PM 2.5 particles smaller than 2.5 micrometers can enter the systemic circulation if inhaled.

“they're so small that they can penetrate into our lungs and why do we care about that size the 2.5 microns that's because it can get through our lungs and into our blood”

“another thing that we we're going to talk a bit about and I've talked quite a bit about this in the past is actually particulate matters um of the 2.5 or smaller variant these are abbreviated p M 2.5 and again it refers to um particulate matters in the air that are smaller than 2.5 micrometers um so again why is that important well there's there's something relevant about a particle that's that small which is that if inhaled it has the potential at least to become systemic and the reason for that has to do with the anatomy of the lung and the size of uh both the Alvar air sacs and the epithelial Linings of them which again it's not necessarily that intuitive that you could breathe something but that it is small enough that it could actually get across a cell barrier at the innermost part of the lungs and enter the systemic circulation just as though it had been injected into you”