The inhalation of these fine dust particles represents a health hazard that was known to the military as long ago as 1974.

BBC: UK to aid Iraq DU removal

Before people started making such a mess, the number of dust particles orbiting earth was tiny, since they would have been dragged quickly into the atmosphere by friction and burnt up.

ECONOMIST: Orbital debris

Collisions between the cosmic rays and the slower-moving protons that exist in the gas and the dust around a supernova should produce subatomic particles called neutral pions.

BBC: Cosmic rays: Fermi telescope settles mystery of origin

Matt Loxham, a medical biologist at Southampton University, collected minute dust particles of different sizes from at an underground train system in an unnamed European city.

BBC: Railway tracks

Wear a filtering mask when cleaning to avoid inhaling dust or airborne particles from cleaning products.

CNN: Sore throat

Although this is a small sample, it suggests that man-made dust particles predominate over naturally occurring ones.

ECONOMIST: Orbital debris

So to get into a human's lungs, they have to ride on mold spores or dust particles.

FORBES: Science Versus Hysteria