| Plexiglass is everywhere. Retail stores, restaurants, schools and offices raced to erect clear plastic shields when the pandemic hit. U.S. sales tripled to roughly $750 million last spring, in the rush for protection from the droplets that health authorities suspected were spreading the coronavirus. Just one hitch: To this day, not a single study has shown that the clear plastic barriers actually control the virus, according to Harvard's Joseph Allen, an indoor-air researcher who calls the plexiglass shields "hygiene theater." For the first months of Covid-19, top health authorities pointed to larger droplets as the key transmission culprits, despite a chorus of protests from researchers like Allen. Tinier floating droplets can also spread the virus, he and others warned, meaning plastic shields can't be counted on to stop them in venues like schools and offices where people still breathe shared air. In recent weeks, authorities affirmed such airborne spread. Plastic makes sense in certain settings, all agree: in front of a cashier who faces many people at close range through the workday, for instance.  A worker cleans a plexiglass shield at a pizza restaurant in San Francisco. Photographer: David Paul Morris/Bloomberg But Allen and other indoor-air experts maintain that for schools and offices, money has been best spent on improved ventilation and air filtration, along with masks. And they argue that cleaner air carries benefits beyond Covid: for mental function, productivity, and to reduce the spread of other germs, like seasonal flu. Recent CDC research found that desk or table barriers in Georgia elementary schools didn't correlate with lower infection rates. Mask mandates and ventilation improvements did. Although many hospital infection-fighters still support plastic shields, some small studies even suggest they may add to transmission by blocking air flow. That raises the ironic possibility that when venues install too much plastic and impede ventilation, they could be contributing to the very risk they're trying to reduce. With U.S. case numbers dropping, so are some plastic shields, according to scattered reports from restaurants, gyms and casinos. As more come down, the plastic industry offers some good news: Those plexiglass shields are 100% recyclable thermoplastic; the main challenge will be gathering it up and shipping it off for a second, post-pandemic life.—Carey Goldberg | 
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