(METROPOLITAN) Lahore most polluted city as smog clouds horizon.

LAHORE -- Lahore was declared the most polluted city of the world on Tuesday as clouds of smog hovering over Punjab's capital city in the morning.

Overall reading of Lahore on the Air Quality Index measured 290 today. In some parts of the city the reading crossed 300 level on the air pollution scale.

The AQI is calculated on the basis of five categories of pollution: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. AQI as high as 151-200 is considered unhealthy, while an AQI rating between 201 to 300 is more harmful and over 300 is extremely toxic.

According to experts, an increase in air pollution used to be witnessed in the winter. A change in the wind speed, wind direction and sliding minimum temperature...