[The_Media_never_lie]

Plutonium dust is about as deadly a material as humans can come in contact with in such a situation. A single one micron dust particle lodged in a lung will kill you.

A micron is a measure of length. I will gladly ingest a micron of plutonium so long as there is no width nor depth involved.

[OregonRancher]

Hmmmmmm. one micron = 39 millionths of an inch???

[Cboldt]

-- A micron is a measure of length --

"Screen size" is customarily a single dimension too, but the particles that get through are three dimensional. Similarly, filters are "micron rated."

That said, I think any radiation or chemical hazard is generally measured by mass.