HVAC systems should not be shut down. Air-flow can be reduced and pressure relationships maintained with the help of special drives on air handling units (variable ventilation systems).
When the unit is started following shut down, bursts of organisms tend to be released and may increase the risk of airlxime infection. Presence of moisture in the system can results in aerosols of fungal spores being released. An out break of fungal endophthalmitis following cataract surgery was traced to such aerosols. The fungus proliferated because the ventilation system wws turned olT routinely and the air was littered before humidification.