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Ventilation Key to Reducing Risk

Flow diagram evaluating potential risk reduction controls. If there is a building HVAC system: A) Demand-controlled ventilation, Disable B) Check max outdoor air intake, Open building dampers to max, if possible, change to the highest rated filter that can be accommodated by system, Check filter installation. If installed and fitted correctly, regularly inspect and replace. If not sealed and fitted properly, fix the filter assembly then regularly inspect and replace. Then measure air change rate of each classroom. Then confirm air change rate exceeds ASHRAE std 62.1 for maximum class size. If it’s not, increase air change rate. Then if you have operable windows, open windows and install a window box fan exhausting outward. If you don’t open windows, place a portable air cleaner in the room. If you don’t have operable windows, move outdoors or repurpose another space for use.   If there is not a building HVAC system: If you have a ceiling fan in classroom, rotate blades counterclockwise. If you have operable windows, open windows and install a window box fan exhausting outward. If you don’t open windows, place a portable air cleaner in the room. If you don’t have operable windows, move outdoors or repurpose another space for use.
Use the flow diagram above to evaluate potential risk reduction controls that may be feasible based on your school’s existing infrastructure.
Adequate indoor ventilation is important in reducing risks associated with COVID-19. Poorly ventilated indoor spaces can lead to increased levels of disease transmission.

Ways to improve school ventilation

  • increase the amount of outdoor air introduced into the building
  • improve the filtration of recirculated air
  • increase the volume of air exchange per hour
  • use supplemental devices such as portable air cleaners and window-mounted box fans. For maximum efficiency, box fans should be pointed outwards, drawing inside air outdoors.

HVAC Systems

It is important that a school’s heating, ventilation and air conditioning (HVAC) system be commissioned prior to reopening to verify its expected performance. Commissioning will confirm that:

  1. filters, dampers, as well as economizers seals and frames are intact, clean, functional and responsive to control signals
  2. temperature and relative sensors are properly calibrated and communicating with the building automation system
  3. air handling systems are providing sufficient airflow to individual rooms and exhaust fans are functional and venting outdoors.

HVAC Systems Best Practices

  • Ventilation systems that use 100% outdoor air eliminate the risk of viral material being recirculated in a building and therefore offer the greatest reduction of risk for indoor disease transmission.
  • Filters in a HVAC’s air handling unit are a first line of defense in removing virus germs from recirculated air. High Efficiency Particulate Air (HEPA) filters and Minimum Efficiency Reporting Value (MERV)-rated filters are effective at removing COVID virus aerosols. MERV13 rated filters are recommended as the top risk reduction strategy for COVID-19. Filters should be upgraded to the highest rated filter that can be accommodated by a school’s HVAC system.
  • It is critical to ensure an HVAC system’s filter cage is maintained. Any gaps in the housing assembly will impair filter efficiency. Filter changes are recommended after a period of low or no building occupancy (i.e. weekend, holiday). When feasible, filters can be disinfected with a 10% bleach solution or another appropriate disinfectant before removal.
  • Increasing the number of air changes per hour (ACH) in an indoor space can also enhance ventilation. ACH should be evaluated for each classroom in relation to the size of the room and occupants’ activities. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has established ventilation standards for different types of school spaces (ASHRAE 62.1 2019 – Ventilation for Acceptable Indoor Air Quality Standards). The amount of air flow into each school room should ideally exceed these minimum standards.
  • Extending operation of an HVAC system also helps improve ventilation. It is recommended that a school building’s HVAC system be turned on at least two hours prior to the arrival of staff, teachers and students and remain running at least one hour after all occupants have departed. The system should also be operated at maximum total airflow to the extent possible.
  • Using displacement ventilation in classrooms and offices is another effective engineering control that provides enhanced air mixing, moving potential contaminant air away from students and teachers.
  • To maintain HVAC operation at maximum air flows independent of the number of students, teachers and administrative staff in the school building, demand-controlled ventilation sequences should be disabled.

Other Ways to Improve School Building Ventilation

Natural Ventilation

For buildings using natural ventilation, opening windows, doors and skylights, or roof ventilators can help improve indoor air flow.

Air Purifiers

Diagram of how to assemble the filters and fan.
Low-Cost Air Purifier

Noise: Depending on the fan used, this unit can be relatively quiet (~35 dB).

What you need:

  • 20” x 20” box fan
  • 20” x 20” x 2: MERV13 or higher rated (i.e. HEPA) filter
  • Tape
  • Caulking putty

How to assemble: Attach the filter to the back of the fan with caulking putty and tape. This air purifier works best if above the ground — placement on a small table is a good option.

Air purifiers with HEPA filters are capable of removing virus, bacteria and fungal spores. Portable air purifier floor units as well as ceiling and wall mounted systems are available with a range of fan sizes and filter ratings. All air purifiers certified by the Association of Home Appliance Manufacturers (AHAM) have a clean air delivery rate (CADR) that can be used to evaluate performance. The CADR rating reflects the volume of air the device is capable of filtering in cubic feet per minute and the efficiency of the filter. The CADR rating reported for smoke is the most comparable to the size of SARS-CoV-2 aerosol germ. It is important to position floor units so that they maximize air filtration based on the configuration and use of a space.
diagram for making a high flow design air purifier
Low-Cost Air Purifier High Flow Design

Noise: Depending on the fan used, this unit can be relatively quiet (~35 dB).

The multiple filters used in this design allow for a higher flow of filtered air (~700 CFM)

What you need:

  • One 20” x 20” box fan
  • Five 20” x 20” x 1”: MERV13 or higher rated (i.e. HEPA) filter
  • Tape
  • Caulking putty
  • Small table

How to assemble: Attach the filters together with tape and secure these filters to the back of the fan with tape. Seal any gaps using caulking putty. When operating the unit, place the fan on a table off the ground.

You can also make your own low-cost air purifiers with materials you may already have. We provide instructions for two options. One only requires an air filter, a box fan, tape, and caulking putty. The other requires five air filters, but allows for a higher flow of filtered air. Depending on the fan used, these units can be relatively quiet (~35 dB)

Ultraviolet Germicidal Irradiation (UV-GI) Devices

UV-GI systems can neutralize airborne viruses and have been installed in high-risk settings, such as health care facilities. These systems use low pressure, mercury lamps which emit non-ionizing electromagnetic radiation. Multiple installation formats are available, including upper-air, in-duct and coil systems. Upper-room air disinfection systems achieve maximal efficiency in spaces with an appropriate fixture and sufficient vertical air exchange in high-ceiling rooms.