The provincial capital of Punjab became the most polluted city in Pakistan and among the most polluted in the world after the city’s Air Quality Index (AQI) went above 400.

The Air Quality Index data was released on Monday and put Lahore on the list of the most polluted cities in the world, while Karachi came in fourth.

The PM2.5 (particulate matter) concentration in Lahore air is currently 75.6 times above the WHO’s annual air quality guideline value, The Nation reported.

Lahore suffers from high levels of air pollution, with the city regularly ranking at the top of pollution rankings of major global cities that includes many Indian cities, including Delhi. The issue of pollution has become a cause of concern among the public.

An AQI between 0 and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".

Air quality in Lahore usually worsens during the winter season from October to February when farmers in the wider Punjab province set fire to the crop stubble in their fields, which produces thick smoke that adds to the smog. At the same time, the weather changes keep the pollutants trapped in the atmosphere for long periods.

Air pollution in Lahore is caused by a combination of vehicle and industrial emissions, the smoke from brick kilns, the burning of crop residue and general waste, and dust from construction sites, factors almost the same as norther India's polluted cities, with Delhi being at the top with AQI of above 400 as well. 

Other factors of air pollution include large-scale felling of trees to build new roads and buildings, The Nation said.