NEW DELHI– Authorities in the Indian capital on Monday closed schools, halted construction work and banned non-essential trucks from entering the city after air pollution reached its worst level this season.

Residents of New Delhi woke up to thick, toxic smog enveloping the city of some 33 million people as air quality became increasingly dangerous. It has become even more serious, according to SAFAR, the country’s main environmental agency, which measures tiny particles in the air that can penetrate deep into the lungs.

The deadly haze covered the capital’s monuments and high-rise buildings, with visibility so low that airlines warned of delays.

In several areas of the city, pollution levels were more than 50 times higher than the safe limit recommended by the World Health Organization. Forecasts indicate poor air quality will continue through the week.

Air pollution in northern India is increasing every year, particularly in winter, as farmers burn crop residues in agricultural areas. Combustion coincides with colder temperatures, which trap smoke in the air. The smoke is then released into cities, where car emissions add to the pollution.

Emissions from industries and the burning of coal to generate electricity are also linked to pollution, which has been is growing steadily in recent weeks.

Since Monday, authorities began implementing Stage 4 of a Graded Response Action Plan, or GRAP 4, based on the severity of air pollution. The first stages of the plan were already in place and the fourth stage includes stricter restrictions.

Classes for all grades except grades 10 and 12 will take place online and no trucks will be allowed into the city except those carrying essential items. Some older, gas-guzzling vehicles were banned from the city and all construction activities were halted. Authorities also urged children, the elderly and people with chronic illnesses or respiratory problems to avoid going out as much as possible.

Over the weekend, farmers in neighboring Uttar Pradesh state set their fields on fire, releasing plumes of gray smoke that winds likely carried to New Delhi and other nearby areas. Despite the poisoned air, many people in the capital continued their usual activities, including morning walks in the city’s beloved Lodhi Garden.

“Everyone has a sore throat,” said Sanjay Goel, a 51-year-old trader in New Delhi. “They should ban burning crop residue…it’s just smoke everywhere.”

The deterioration of air quality in the capital has also sparked outrage among residents on social media. Many complained of headaches and dry coughs, calling the city “apocalyptic” and a “gas chamber.” Others urged authorities to resolve the public health crisis once and for all. Several studies estimate that more than a million Indians die each year from pollution-related diseases.

The authorities invoked similar measures in the past and sometimes deployed water sprinklers and smog guns to try to control the haze. But critics say there needs to be a long-term solution that significantly reduces the pollution itself, instead of measures to mitigate the effects after it has already hit the region.