Sales of plexiglass tripled to roughly $750 million in the U.S. after the pandemic hit, as offices, schools, restaurants and retail stores sought protection from droplets that health authorities suspected were spreading the coronavirus.

There is just one hitch: Not a single study has shown that the clear plastic barriers actually stop the spread of the virus, said Joseph Allen of Harvard’s Chan School of Public Health.

“We spent a lot of time and money focused on hygiene theater,” said Allen, an indoor-air researcher. “The danger is that we didn’t deploy the resources to address the real threat, which was airborne transmission—both real dollars but also time and attention.”

“The tide has turned,” he said. “The problem is, it took a year.”

For the first months of the Covid-19 pandemic, top health authorities pointed to larger droplets as the key transmission culprits, despite a chorus of protests from Allen and other researchers. Tinier floating droplets also can spread the virus, they warned, meaning plastic shields can’t stop its transmission. Not until last month did the World Health Organization and U.S. Centers for Disease Control and Prevention fully affirm airborne transmission.

Plastic shielding had created “a false sense of security,” said building scientist Marwa Zaatari, a pandemic task force member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

“Especially when we use it in offices or in schools specifically, plexiglass does not help,” Zaatari said. “If you have plexiglass, you’re still breathing the same shared air of another person” in the room.

Recent CDC research found that desk or table barriers in Georgia elementary schools did not correlate with lower infection rates. Mask mandates and ventilation improvements did.

An April study published by the journal Science suggested that desk shields might even slightly raise the risk of Covid-like symptoms. And a prepublication paper from Japan late last month linked plastic shielding with infections in a poorly ventilated office.

Such studies raise the ironic possibility that when venues install too much plastic and thus impede ventilation, they could be raising the very risk they’re trying to reduce.

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