevator.

The team also modeled the effects of masks and different weather conditions on the movement of viral particles inside the elevator.

Next, researchers looked at how an air purification system, with different combinations of inflow and outflow positions, would influence the movement of air inside the elevator.

"We quantified the effect of air circulation on airborne virus transmission and showed that installing an air purifier inside an elevator alters the air circulation significantly but does not eliminate airborne transmission," study co-author Dimitris Drikakis, researcher at the University of Nicosia in Cyprus, said in a news release.

The simulations showed the risk of viral transmission inside the elevator was actually lowest in elevators with the least air circulation.

"This is due to reduced flow mixing inside the elevator," said Talib Dbouk, study co-author and Nicosia researcher. "Regulatory authorities should thus define the minimum ventilation required depending on the type of building."

According to the study's two authors, the model did not consider differences in the effectiveness of the virus-killing mechanism in different air purifiers.

While in most large spaces, the benefits of air purification outweigh the pitfalls of increased air circulation, in some small spaces, the latest research suggests more stagnant air may be best.

"Our results show that installing an air purifier may increase the droplet spread," Drikakis said. "The air intake integrated inside the purifier equipment induces flow circulation that can add to the transport of contaminated saliva droplets in the cabin."

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