This chart is great, not only at showing the risk categories but also how to lower risk of transmission of COVID-19:
To reduce risk you must try to move from one block to a block on the left and/or above; from high occupancy to low occupancy, from no mask to mask. And regarding masks, it is interesting that it actually results in a lower risk for prolonged contact than no mask for short time.
The chart is from this paper, on the research of risk of transmission.
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