We can keep kids in class by:
Monitoring Indoor Air Quality
Introducing Outdoor Air with Ventilation
Adding Portable HEPA Filters
Targeting 6+ Air Changes per Hour
Core ventilation and HEPA filtration recommendations from CDC
CDC indicates in point 9 of their ventilation FAQ that CO2 monitoring can provide information on whether ventilation is adequate. CDC goes on to suggest:
This is almost exactly what one large engineering campus in Research Park did, setting their Trane Demand-Control Ventilation system to a set-point of 600 ppm. The result was nearly outdoor levels of fresh air inside their campuses -- uniformly between 409 and 475 ppm of CO2.
If businesses see fit to protect their engineers this way, why not our future engineers?
Essential to Knowing if Engineering Controls are Operational
In EPA's Webinar, You Can't Manage What You Don't Measure, we learned about schools that are using Indoor Air Quality (IAQ) Monitoring as a basis for knowing whether they are successful at providing clean air in classrooms.
Boston Public Schools, for instance, provides an Indoor Air Quality (IAQ) Sensor Dashboard allowing parents and community members to see in near real time what the key air quality parameters are in classrooms.
This level of transparency provides trust, and has another purpose. Without monitoring, it is possible that unnoticed and unexplained system failures would yield unanticipated rates of infection, leading to erroneous and unnecessary fatalism about Covid, undermining confidence in engineering controls to solve the problem.
If a safety system system is unplugged in the corner and forgotten about, then it isn't going to do any good. We need monitoring so we can know that ventilation continues to work.
Introducing Outdoor Air
Schools need to maximize the volume of outdoor air introduced to classrooms. A study in Italy found that with mechanical ventilation achieving an average of 6 Air Changes per Hour (ACH), transmission of Covid was reduced by 82.5% (a factor of six). A pre-print study can be found here.
Our parents guide, which can be had by contacting us, contains details on how to ask your school about ventilation, outdoor air, air changes, and airflow rates to compare your school's ventilation to this.
Portable HEPA Filters
Supplemental Air Filtration to Achieve Sufficient Air Change Rates
Studies have come out showing that HEPA filters can reduce the risk of transmission of Covid in public places like hospitals and schools. Most notably, 6 Air Changes per Hour (ACH) can result in a factor of six reduction in exposure (82.5%):
Oct 4, 2020: Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS-CoV-2
Staying for 2 h in a closed room with a highly infective person, we estimate that the inhaled dose is reduced by a factor of six when using air purifiers with a total air exchange rate of 5.7 /h.
For schools that can't bring in adequate outdoor air, portable HEPA filters can cost less than a dollar per square foot to procure, and can keep more kids in class and learning.
At Least 6 ACH
The More, The Better
There is no consensus on what constitutes an acceptable level of risk, so there is no hard-and-fast law requiring a particular level of risk reduction yet. However, there is actionable information on what schools should reasonably be doing to significantly reduce risk, as seen above.
For that reason, experts like Bill Bahnfleth, the former president of ASHRAE and the current Chair of its Epidemic Task Force, are recommending 6 or more Air Changes per Hour (ACH).
This expert recommendation for 6 or more ACH is consistent with the excellent findings with 6 ACH of outdoor air and from HEPA filtration above.
Following New Research
A new pre-print study from Devabhaktuni Srikrishna asks the question of how many air changes per hour (ACH) are needed to achieve the protective level of N95 mask filtration with ventilation. Findings show a bare minimum of 12 ACH may be needed to provide equivalent ventilation of an N-95 mask for "far field" exposure only. In some cases, 12-30 ACH may be better, or in other words, the more ACH, the better.
Most schools currently have less than 6 ACH. We will be following this new research closely for school recommendations and pandemic-era best practices for ACH.