Before we talk about PM 2.5 we need to talk about size. A micron is a unit of measurement and used to refer to the size of a particle. A healthy human eye with no defects can see particles as small as 10 microns but there are many particles in the air that are far smaller than this.
PM 2.5 is given to the type of particles that are so small that they can even enter the blood stream via your lungs. They have a diameter of 2.5 micrometers or less. It’s the most damaging type of particle because of this and also one of the most difficult to filter against because it’s so small.
The concentration of PM 2.5 particles tends to be worse in the large cities and particularly on days when there is little or no wind.
In a 2013 study on 312,949 people in nine European countries, it was found that for for every additional 10 microgram/m3 of PM 2.5 particles, the rate of lung cancer increased by 36%.
Modern diesel engines are a particularly big source of PM 2.5 and smaller particles, which can carry soot and other harmful substances in to the bloodstream and leave deposits on the body organs.
India has set a limit of 50 micrograms/m3 of PM 2.5 particles (the US has set a limit of 12 micrograms/m3), but many cities regularly exceed this with Delhi often reporting 200 micrograms/m3 or higher.