Air pollution: Delhi is dirty, but how do other cities fare?

The WHO last week named India's capital the worst of 1,600 cities for PM2.5 particles. What are the other measures of pollution, and do the experts agree?

Last year, a study by the World Health Organisation found that Delhi had the dirtiest atmosphere of 1,600 cities around the world for PM2.5 particles.

PM stands for particulate matter - solid particles or liquid droplets in the air. When smaller than 2.5 micrometers (that's 100 times thinner than a human hair), they are known as PM2.5. These are created by such things as vehicle emissions, forest fires and smelting and processing metals.