World Health Organization has said that India is home to four of the five cities in the world with the worst air pollution. <br/><br/>But while WHO experts acknowledge India faces a huge challenge, many countries are so bad that they have no monitoring system and cannot be included in its ranking. <br/><br/>The dirtiest air was recorded at Zabol in Iran, which suffers from months of dust storms in the summer. The next four are all Indian cities – Gwalior, Allahabad, Patna and Raipur. India's capital New Delhi is the ninth worst city, measured by the amount of particulate matter under 2.5 micrograms found in every cubic metre of air.<br/><br/><br/>Maria Neira, head of public health, environmental and social determinants of health at the WHO, praised India's government for developing a national plan to deal with the problem when others have been unable to.