NEW DELHI (AP) — New Delhi and other cities around South and Southeast Asia were shrouded in thick smoke as air pollution levels soared Thursday.

Air pollution in the region is worsened, especially in the winter months, when the burning of crop residues in agricultural fields coincides with cold temperatures that trap smoke. The smoke is blown into cities where there are more people and where automobile emissions further contribute to pollution. Emissions from uncontrolled industries and the use of coal to generate electricity are also linked to poor air quality in urban areas.

Various studies estimate that more than one million Indians die every year from air pollution-related diseases.

According to India’s main environmental monitoring agency SAFAR, New Delhi’s air quality has fallen into the serious category. It measures particulate matter in the air that can enter the lungs.

In many parts of the city, levels were more than 50 times the safe limit recommended by the World Health Organization. Forecasters have warned that air quality will worsen next week before the arrival of cold winds that could disperse polluted air.

The air quality index level in Lahore, Pakistan, on the border with India, was rated as hazardous, according to Swiss group IQAir, which tracks global air quality.

Levels in Hanoi, Vietnam, were unhealthy, according to IQAir.

The main causes of bad weather in Hanoi are transportation, industry and construction, Nguyen Hoang Anh, deputy head of the environmental quality management section of the ministry’s Pollution Control Department, told state media on Monday.

There are about 1 million cars and about 7 million motorcycles in Hanoi. Many are old and do not meet emissions standards. Construction projects also do not prevent dust from escaping, and factories use fossil fuels for energy, contributing to the city’s polluted air.

Bangkok’s air quality was unhealthy for vulnerable groups on IQAir. The Bureau of Meteorology said ventilation rates have been poor recently and the atmospheric inversion layer has caused airborne particles to accumulate.