According to Central Pollution Control Board (CPCB), which...

Particles in the size range $1-10 \mu \mathrm{m}$ have measurable settling velocities but are readily stirred by air movements whereas particles of size 0.1-1 $\mu \mathrm{m}$ have small settling velocities. Those below $0.1 \mu \mathrm{m}$, a submicroscopic size found in Urban air, undergo random Brownian motion resulting from collisions among individual molecules. Of all the different types of particulates in the atmosphere, the presence of trace elements such as cadmium, lead, nickel and mercury may constitute the greatest health hazard.