The most precise test would be somehow to have a sample of air deep in the lungs. That would be the most precise, because you can inhale some but it does not all go into the lungs. Some of it is brought up in the sputum and it goes into the gut and has no effect. But the actual dose, to be very precise, would be, and so on, the little probe that goes right down deep into the alveoli. That's the way you measure that.
On February 14th, 2013. See this statement in context.