Ventilation Guidance for Schools: COVID-19
The information below is for school administrators and facilities managers who may have questions about ventilation and COVID-19. Buildings with complex heating, ventilation, and air conditioning (HVAC) systems typically work with HVAC professionals to evaluate and/or improve the flow of air. Note that the indoor air considerations listed below also apply to tents put up for classroom or other educational events. The definition of tents is having at least two or more sides enclosed.
While ventilation is important for good indoor air quality, consider it as part of a larger effort to provide a healthy school environment during the pandemic.
COVID-19 spreads mainly between people who are in close contact with one another. When indoors, there is less airflow to disperse and dilute viral particles when exhaled, so the risk of spread of COVID-19 to another person nearby is higher than being outdoors.
Protect yourself and others and promote health
Improvements to indoor air alone will not stop the spread of COVID-19, but proper ventilation used with other actions can help reduce the spread of the disease. Use ventilation considerations in conjunction with other actions, such as wearing a face covering, staying at least 6 feet away from others, frequent hand hygiene, and cleaning and disinfecting surfaces that are touched a lot.
Ventilation changes should be part of the indoor air quality assessment for your buildings. When making changes to bring more outside air into buildings, school administrators and facilities managers should always consult with the person appointed to manage the indoor air quality program.
Ventilate with outdoor air
Large buildings, such as schools, rely on heating, ventilating, and air conditioning (HVAC) systems to bring in fresh air from outside. In recent years, adjusting HVAC systems to bring in less outdoor air lowered heating and cooling energy costs. However, during the pandemic, increasing the amount of outdoor air brought into buildings reduces the amount of virus in the air, and minimizes recirculated air. The best time to make changes and upgrade an HVAC system is when school is not in session. HVAC systems need to function fully when people are in school buildings.
- Operate all HVAC systems in the mode for an occupied building for a minimum of one week before people occupy the building to ensure correct operation of the system. See Checklist No. 2 in Startup Checklist for HVAC Systems Prior to Occupancy, on page 5 of ASHRAE Epidemic Task Force, Schools & Universities (PDF).
- Open system dampers for the HVAC system you have already to allow for maximum outdoor intake. Increase the intake of air to above the ASHRAE minimum, to promote health while maintaining indoor comfort for people in the building, as defined by the design temperature and relative humidity.
- Minimize re-circulation of air within the building and instead bring in more air from outside.
- Demand control ventilation (DCV) should be disabled during the pandemic.
- Outdoor intakes should be more than 10 feet from the outdoor exhaust vents.
- Maintain negative pressure in the health office and bathrooms, to the extent possible.
- Each day the building is occupied, ventilation systems should be started at least two hours before people enter the building, and kept on for at least two hours after people have left the building.
- Opening windows may help to bring in more fresh air to increase natural ventilation, but the amount depends on temperature and pressure differences between the indoor and outdoor air. To further increase natural ventilation, a fan in a window may help. However, in buildings with HVAC systems, opening windows could affect mechanical ventilation in some areas and change airflow directions. Consult with an HVAC professional to change natural ventilation.
- Keep in mind that pollen, traffic-related air pollution, and other factors can trigger asthma attacks in some people. Air filtration may help relieve potential issues that bring outdoor air into a building can make worse.
Increase filter efficiency
Optimize your current HVAC system. ASHRAE recommends filters with a Minimum Efficiency Reporting Value (MERV) rating of 13 or higher for SARS-CoV-2.
- Upgrade system filters to a MERV 13 filter or to the highest MERV-rated filter that your HVAC system allows.
- Regularly inspect filters to make sure they are installed and fit correctly, with no gaps or air bypass.
- Change or maintain filters based on manufacturer recommendations.
Supplement with portable air cleaners
Portable air cleaners can be used in addition to an HVAC system’s filtration of air. It is important to use only the type with HEPA filtration. Ionizing units and air cleaners that produce ozone can be harmful to health.
- Select only filtering types of air cleaning devices, with high efficiency particulate air (HEPA) filters.
- If you need to prioritize the use of HEPA filters, the health office should be at the top of the list.
- Devices selected should be based on the size of the room, using the clean air delivery rate (CADR). This rate reflects the amount of air that a unit can process per unit of time.
- Consider using multiple units in various areas of the room.
- Airflow patterns and where people are located in the room should be considered.
- Consider noise generated from the air cleaning device.
- Some portable and in-duct air cleaning devices are capable of producing ozone, which could be harmful to health.
- Harvard: Experts offer advice on air purifiers for classrooms
Access the portable air cleaner calculator.
- California Air Resources Board: Air Cleaners & Ozone Generating Products
- In complex systems, evaluate ventilation and filters to optimize HVAC performance or before making major changes and verify changes afterwards to confirm expected results, called “commissioning” and “re-commissioning.”
- Evaluate with an expert how the HVAC system functions.
- To prevent blocking airflow, keep a 3-foot clearance around unit ventilators in classrooms.
- Make sure that supply air diffusers, exhaust, and return grills in classrooms are not blocked. They should be clear, clean, and dry.
- For rooms with high occupancy rates, use carbon dioxide (CO2) as a proxy for ventilation effectiveness. Carbon dioxide levels are easy to check with a low-cost, auto-read instrument. Ideally, keep CO2 levels at or below 800 ppm during the pandemic.
- Maintain indoor air relative humidity levels between 40% and 60% during spring and fall. Winter conditions may warrant lower levels of relative humidity to prevent excess moisture on windows and other surfaces. Too much humidity can allow for the presence of dust mites and mold, which can be triggers for people with allergies and/or asthma.
- It is unknown how effective ultrasonic waves, high intensity UV radiation, LED blue light, and other alternative disinfection methods are against the virus that causes COVID-19.
Resources and guidance
- Harvard: Schools for Health Risk Reduction Strategies for Reopening Schools (PDF)
- CDC: Operating schools during COVID-19: CDC’s Considerations
- EPA Supports Healthy Indoor Environments in Schools During COVID-19 Pandemic
- ASHRAE Journal: Guidance for Building Operations During the COVID-19 Pandemic (PDF)
- Taylor & Francis Online: Integrating IAQ control strategies to reduce the risk of asymptomatic SARS CoV-2 infections in classrooms and open plan offices