Two days after spending an hour (wearing an N95 respirator) in a seminar room full of mostly unmasked people, I tested positive for covid. I isolated at home for a week but apparently didn’t isolate enough from my wife. Fortunately, we both had fairly mild symptoms, presumably thanks to being up-to-date with vaccinations.

Then the generally-reliable New York Times published a commentary about a meta-analysis by the generally-reliable Cochrane Collaboration, apparently showing that mask mandates don’t reduce covid spread. I’ve seen so many people “wearing masks” (often cloth masks that don’t block aerosols) with their noses exposed, that I’m not too surprised. I can’t control what other people do, but should I keep wearing an N95? Based on this expert commentary, I will.

But what would really help is improving ventilation. A commentary in Science notes that respiratory disease costs the US 50 billion dollars a year. They argue that (by analogy with restaurant inspection certificates):

“Wide use of monitors displaying the state of IAQ [indoor air quality] must be mandated… visible displays will help keep building operators accountable for IAQ and will advance public awareness, leading to increased demand for a safe environment.”

I was told that Georgia Tech has CO2 monitors in most rooms on campus, which grad students used to identify safer times to work in the lab, during peak infection months. This excellent review in the Journal of Fluid Mechanics suggests that, if CO2 exceeds 750 ppm, you’re breathing too much of other people’s exhaled air. Hot breath rises, so:

“displacement ventilation, which encourages vertical stratification and is designed to remove the polluted warm air near the ceiling, seems to be the most effective at reducing the exposure risk.”