If you’ve spent any time inside bars or restaurants during the pandemic, you’ve almost certainly encountered a space chopped up by a series of plexiglass dividers. These see-through plastic barriers, typically set up between tables and at bar tops or takeout counters, are supposed to provide protective boundaries, safeguarding people from the spread of COVID-19. But according to a new report from the New York Times, these ubiquitous features of pandemic life may not work the way that many assumed — especially the countless restaurateurs who’ve spent hundreds or thousands of dollars installing them.
SARS-CoV-2, the virus that causes COVID-19, is spread primarily through invisible aerosols that are exhaled when speaking (or, especially, singing), which can hang in the air for hours in spaces with poor ventilation. The higher the concentration of viral particles in a given space, the more likely one is to become infected — which is why delta, which produces extremely high viral loads in infected people, is so contagious. Dilution of viral particles through adequate ventilation is the most effective preventative measure against the spread of the disease in a given space — outdoors obviously being the most ventilated space of all. (This is why airplanes, which typically replace their air every two to three minutes, are relatively safe compared to virtually any other seemingly confined indoor space.)
And while physical barriers are effective at blocking big droplets released from a person’s nose or mouth when they cough or sneeze, they’re essentially useless at blocking the spread of aerosols within a contained room. In a pre-print paper (meaning its it has not yet been peer reviewed) published on July 30, British researchers determined that “screens are unlikely to provide any direct benefit in reducing exposure to the virus from droplets or aerosols when people are already located at 2 meters or greater or where they are not face to face.”
Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and an expert on viral transmission, told the Times to think about exhaled aerosols in terms of a plume of cigarette smoke. With a plastic barrier, “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.”
In other words, plexiglass screens may provide protection from the bigger, wetter droplets expelled during a sneeze, but they provide relatively little protection from aerosols. (A caveat is that plastic barriers may be useful in certain circumstances, like for bus drivers, who are typically closed off entirely from passengers, and therefore aren’t likely to breathe the same air.) Worse, barriers may also disrupt the normal airflow in a room, creating pockets of space where aerosols can accumulate.