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Are you able to square the levels of CO2 on planes, which are 1000-1800 ppm, so over 2-3x atmospheric air, with the 12-15 ACH? I don't understand how those two things can be true.

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ACH often refers to filtration and not ventilation; that is, recycled air counts for ACH calculations but won't decrease CO2 because that's not scrubbed by the filters. So it's possible you can have high CO2 levels but also no virus or particulate matter in the air; this is why some places will report PM2.5 as well as CO2, as a proxy for filtration. To make it more complicated, you can also use UV-C to inactivate pathogens, which may not decrease PM2.5 and won't decrease CO2.

I'm not saying this is the case necessarily for planes, but I'm just trying to provide context for how proxy measures of air quality may not tell the full story.

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