I also loved this post about viral load, with its specific numbers, and had that exact question. The exposure x time formula to reach the threshold of a viral load of 1000 particles must itself occur within some larger but limited time-frame. If the formula gets you a viral load in a few hours, you get sick. But presumably if you get a viral load of 100 particles once a month for 10 months—to take an obvious example—you won't get sick. I guess the basic question is, how quickly do you have to reach the viral threshold for it to overcome your body's ability to neutralize the virus?