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Physics study reveals one life-saving reason to take the stairs

Croisy / Edvard Nalbantjan / Shutterstock / Inverse

Cramped, old elevators can be rough rides at the best of times, but in the time of Covid-19, they have become especially worrisome. And thanks to a team of engineers from Cyprus, just how unsafe such elevators are all-too real.

Air filtration and ventilation are the gold standards of good pandemic hygiene because of their ability to effectively disperse and even sanitize infectious aerosol droplets which spread Covid-19.

But in a new study published this week in the journal Physics of Fluids, a research duo has discovered that these safety precautions may actually make elevators even more dangerous.

Some companies are fitting offices out with new-and-improved air filter systems, but these same measures may not provide as much protection from Covid-19 as you think.picture alliance/picture alliance/Getty Images

Why it matters — When it comes to Covid-19 safety, anticipating and mitigating infectious elements is the name of the game. Having a better understanding of how infections spread in common, cramped environments like elevators can help regulators find solutions beyond just sticking an air filter in and hoping for the best.

Every time an unmasked person coughs, thousands of potentially infectious droplets of saliva are flung into the air and left to ride the wind in search of another host to land on. The more open air the space, the less risk of infection from these saliva droplets. But in enclosed environments, like a tiny nine-square-foot elevator, the risk of infection begins to climb.

In just five seconds, a droplet of saliva can travel up 18 feet, report the authors.

The big idea — Wearing a mask reduces the risk from droplets but, especially in residential elevators, riders may not always heed this advice.

In addition to existing ventilation in elevators that make air safe to breathe, installing air purifiers that use ultraviolet radiation to sanitize droplets has become a popular solution. But the researchers behind this new study say these new safety systems are ignoring a key component that could negatively impact Covid-19 transmission: turbulence.

As air purifiers and ventilation systems circulate air in this environment, they in turn create nearly undetectable movement in the air which acts like an ocean wave to carry infectious particles further than if the air had been still.

Dimitris Drikakis is a lead author on the study and a professor of engineering and medical science at the University of Nicosia. He says that, while air purifiers might alter the environment's air circulation, it doesn't necessarily improve its transmission risk.

"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," Drikakis said in a statement.

What they did — To model how this increased transmission might occur, the researchers designed a simulated elevator and trialed four different scenarios with either varying levels of ventilation or the addition or subtraction of an air purifier.

Using a dummy and a simulated elevator, researchers investigated how air purifiers might really affect Covid-19 transmission.Physics of Fluids

Inside these simulated elevators, the researchers placed a "dummy" human with a mild cough. As the dummy expelled droplets during its cough, the team used fluid dynamics to model how different ventilation environments might affect transmission.

The results were pretty unsettling.

What they discovered — While you might expect higher levels of ventilation to improve transmission risk, the researchers found that more ventilation actually increased transmission, as did the addition of air purifiers.

"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."

Their model revealed that the lowest levels of ventilation were actually more effective at curbing transmission.

In addition to impacting small, enclosed environments like elevators, the researchers also right that similar principles could be applied to small residential rooms, aircraft cabins, and even spacecraft.

What's next — These results may be alarming, but it's important to remember that they are idealized, simulated results, meaning they might not exactly capture how this would all go down in the real world. One simplification, for example, is that the researchers did not factor in the UV sanitation aspect of air purifiers, which could affect their ultimate impact.

For now, the researchers argue that it is important to investigate how these environments can be sanitized and ventilated without also kicking up potentially dangerous turbulence.

As for what that means for us, maybe it's time to start taking the stairs.

Abstract: The impact of air ventilation systems on airborne virus transmission (AVT), and aerosols in general, in confined spaces is not yet understood. The recent pandemic has made it crucial to understand the limitations of ventilation systems regarding AVT. We consider an elevator as a prototypical example of a confined space and show how ventilation designs alone, regardless of cooling or heating, contribute to AVT. Air circulation effects are investigated through multiphase computational fluid dynamics, and the performance of an air purifier in an elevator for reducing AVT is assessed. We have investigated three different flow scenarios regarding the position and operation of inlets and outlets in the elevator and a fourth scenario that includes the operation of the air purifier. The position of the inlets and outlets significantly influences the flow circulation and droplet dispersion. An air purifier does not eliminate airborne transmission. The droplet dispersion is reduced when a pair of an inlet and an outlet is implemented. The overall practical conclusion is that the placement and design of the air purifier and ventilation systems significantly affect the droplet dispersion and AVT. Thus, engineering designs of such systems must take into account the flow dynamics in the confined space the systems will be installed.
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