British researchers have conducted modeling studies simulating what happens when a person on one side of a barrier — like a customer in a store — exhales particles while speaking or coughing under various ventilation conditions. The screen is more effective when the person coughs, because the larger particles have greater momentum and hit the barrier. But when a person speaks, the screen doesn’t trap the exhaled particles — which just float around it. While the store clerk may avoid an immediate and direct hit, the particles are still in the room, posing a risk to the clerk and others who may inhale the contaminated air.
“We have shown this effect of blocking larger particles, but also that the smaller aerosols travel over the screen and become mixed in the room air within about five minutes,” said Catherine Noakes, professor of environmental engineering for buildings at the University of Leeds in England. “This means if people are interacting for more than a few minutes, they would likely be exposed to the virus regardless of the screen.”
Dr. Noakes said erecting barriers may seem like a good idea but can have unintended consequences. She conducted a study published in 2013 that looked at the effect of partitions between beds in hospitals. The study showed that while some people were protected from germs, the partitions funneled the air in the room toward others.
So while a worker behind a transparent barrier might be spared some of the customer’s germs, a worker nearby or customers in line could still be exposed. Dr. Noakes said most screens she has seen are “poorly positioned and are unlikely to be of much benefit.”
“I think this may be a particular problem in places like classrooms where people are present for longer periods of time,” Dr. Noakes said. “Large numbers of individual screens impede the airflow and create pockets of higher and lower risk that are hard to identify.”
To understand why screens often have little effect on protecting people from aerosol particles, it helps to think about exhaled breath like a plume of cigarette smoke, Dr. Marr said.
“One way to think about plastic barriers is that they are good for blocking things like spitballs but ineffective for things like cigarette smoke,” Dr. Marr said. “The smoke simply drifts around them, so they will give the person on the other side a little more time before being exposed to the smoke. Meanwhile, people on the same side with the smoker will be exposed to more smoke, since the barriers trap it on that side until it has a chance to mix throughout the space.”
Most researchers say the screens most likely help in very specific situations. A bus driver, for instance, shielded from the public by a floor-to-ceiling barrier is probably protected from inhaling much of what passengers are exhaling. A bank cashier behind a wall of glass or a clerk checking in patients in a doctor’s office may be at least partly protected by a barrier.
Understand the State of Vaccine and Mask Mandates in the U.S.
A study by researchers with the National Institute for Occupational Safety and Health in Cincinnati tested different sized transparent barriers in an isolation room using a cough simulator. The study, which hasn’t yet been peer-reviewed, found that under the right conditions, taller shields, above “cough height,” stopped about 70 percent of the particles from reaching the particle counter on the other side, which is where the store or salon worker would be sitting or standing.
But the study’s authors noted the limitations of the research, particularly that the experiment was conducted under highly controlled conditions. The experiment took place in an isolation room with consistent ventilation rates that didn’t “accurately reflect all real-world situations,” the report said.
The study didn’t consider that workers and customers move around, that other people could be in the room breathing the redirected particles and that many stores and classrooms have several stations with acrylic barriers, not just one, that impede normal air flow.
While further research is needed to determine the effect of adding transparent shields around school or office desks, all the aerosol experts interviewed agreed that desk shields were unlikely to help and were likely to interfere with the normal ventilation of the room. Depending on the conditions, the plastic shields could cause viral particles to accumulate in the room.
“If there are aerosol particles in the classroom air, those shields around students won’t protect them,” said Richard Corsi, the incoming dean of engineering at the University of California, Davis. “Depending on the air flow conditions in the room, you can get a downdraft into those little spaces that you’re now confined in and cause particles to concentrate in your space.”
Aerosol scientists say schools and workplaces should focus on encouraging workers and eligible students to be vaccinated, improving ventilation, adding HEPA air filtering machines when needed and imposing mask requirements — all of which are proven ways to reduce virus transmission.
This content was originally published here.