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Indoor Air Quality (IAQ) Section

Aerosol Transmission and Airborne Diseases

Inadequate ventilation in California classrooms is common and is linked to higher illness absence. This is a cause for increased concern during the current COVID-19 pandemic caused by SARS-CoV-2. Growing evidence suggests that viral transmission can increase in crowded, poorly ventilated indoor spaces, through small aerosols that can remain airborne.

We have developed:
  • a report on modeling the potential benefits for schools of adequate outdoor air ventilation and air filtration in reducing long-range airborne transmission of respiratory infections,
  • a peer-reviewed journal paper on a more comprehensive multiplicative modeling framework addressing both short-range and long-range airborne transmission exposure risk reductions for various indoor and outdoor scenarios, and
  • additional useful information on practical considerations for ventilation and air filtration inspection and improvement for reopened schools.

This work is part of the Air Quality Section (AQS)’s efforts to better understand aerosol transmission and to promote adequate ventilation and filtration in schools, which would have broad long-term health benefits.

More information on school ventilation and air filtration can be found at:

CDC’s Guidance for Ventilation in Schools and Childcare Programs

EPA’s Program for Healthy Indoor Air Quality in Schools

Practical Implications

Multiple protective strategies can help to substantially reduce the risk of long-range airborne transmission of SARS-CoV-2 in classrooms. These include:
  • Mask wearing: All individuals (teachers, students, staff, etc.) should wear masks— under all ventilation rate or air filtration conditions in the classroom, this practice reduces both short-range and long-range airborne transmission risk comparing to not wearing mask.
  • Outdoor air ventilation: The system should provide at least the code-required minimum ventilation rate (per California Title 24). In classrooms with no ventilation and no filtration, the risk of long-range airborne infection would be over six times as high as that for classrooms with code-required ventilation and a MERV 8 filter.
  • Filtration: Ventilation system filters should be MERV-rated at MERV 13 or better. They should also be properly installed (i.e., no gaps that would allow air to bypass the filter) and properly maintained (i.e., replaced as often as recommended). MERV-rated filters can provide substantial protection from long-range airborne infection, especially if ventilation is poor.
  • In-room (portable) air cleaners: Air cleaners used to reduce the risk of long-range airborne transmission should provide high-efficiency filtration and a sufficient “clean air delivery rate” (CADR) (i.e., at least 2/3 of the floor area). Such air cleaners can provide substantial additional protection, especially in naturally ventilated classrooms (in which air is supplied only through open windows or doors) or in classrooms with non-functioning or poorly functioning ventilation systems. Multiple devices per classroom may be necessary for sufficient total air cleaning.

Reports and supporting information

The role of building ventilation and filtration in reducing risk of airborne viral transmission in schools, illustrated with SARS-CoV-2 (2020)(PDF)

Ventilation and Filtration to Reduce Long-Range Airborne Transmission of COVID-19 and Other Respiratory Infections: Considerations for Reopened Schools (2021) (PDF)

Window Openable Area Inspection Datasheet (SI Units) (Excel file, see Appendix B for details)

Window Openable Area Inspection Datasheet (English Units) (Excel file, see Appendix B for details)

CO2 Decay Ventilation Measurement Datasheet (SI Units) (Excel file, see Appendix E for details)

CO2 Decay Ventilation Measurement Datasheet (English Units) (Excel file, see Appendix E for details)



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