Four out of the five most polluted cities in the world are now in India, according to a recent survey released by the World Health Organization (WHO). Zabol, in Sistan and Baluchestan province of Iran overtook Delhi to become the city with the dirtiest air primarily due to dust storms in the summer. The other four cities that followed on the list include Gwalior, Allahabad, Patna and Raipur.

Air pollution levels are measured by the amount of Particulate Matter (PM) under 2.5 micrograms found in every cubic metre of air. The term PM refers to the mixture of solid particles and liquid droplets found in the air. Some of these particles are detected through the naked eye, while some require electron microscopes to be detected.

The Indian capital, which is known for its high pollution levels, was placed ninth on the list. Zabol clocked a so-called PM2.5 measure of 217, while Delhi had an annual average PM2.5 reading of 122. However, in 2014, Delhi was ranked as the most polluted city with its PM2.5 measuring 153.

In the long run, a tiny PM can cause strokes, lung cancer and heart diseases, including causing symptoms of heart attacks that can lead to fatalities. According to WHO, the quality of outdoor air attribute to around three million of the seven million premature deaths, which occur each year due to air pollution.

However, experts admit that although India does face a huge challenge, there are many countries that do not even have any monitoring system in place and, hence, had been left out of the rankings.

The survey of 3000 urban areas by the organisation – almost double of what was included in their 2014 study – showed that only 2% of the cities in poor countries had air quality that is on par with WHO standards. Around 44% of the richer countries are reported to meet WHO standards.

Delhi air pollution
As many as 15 Indian cities showed levels of air pollution that greatly exceed the prescribed national standards - Representational image PRAKASH SINGH/AFP/Getty Images

Maria Neira, a public health official of WHO said: "Probably some of the worst cities that are the most polluted ones in the world are not included in our list, just because they are so bad that they do not even have a good system of monitoring of air quality, so it's unfair to compare or give a rank.