In welcoming students back to school, both private and public institutions have said much about their ability to keep surfaces clean. That worries me. The new coronavirus spreads through both droplets and aerosols, both types of spreading recognized since the end of May. The more people speak and shout and laugh and sing and breathe hard, the more virus particles they release into the air. Both pre-symptomatic and asymptom- atic people can be infectious, and some of those will be superspreader, releasing more infectious material than others. Uncovered flushing toilets also send aerosols into the air. Since aerosols are tiny virus particles that remain in the air for a long time, only fresh air can dilute them. Recirculating cooled or heated air without mixing it with fresh air is recirculating virus-laden air. Good filters might help, but I doubt that they are adequate in many building HVAC systems.