Many companies devised strategies based on WHO guidance that large respiratory droplets of the virus could infect people when first emitted and after they landed on surfaces. Now the concern over infection is focused on the idea that tiny droplets could linger in the air for hours. Companies are reviewing if they have gone far enough with policies on mask-wearing, sealing off conference rooms and upgrading ventilation systems. Some, like retailers who have installed plexiglass barriers in their stores between cashiers and customers, are wondering what else they can do if the larger droplets those barriers aim to contain are just one piece of respiratory transmission, consultants said.