The air you breathe is not just made of good stuff. There is also dust, dirt, soot and smoke: all of which contains tiny, mostly toxic, particles. This is what scientists call particulate matter or PM. The smallest particles are the worst. They are 2.5 micrometers in diameter, far thinner than a strand of human hair.

In high concentrations, they can be deadly.

PM2.5 levels in Delhi-NCR peaked at 30 times more than the prescribed limit in the week after Diwali.

PM2.5 concentration in Delhi-NCR