Central HVAC Systems generally provide heating, cooling, and ventilation (outdoor air) to a classroom. Central HVAC systems are generally equipped with filters that have a minimum efficiency rating value (MERV). Higher MERV ratings remove a greater percentage of small particles. A standard pleated HVAC filter will have a MERV8 rating. The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) recommends installing MERV13 filters to capture airborne viruses. A typical classroom of 960 ft2 will have an HVAC system supply air filtration rate of about 1,000 cubic feet per minute (CFM), meaning that the all the air in the classroom will be filtered six times per hour. Note that the HVAC fan must be running to provide filtration.
Portable Air Cleaners with High Efficiency Particulate Air (HEPA) filtration are widely available. They are measured in terms of their “Clean Air Delivery Rate” (CADR) which is a measure of the cleaning speed of the system in removing particles in a controlled environment. Portable air cleaners have CADR’s ranging from about 25-400 CFM (depending on size), and more than one could be used in a classroom to increase the amount of filtration provided.
An interactive tool published by the California Department of Public Health (CADPH) to estimate the change in relative risk of long-range airborne virus transmission under a range of filtration methods was used to predict the impact of portable air cleaners.