As the gases rise in the smoke stack, they cool and some of the gas molecules come together to form additional fine particles. The resulting particles are exceedingly small when they are emitted into the environment. Fine particles behave entirely differently from coarse particles and, as we will see, are much more dangerous to humans. Fine particles are also much more difficult and expensive to control. They are also invisible, so when they are not controlled, there is no way to know it except by monitoring with the proper instruments.
Incinerators emit large numbers of particles, despite the best available control technology.
Half of all the particles emitted will have a diameter less that 2 um, and the majority of these will have a diameter of 0.3 um. It is difficult to imagine how small these particles are. You can fit 40,000 particles with a diameter of 2 um on the dot. For regulatory purposes, coarse particles are considered to be the same as fine particles, as if they were all equivalent. There are 437.5 grains in an ounce.