ASHRAE recommends you review HVAC programming to provide flushing two hours before and post occupancies. This includes operating the exhaust fans as well as opening the outside air dampers. For buildings without the capacity to treat large quantities of outside air and when outside air conditions are moderate, open all windows for a minimum of two hours before reoccupation.
Where increasing fresh air intake is not possible or is limited, add gas phase filtration to reduce contaminants.