How can I best manage an all-air system (heating and air-conditioning) serving a single working space to reduce the risk of the spread of COVID-19?

It is the case for supermarkets (always open, even in an emergency period), or for some public places frequented by many people at the same time (such as shops, shopping centres, restaurants, bars, cinemas, theatres and gyms (closed during the emergency period), that crowding determines the greatest risk of the virus spread. This is true for both those who stay in the environment in their working hours, and for those who enter and stay only the time necessary to fulfil their needs. When the activities that are currently closed are reopened, it is likely that, for a certain period of time, people flow into the public places will be strictly controlled, as is the case in supermarkets today. In all cases, it will be essential to increase the outdoor air flow rate to reduce the risk. This can be done by carrying out what is proposed in the AiCARR document [1]. In particular, internal recirculation should always be closed to increase the outdoor air flow rate. Where this is not possible  due to the configuration of the system, as reported in [1] in the case of roof top HVAC units, the presence of recirculated air does not increase the risk of contagion in the public place.