1/ Recommend getting an affordable CO2 meter (see post #2) & do this experiment (here: supermarket), post results. 1000ppm a bit high, but can get much much higher (and thus very risky).

But filtering (HVAC, HEPA) don't remove CO2. So places w/ filters, CO2 overestimates risk) https://twitter.com/mdc_martinus/status/1292113878936150016

2/ In post below I show one v useful CO2 meter and describe how to use it to measure ventilation rate (same meter, different experiment -- #1 is just measuring CO2 in indoor spaces such as offices, schools, shops etc) https://medium.com/@jjose_19945/how-to-quantify-the-ventilation-rate-of-an-indoor-space-using-a-cheap-co2-monitor-4d8b6d4dab44?source=friends_link&sk=6cda52f5682a4a450a10691f07d1ad2c

3/ Important to check that meter reads ~415 ppm outdoors (adjust otherwise), these cheaper analyzers do drift around.

$159 on Amazon for that particular one, I think cheaper in US here: https://naltic.com/aranet4-co2.html

Some cost, but can share with others, across school, business...