Plexiglass shields are everywhere, but it’s not clear how much they help

Plexiglass shields have become ubiquitous at offices, grocery stores and restaurants across the country in the coronavirus age. They were even installed on the vice presidential debate stage last week.

Given that they’re just about everywhere, you may wonder how effective they actually are.

Businesses and workplaces have pointed to plexiglass dividers as one tool they are using to keep people safe against the spread of the virus. But it’s important to know there’s little data to support their effectiveness, and even if there were, the barriers have their limits, according to epidemiologists and aerosol scientists, who study airborne transmission of the virus.

The Centers for Disease Control and Prevention (CDC) has offered guidance to workplaces to “install physical barriers, such as clear plastic sneeze guards, where feasible” as a way to “reduce exposure to hazards,” and the Labor Department’s Occupational Safety and Health Administration (OSHA) has issued similar guidance.

That’s because the plexiglass shields can in theory protect workers against large respiratory droplets that spread if someone sneezes or coughs next to them, say epidemiologists, environmental engineers and aerosol scientists. Coronavirus is thought to spread from person to person “mainly through respiratory droplets produced when an infected person coughs, sneezes or talks,” according to the CDC.

But those benefits haven’t been proven, according to Wafaa El-Sadr, professor of epidemiology and medicine at Columbia University. She says there have not been any studies that examined how effective plexiglass barriers are at blocking large droplets.

Moreover, the bigger problem is that even if they do, that’s not the only way that the coronavirus spreads. Last week, the CDC released new guidelines saying that the coronavirus can spread through aerosols — tiny particles containing the virus that float in the air and can travel beyond six feet — that are released when people breathe, talk or sneeze.

Most droplets people release when they talk or breathe are in a “size range that will flow past the barrier,” said Pratim Biswas, an aerosol scientist at Washington University in St. Louis.

The dividers “do not address all possible modes of transmission, such as aerosol transmission, or fully protect anyone from Covid-19,” the University of Washington’s Environmental Health and Safety Department said in a July review of the benefits and limitations of plexiglass barriers at campus facilities.

There’s also another problem in some cases: the size of the barriers. Marissa Baker, an assistant professor at the University of Washington, has been conducting a separate study of coronavirus safety measures at nine grocery stores in Seattle and seven in Portland, Oregon, each month since May.

She has observed that plexiglass shields at cash registers and self-checkout stations are often too small to even prevent droplet transmission between customers and workers.

“Some are smaller and don’t even cover the nose of a tall individual,” she said. “The airborne particles are going to be able to get anywhere, regardless of the barriers.”

Restaurants and retail industry trade groups say they view the barriers as one potential way to combat the spread of Covid-19, in addition to other measures.

“We know so much more now, and we follow the scientific data to guide our decisions. Based on the recommendations by public health authorities, it would be important to look at layered controls in any environment,” a spokesperson for FMI, a trade group for the grocery industry, said in an email. Mask wearing and maintaining social distancing “are still two effective means to mitigate risk of contracting the virus.”

The National Restaurant Association said in a statement that plexiglass shields and barriers are a tool that “when combined with other best practices — like face coverings, appropriate social distancing, and handwashing — provide an additional level of safety.”

Depending on where you work or where you visit, such measures may be less than realistic, though. For instance, cashiers and waiters can’t do their job without being in close proximity to customers.

That’s why upgrading ventilation systems is important to reduce the virus’ airborne spread in indoor settings, experts say.