What you breathe in adds up with every minute, and faster when you breathe hard. A mask divides it: 8 hours in an N95 adds up to about 48 minutes with no mask.
time × how hard you breathe ÷ protection factor = time unmasked, sitting still
Each bar is what you breathe in, shown as time with no mask, sitting still.
Breathing the same air for twice as long means twice the dose. A mask divides every minute: an hour in an N95 counts like 6 minutes unmasked, a 12-hour flight like 72 minutes.
Hard exercise moves over ten times as much air as sitting, so an hour at the gym can count for more than a day at a desk.
Adult averages, litres per minute (US EPA).
The clock assumes the air stays the same. Crowded, stuffy rooms and smoke hold more particles, so each minute counts for more. Fresh air, open windows and distance slow it down.