Fume particle aerodynamics

Fume particles, and particulate aerosols in general, are sorted by
aerodynamic size, the only practical way to handle such materials.
Spherical particles have predictable settling rates based on diameter and
density, but welding fume is complex, being made up of small spheres
(nanometer scale) agglomerated into larger clumps. The aerodynamic
settling times for these particles will be longer, but they are complex, and
their behavior will be less predictable, both in the air and in the human
body.

Micrograph on left from Jenkins, Mendez, and Eagar, "Effect of Arc Welding
Electrode Temperature on Vapor and Fume Composition," Trends in Welding
Research conference, Pine Mountain, Georgia, June 2005