I guess what makes me feel that doesn't fit perfectly is the "continuous contact for 15 minutes" rule. If straight up load was the main driver then I'd have tho

I guess what makes me feel that doesn't fit perfectly is the "continuous contact for 15 minutes" rule. If straight up load was the main driver then I'd have thought time would be a less important factor. But I can see if you simply assume load is proportionate to time in contact then you can model it as purely a function of total viral load.