Definition

The fine inhalable particles that are less than or equal to 2.5 micrometers in diameter are collectively known as PM2.5. They are more than 100 times thinner than a human hair (too small to be detectable by the human eye) making them more harmful than larger particles as they can penetrate deeper into the human respiratory tract when inhaled.

PM2.5 in the atmosphere consists of toxic organic compounds and also heavy metals released from various sources. Once released, they stay in the air for a long time i.e. days or weeks and can travel hundreds of miles.