1/ More results (year 1) of our Healthy High School PRIDE study. Here we show a cumulative distribution plot of average rebreathed fraction (RF) of air in classrooms during the occupied day. RF is actually converted to a percentage on the vertical axis. 
2/ Note that a rebreathed fraction of 0.03 (3%), for example, means that 3% of every inhaled breath originated from the collective respiratory systems of others in the indoor space (including anyone infected with COVID-19).
3/ In my recent USEPA webinar on layered risk reduction strategies for schools I argued for a maximum RF < 0.008 (0.8%) for classrooms during this pandemic. That translates to an average CO2 concentration of less than 700 ppm.
4/ The median RF for our study was 0.025 to 0.027 (2.5 to 2.7%) for 2 seasons, values consistent with our findings for schools in the same part of TX 15 yrs earlier. Sadly, not much has changed. Woeful ventilation persists (primarily to lower energy costs for ISDs).
5/ Only one classroom had a RF of less than 0.008 (0.8%). Note that 20% of classrooms had RF > 0.03 (3%) with a maximum value of nearly 0.045 (4.5%).
6/ Many K-12 classrooms across US are under-ventilated (some extremely) during "normal" times. In fact, saying that we should double ventilation during the pandemic might only get us to where we should be during pre-pandemic times, e.g., in accordance with ASHRAE 62.1-2019.
7/ During this pandemic we should strive for classroom ventilation at 2x ASHRAE 62.1 2019 or more, if possible. Post-pandemic we should stress the benefits of appropriate ventilation of schools to improve #IAQ, reduce absences during cold & flu season, and improve performance.
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