LAHORE: The level of toxic smog that has been gagging life in Lahore for weeks dropped to "very unhealthy" category, but only briefly, as the air quality returned back to "hazardous" after a slight improvement on Tuesday. 

The city ranked second on the Swiss group's list of the most polluted cities with an air quality index (AQI) of 395 around 9:30am. 

Meanwhile, the (PM2.5) pollutants the fine particulate matter in Lahore's air that causes the most damage to health — was 54.5 times higher than the World Health Organisation's (WHO) annual air quality guideline value.

Earlier, the AQI in the megapolis dropped to 252 in the "very unhealthy" category between 7am and 8am. The improvement was recorded for the second time in two weeks. 

As per IQ Air, 300 is the threshold considered "hazardous" for human health.

The city of 14 million people close to the border with India has led the global air pollution charts, registering unprecedented levels of smog earlier this month.

However, New Delhi has replaced Lahore as the most polluted city in the world on the air quality monitor's list, with readings hitting their highest of this pre-winter season.

As of today, the Indian capital had an AQI of 431 around 9:30am.

Punjab, home to more than half of Pakistan's 240 million people, has been reeling under the toxic smog for weeks as temperatures go down with the arrival of winter.

The provincial authorities closed schools in its major cities on November 6, and on Friday extended the closure to November 24 in Multan and Lahore, the worst smog-hit cities in the country.

All outdoor sports in schools and other activities also remain banned in the province until January, with continuing crackdown operations on polluting rickshaws, barbecues and construction sites in pollution hot spots.

Seasonal crop burn-off by farmers on the outskirts of the city also contributes to toxic air the WHO says can cause strokes, heart disease, lung cancer and respiratory diseases.

While smog-laden air gags life in Punjab, port-city Karachi also struggles with poor air quality, ranking as the fifth most polluted cities in the world today. The city an AQI of 200.