Benchmarks for safe indoor air

What are the benchmarks for fresh clean airflow our classrooms need to meet for safe indoor air? 

The Oregon Health Authority recommended 3 to 6 air changes per hour to PPS.  But at 3 changes of air per hour, it still takes 46 minutes for the HVAC system and air purifier to clear out 90% of the respiratory aerosols hanging in the air.

That is why the Lancet Task Force on Safe Work, Safe School, and Safe Travel, and a host of leading experts in safe air, recommend at least 6 changes of air an hour as best for safety from all airborne respiratory infections.  At 6 changes of air, it takes only 23 minutes to clear out 90% of aerosols. 

And the higher we go above 6, it’s even quicker, making classroom air better and safer for kid and teachers. If we get to 12 and higher air changes, we can get aerosols, PM 2.5, and other pollutants cleared out in under 12 minutes.

Meeting a minimum of 6 times an hour, and ideally getting closer to 12 times, where fresh clean air comes into the classroom has a host of benefits beyond clearing out viruses, too. Changing the air out between 6 to 12 times an hour clears out asthma triggers and particular matter that causes lung cancer. This level of ventilation and air filtration also helps kids concentrate and learn better.

We can get fresh and cleaned air coming in 6 to 12 times an hour from a combination of methods. Outdoor air that has gone through the superstar air-cleaning MERV 13 filter. Recirculated air that has gone through that MERV 13. And/or an air purifier that moves air through either a MERV 13 or a HEPA filter.

The California Department of Public Health recommends 6 to 12 changes of fresh, clean air each hour for schools.

What other leading experts recommend 6 to 12 air changes?

Dr. María Neira
Director of the Public Health, Environment and Social Determinants of Health Department at the World Health Organization

Dr. Neira recommends 6 changes of air an hour:

Richard Corsi, Ph.D., PE.
Dean of Engineering at University of California, Davis

Dr. Corsi is one of the world’s leading indoor environmental engineers, and he was our very own Dean of Engineering at Portland State University before he moved to Davis. Dr. Corsi says:

“I do not believe aiming for 4 or 6 ACH in schools or other indoor spaces is sufficiently aspirational & have written about how a combo of increased outdoor air supply and use of #CorsiRosenthalBox es can get us close to 10 equivalent ACH in classrooms.”

William Bahnfleth
Professor of Architectural Engineering, Penn State University
President of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers 2013-2014
Chair of the American Society of Heating Refrigeration, and Air-Conditioning Engineers Epidemic Task Force

“Before Delta emerged, a common recommendation was 4 eACH is OK for schools and 6 is excellent. Today I am recommending 6 confirmed and enhance that if feasible …”


Lancet Task Force on Safe Work, Safe School, Safe Travel

California Department of Public Health Indoor Air Quality Section Ventilation and Filtration to Reduce Long-Range Airborne Transmission of COVID-19 and Other Respiratory Infections: Consideration for Reopened Schools