If outdoor air is considered satisfactory, and air pollution poses little or no risk, all facilities, classrooms, and offices should be ventilated for at least a 24-hour period. Heating, ventilation, and air conditioning (HVAC) system filters should be replaced as soon as possible, and, if necessary, replaced frequently thereafter to remove additional ash and odorous particle build-up. 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 minimum efficiency reporting value or 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. Ash and other particles should be gently swept. High efficiency particulate air or HEPA filters should be installed on all vacuums. Hard surfaces, such as hard floors, desks, and bookcases should be cleaned with a wet cloth or towel. Objects that the students use, such as toys, athletic equipment, and drinking fountains that may have been exposed to settled ash or dust should also be thoroughly cleaned. In no instance should ozone-generating devices be used to “deodorize” classrooms because ozone can react with chemicals in the air and on indoor surfaces to create formaldehyde, ultrafine particles, and other harmful substances.