Air Change Rates 101: Ventilation, Air Filtration are Building Blocks for K-12 Covid-19 Mitigation Efforts

Just this month, the CDC updated its guidelines to reflect what scientists and air quality experts have been saying for quite some time — airborne transmission of SARS-CoV-2 can occur indoors with virus particles suspended in the air. In schools across the country, school administrators and facility managers include indoor air quality as a key area of focus in their COVID-19 risk mitigation efforts.

The ASHRAE Epidemic Task Force published guidance for schools with practical information and checklists to minimize the risk of spreading SARS-CoV-2 in the learning environment. While the information can feel highly technical, the goal is quite simple — focus on keeping clean air flowing through the building.

It Starts with the Air Change Rate. What is It? And Why Does it Matter?

ASHRAE defines Air Change Rate as the airflow in volume units per hour divided by the building space volume in identical volume units. A less technical definition? Replacing the air in a defined space with clean air. Air Change Rate is typically expressed in Air Changes per Hour (ACH). If the ACH in a classroom is 4, that means the air is being replaced with clean air every 15 minutes. ACH matters — especially during the Covid-19 Pandemic — because as the ACH increases, the likelihood of exposure to airborne viruses and other harmful pathogens decreases.

What’s a “Good” ACH for Schools?

With ASHRAE’s guidance in mind, the Harvard T.H. Chan School of Public Health suggests schools aim for 4-6 air changes per hour in typical classrooms. For schools struggling to meet that threshold, there are ways to boost the number of air changes.

How to Boost Air Changes in Classrooms

Many schools struggle to meet the 4-6 ACH benchmark, especially if their building has poor ventilation or outdated HVAC systems. So what are their options to boost air change rates in their classrooms to keep their students, administrators and staff healthy?

• Increase Outdoor Ventilation. Working with facilities managers, schools can consider increasing air changes via natural methods like opening doors and windows or mechanical means such as opening the dampers in the HVAC system. It’s important to note that while opening windows is a quick fix, it’s not a reliable, long term strategy. The amount of air coming into a window varies depending on outdoor winds, temperature gradients and whether fans are used to exchange the air. In certain conditions, increasing outdoor air isn’t an option. For example, the recent wildfires on the West Coast impacted schools’ ability to rely on outdoor air to dilute stagnant indoor air. Also, uncomfortably cold weather, stifling heat or excessive noise pollution may also play a role.
• Increase Filtration. If a school has an HVAC system, cleaning recirculated air may be achieved by installing a MERV-13 rated filter or higher on the HVAC system. ASHRAE recommends MERV-13 or higher for COVID-19 mitigation. Again, it’s important to work with facilities managers and HVAC professionals, because not all HVAC systems can accommodate these high-efficiency filters. If increasing the outdoor air ventilation is a challenge, and the building can’t install a MERV-13 rated filter or higher, ASHRAE recommends increasing the mechanical filtration to the maximum possible and supplementing with local or in-room HEPA filters to reach the ACH target.
• Supplement with local HEPA Air Purifiers.  Schools can supplement existing methods for air changes by adding local HEPA air purifiers to remove airborne virus particles.

The good news is that solutions exist for schools to quickly and cost effectively achieve 4-6 ACH in their classrooms. What’s tricky, however, is that not all of these solutions are created equal.Some are only short-term fixes and are difficult to operate and manage at scale. To maximize your budget and future-proof your school, look for permanent, long-term air cleaning solutions that will not only mitigate COVID-19 risks today, but have the staying power to reduce the risk of airborne viruses for your students, administrators and staff in years to come.

Click below to read more in our guide to improving classroom air quality.