Thai officials shut hundreds of schools in Bangkok on Wednesday as toxic haze hung over the city, making the capital’s air among the worst in the world.

The metropolis of about 10 million people closed 437 government schools until Friday, its local authority said. The air quality index, or AQI, exceeded 170 around lunchtime, the seventh worst urban reading globally, according to data from AirVisual, which tracks pollution levels.

Prime Minister Prayuth Chan-Ocha, leader of Thailand’s military government, said he’s also seeking temporary closure of some factories during pollution spikes to tackle the problem.

Bangkok is grappling with a severe seasonal deterioration in air quality for the second year in a row. Officials have blamed everything from exhaust fumes, factories and rubbish burning to smoke billowing from rural regions as farmers set fields alight to clear them.

The city has resorted to desperate measures to try and clear the visible grey haze, including spraying water off skyscrapers and flying rainmaking sorties to wash away the pollution, but with little success.

The smog spike is unwelcomed for a city that relies on tourism for economic growth. Bangkok received more than 20 million international overnight visitors in 2017, the highest figure in the world, according to a gauge compiled by Mastercard.