NEW DELHI, NOVEMBER 14

Thick smog shrouded New Delhi and other cities around South and Southeast Asia as air pollution levels soared Thursday.

Air pollution in the region worsens particularly in winter when the burning of crop residue in agricultural areas coincides with cooler temperatures that trap the smoke.

The smoke is blown into cities, where there are more people and where auto emissions further add to the pollution. Emissions from industries without pollution controls and the use of coal to produce electricity are also linked to poor air quality in urban areas.

Several studies have estimated more than a million Indians die each year from air pollution-related diseases.

New Delhi's air quality fell into the severe category, according to SAFAR, India's main environmental monitoring agency. It measures particulate matter in the air that can enter the lungs.

In many areas of the city, the levels were more than 50 times higher than the World Health Organization's recommended safe limit. Forecasters warned air quality will worsen before the arrival of cold winds next week that could blow away the smog.

Lahore, Pakistan, which is on the border with India, had an air quality index level considered hazardous, according to the Swiss group IQAir, which tracks global air quality.

The levels in Hanoi, Vietnam, were unhealthy, according to IQAir.

Transport, industry and construction were the main causes of bad air in Hanoi, Nguyen Hoang Anh, deputy head of the environmental quality management division at the ministry's Pollution Control Department told state media on Monday.

Hanoi has around 1 million cars and nearly 7 million motorbikes. Many are old and don't meet emission standards. Construction projects also don't stop dust from escaping, and factories use fossil fuel for power, contributing to the city's smog.

Bangkok's air quality was unhealthy for sensitive groups on IQAir. The Meteorological Department said air ventilation rates have been poor recently and an atmospheric inversion layer caused airborne particles to accumulate.