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Covid-19 precautions have turned many parts of our world into a giant salad bar, with plastic barriers separating sellers from buyers, dividing customers in nail salons and protecting students from their classmates.
Intuition tells us that a plastic shield would be protective against germs. But scientists who study aerosols, airflow, and ventilation say that most of the time barriers don’t help and probably give people a false sense of security. And sometimes the barriers can make things worse.
Research suggests that in some cases a barrier protecting an assistant behind a cash register can redirect germs to another worker or customer. Rows of clear plastic guards, like the ones you might find in a nail salon or classroom, can also obstruct normal airflow and ventilation.
Erecting plastic barriers can alter the airflow in a room, disrupt normal ventilation, and create dead zones where viral aerosol particles can accumulate and become highly concentrated.
Under normal conditions in shops, classrooms and offices, particles of exhaled air disperse, carried by drafts and, depending on the ventilation system, are replaced by fresh air approximately every every 15 to 30 minutes. But erecting plastic barriers can alter the airflow in a room, disrupt normal ventilation, and create “dead zones” where virus aerosol particles can accumulate and become highly concentrated.
“If you have a forest of barriers in a classroom, it’s going to interfere with proper ventilation of that room,” says Linsey Marr, professor of civil and environmental engineering at Virginia Tech and one of the world’s foremost experts in the field. viral transmission. “Everyone’s sprays are going to get trapped and stuck there and build up, and eventually they’ll spill out beyond your own desk.”
There are certain situations in which transparent screens can be protective, but it depends on a number of variables. Barriers can prevent the large droplets ejected during coughing and sneezing from splashing onto others, which is why buffets and salad bars often have transparent sneeze guards over the food.
But Covid is largely spread through invisible aerosol particles. While there isn’t much real-world research on the impact of transparent barriers and the risk of disease, scientists in the US and UK have started to study the issue, and the results are not reassuring.
A study published in June and led by researchers at Johns Hopkins University in Baltimore, for example, showed that desk screens in classrooms were associated with an increased risk of coronavirus infection. In a Massachusetts school district, researchers found that plexiglass partitions with side walls in the main office were obstructing airflow. A study of schools in Georgia found that office barriers had little impact on the spread of the coronavirus compared to improvements in ventilation and masking.
Although a barrier can help a salesperson avoid an immediate and direct hit, particles are still in the room, posing a risk to the helper and others.
Before the pandemic, a study published in 2014 found that office dividers were among the factors that may have contributed to disease transmission during a tuberculosis outbreak in Australia.
British researchers have conducted modeling studies simulating what happens when a person on one side of a barrier – such as a customer in a store – exhales particles when speaking or coughing under various ventilation conditions. The screen is more effective when the person is coughing because the larger particles have greater momentum and hit the barrier. But when a person speaks, the screen doesn’t trap exhaled particles – which just float around them. Although the store worker can avoid an immediate and direct hit, the particles are still in the room, posing a risk to the assistant and others who can inhale the contaminated air.
“We have shown this effect of blocking larger particles, but also that the smaller aerosols pass through the screen and mix in the ambient air in about five minutes”, explains Catherine Noakes, professor of environmental engineering for buildings. at the University of Leeds, England. “This means that if people interact for more than a few minutes, they would likely be exposed to the virus regardless of the screen.”
Noakes says putting up barriers may sound like a good idea, but it can also have unintended consequences. She conducted a study published in 2013 that looked at the impact of partitions between beds in hospitals. The study showed that although some people were protected from germs, partitions carried air from the room to others.
So while a worker behind a transparent barrier might be spared some customer germs, a nearby worker or queuing customers could still be exposed. Noakes says most of the screens she’s seen are positioned incorrectly and probably won’t be of much use.
Most researchers say screens are most likely useful in very specific situations. A bus driver, for example, protected from the public by a floor-to-ceiling barrier is likely to be protected from inhaling much of what passengers breathe out.
“I think this can be a particular problem in places like classrooms where people are present for longer periods of time,” says Noakes. “A lot of individual screens obstruct airflow and create pockets of higher and lower risk that are difficult to identify. “
To understand why screens often have little impact on protecting people from aerosol particles, it’s helpful to think about exhaled breath such as a plume of cigarette smoke, according to Marr. “One way to think of plastic barriers is that they are good at blocking things like spit balls but ineffective for things like cigarette smoke,” she says. “The smoke just drifts around them, so they’ll give the person on the other side a little extra time before they’re exposed to the smoke.” During this time, people on the same side as the smoker will be exposed to more smoke, as the barriers trap her on that side until she has a chance to mingle throughout the space.
Most researchers say screens are most likely useful in very specific situations. A bus driver, for example, protected from the public by a floor-to-ceiling barrier is likely to be protected from inhaling much of what passengers breathe out. A bank teller behind a glass wall or a receptionist checking in patients in a doctor’s office can be at least partially protected by a barrier.
A study by researchers at the US National Institute for Occupational Safety and Health tested transparent barriers of various sizes in an isolation room using a cough simulator. The study, which has yet to be peer-reviewed, found that, under the right conditions, taller screens, above ‘cough height’, prevented around 70% of particles from reach the particle counter on the other side, where the store or salon employee would be sitting or standing.
If there are aerosol particles in the classroom air, the screens around the students will not protect them. Depending on the airflow, a downdraft in the small spaces you are now confined to can cause particles to concentrate in your space.
But the study 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 constant ventilation rates that did not “accurately reflect all real-world situations,” the report said.
The study does not consider that workers and customers are on the move, that other people might be in the room breathing the redirected particles, and that many stores and classrooms have multiple stations with acrylic barriers, not one. alone, which impede the normal circulation of air.
While more research is needed to determine the impact of adding transparent screens around school or office desks, all aerosol experts interviewed agree that the screens are unlikely are useful and likely interfere with normal room ventilation. Depending on the conditions, the plastic shields could cause virus particles to build up in the room.
“If there are aerosol particles in the classroom air, these shields around the students won’t protect them,” says Richard Corsi, incoming dean of engineering at the University of California, Davis. “Depending on the airflow conditions in the room, you can get downdraft in those small spaces that you are now confined to and cause particles to concentrate in your space. “
People shouldn’t panic when they see transparent barriers, but neither should they view them as completely protective. Workers and students who have transparent shields around them should continue to wear masks to reduce risk
Aerosol scientists say schools and workplaces should work to encourage eligible workers and students to get vaccinated, improve ventilation, add Hepa air filter machines in case need and impose mask requirements – all proven ways to reduce transmission of the virus.
The problem, experts say, is that most of the people tasked with erecting barriers in offices, restaurants, nail salons and schools don’t do so with the help of engineering experts who can assess the condition. air flow and ventilation for each room.
People shouldn’t panic when they see transparent barriers, but neither should they view them as completely protective. Workers and students who have transparent shields around them should continue to wear masks to reduce risk, according to Corsi. “The circulation of air in the rooms is quite complicated,” he says. “Each room is different in terms of the arrangement of the furniture, the height of the walls and ceilings, the vents, the location of the shelves. All of these things have a huge impact on the actual flow and distribution of air in a room, because every classroom or office space is different. – New York Times
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