Actually that's not new, and it's not a "buzzword" either. A brief lecture:

My late toxicology professor used to say back in the 90's about lab air pollution experiments, "Rats can't inhale rocks". In other words, really large particles don't make it into the nose. Particles big enough to stay in the airstream are trapped in the mucus of the upper airways starting at the nose, and usually swallowed or ejected. Really tiny particles (less than 0.5 um) go in and out of the lungs with the air. But those around 2.5 um get sucked in and stick in the alveoli, the microscopic lung sacs where the actual exchange of oxygen and CO2 takes place.

That is why that particular size is important. Especially if they are carcinogenic materials (chemical or alpha-emitting radioactive) that stay in the lung for life and keep building up with exposure over time.