The World Health Organization's (WHO) latest study on the most polluted places on the planet is out, and it's damning for one country in particular. Of the 10 most polluted cities in the world, nine are in India. Whittled down from the WHO's study of over 4,000 cities in 100 countries, CNBC takes a look at the 10 most polluted cities in the world based on a comparison of the average amount of PM2.5 — a particle considered so small that it can enter the lungs and cause serious health problems.

1. Kanpur, India

AFP/Getty Images

The city of Kanpur in northern India has an annual PM2.5 level of 173 micrograms per cubic meter, far higher than the WHO's recommendation of no more than 10 mcg per cubic meter.

2. Faridabad, India

Sanjeev Verma/Hindustan Times via Getty Images

The largest and most populous city in the state of Haryana, Faridabad borders India's capital New Delhi. Heavy traffic is just one factor that contributes to its annual average of 172 micrograms per cubic meter of PM2.5.

3. Varanasi, India

Kaveh Kazemi/Getty Images

Home to the Ganges river, Varanasi is India's spiritual home. Hindu pilgrims bathe in the river, but there is increasing concern that the air quality in the holy city has reached a crisis point. The city recorded 151 micrograms per cubic meter of the most dangerous particles.

4. Gaya, India

SUMAN/AFP/Getty Images

Another holy city with a myriad of both Hindu and Buddhist temples. Gaya notched 149 micrograms per cubic meter.





5. Patna, India

Arpan Basu Chowdhury/Pacific Press/LightRocket via Getty Images

A sprawling ancient city on the banks of the Ganges, Patna is noisy and congested city and is used as a transport hub. It recorded 144 micrograms of PM 2.5.





6. Delhi, India

Nasir Kachroo/NurPhoto via Getty Images

Formerly the world's most polluted city, Delhi has partially cleaned up its act but still averages PM2.5 levels of 143.





7. Lucknow, India

Pawan Kumar/Reuters

Famed for its Mughal architecture and colonial-era buildings, Lucknow's parks aren't enough to keep it out of the top 10. Lucknow recorded a PM2.5 average of 138.





8. Bamenda, Cameroon

Reuters

Unlike India's smog-filled cities, this Cameroonian city's air quality is compounded by extensive deforestation. It recorded an average of 132.





9. Agra, India

LUDOVIC MARIN / Getty Images

No other monument springs to mind quicker than the Taj Mahal. Yet the city, in India's Uttar Pradesh state, is falling behind rapidly. Polluting cooking fuels as well as cars contribute to the poor air quality. Agra's average PM2.5 level is 131.





10. Muzaffarpur, India

Nasir Kachroo/NurPhoto via Getty Images