Adequate ventilation and air filtration are critical. HVAC system filters should be changed frequently. Schools would benefit by changing to higher efficiency filters—as high as can be used safely in existing HVAC systems. Filters with at least a MERV rating of 8, and preferably higher, should be used because higher efficiency filters will remove more of the smaller particles from the air that pose health risks. Check with HVAC system manufacturers regarding suitable filters for the systems.