LAHORE: The air quality in Lahore dropped to "hazardous" level due to high levels of toxins in the atmosphere, making the Punjab capital the most polluted city in the world, IQAir data released on Tuesday showed. 

Citizens are urged to practice caution to stay safe from effects of pollution while provincial authorities rush to tackle the worsening smog crisis.  

The latest data showed that Lahore has crossed the 400 US AQI mark which means its air quality is hazardous, the worst possible.

Speaking to Geo.tv, environmentalist Yasir Husain said that healthy air is below 50 US AQI, which measures PM2.5 particles. This means Lahore's air is 80 times more polluted than a healthy level, he added.

"With the season direction of air flows have changed. In winter, wind flows from Pakistan to India. Agricultural waste burning, vehicle exhaust and power production cause the most air pollution.

Due to a slower pace of wind in winter, the polluted air stays in the city, combining fog and pollutants from burning into deadly smog.

Here is the air quality classification according to IQAir:

An AQI of 0-5 is considered "good", 51-100 is considered "moderate", 101-150 is "unhealthy for sensitive groups", 151-200 is simply "unhealthy", 201 - 300 is "very unhealthy", while more than 301 degrees indicates "hazardous".

The second most polluted city in the IQAir's list is New Delhi, with a very unhealthy air quality as the AQI level in the Indian capital is 285.