Thank you for your helpful article. If masks & hand sanitizers are prevention for the hands, nose & mouth to SARS-COV2, then layers of prevention such as windows are the "holes" & "skin" of your house to the pandemic.

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There is no evidence that HEPA's work conclusively, but it makes rational sense. Small studies show that it can trap SARS-COV2 even though it is only 0.15 microns. We can debate this like masks till we all fall down, or we can shell out +$150 for a HEPA air purifier and be done with worrying.

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One more "mask" you can purchase for your house is a carbon dioxide meter. There is no proven studies yet, but if your room has greater than 800 parts per million (ppm) of CO2, there it is good evidence that your window needs opening or air needs purifying.

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Simple as that. Too bad poor people can't shell out $1000 for air-purifiers, masks, CO2 monitors and hand sanitizers.

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Here are some references:

From an engineer's perspective:

https://www.fastcompany.com/90538531/this-is-the-safest-indoor-space-to-prevent-the-spread-of-covid-19-according-to-a-mechanical-engineer

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.A model used in a scientific study:

https://www.amazon.com/GZAIR-Temperature-Relative-Mountable-0-5000ppm/dp/B08644N7QD/ref=sr_1_10?dchild=1&keywords=carbon+dioxide+meter&qid=1598372478&sr=8-10