Quite a simple concept, it's particulate matter in the air, the greater the density the worse the effect on humans, animals, and ecology. It would be difficult to measure the effect of one specific particulate's effect.
Particle pollution — also called particulate matter (PM) — is made up of particles (tiny pieces) of solids or liquids that are in the air. These particles may include:
Dust
Dirt
Soot
Smoke
Drops of liquid
Some particles are big enough (or appear dark enough) to see — for example, you can often see smoke in the air. Others are so small that you can’t see them in the air.
"The air pollution disasters in London in 1952, the Meuse valley in 1930, and in Donoroa, Pennsylvania, in 1948 made it clear that extremely high levels of particulate-based smog could produce large increases in the daily mortality rate."