NEW DELHI (AP) — Authorities in India’s capital shut schools, halted construction and banned non-essential trucks from entering the city on Monday after air pollution shot up to its worst level this season.

Residents of New Delhi woke up to thick, toxic smog enveloping the city of some 33 million as the air quality became increasingly hazardous. It rose further into the severe category, according to SAFAR, the country’s main environmental agency, which measures tiny particulate matter in the air that can enter deep into the lungs.

Air pollution in northern India rises every year, particularly in winter, as farmers burn crop residue in agricultural areas. The burning coincides with colder temperatures, which trap the smoke in the air. The smoke is then blown into cities, where auto emissions add to the pollution.