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Indoor Air Quality (IAQ) Section

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 the potential benefits for schools of adequate outdoor air ventilation and air filtration in reducing long-range airborne transmission of respiratory infections. The report includes results of a modeling process, based on specific assumptions, to identify factors that could influence long-range airborne transmission of SARS-CoV-2 in classrooms. This work is part of the IAQ Section’s efforts to promote adequate ventilation and filtration in schools, which would have broad long-term health benefits.

Practical Implications

Our modeling identified multiple protective strategies that could 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—this practice reduces airborne transmission risk by more than half, independent of the rate of ventilation or air filtration in the classroom.
  • 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.

Report and supporting information  

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

Appendix 1:  Identified References with Brief Descriptions (Excel file)

Appendix 2:  Quanta Generation Rates for Aerosol Transmissible Diseases (PDF)  

Appendix 3:  Interactive model (Excel file)

Appendix 4: Ultraviolet (UV) Dose for Aerosol Transmissible Viruses and Information on UV Germicidal Irradiation (UVGI) (PDF)

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