Covid precautions have turned many parts of our world into a giant salad bar, with plastic barriers separating sales clerks from shoppers, dividing customers at nail salons and shielding students from their classmates. Intuition tells us a plastic shield would be protective against germs. But scientists who study aerosols, air flow and ventilation say that much of the time, the barriers don’t help and probably give people a false sense of security.
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