Acrid haze from Indonesian forest fires pushed air pollution to unhealthy levels across parts of Southeast Asia, prompting school cancellations and official warnings for millions of residents to stay indoors, according to Bloomberg.

Air quality ranged from “very unhealthy” to “hazardous” in parts of the Philippines and much of Borneo, according to Swiss monitor IQAir, as authorities stepped up their responses to a worsening cross-border crisis.

While forest fires are a seasonal occurrence across Southeast Asia, prolonged dryness and hotter temperatures brought by the El Niño weather pattern have heightened the risk of more widespread and longer-lasting emergencies. The number of fire hotspots in Indonesia spiked to nearly 10,000 last month, 10 times more than in the last two years, and more than 200,000 hectares have been scorched so far this year, according to official data.

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Quezon City, the most populous and largest city in the Philippines’ capital region, canceled in-person public school classes on Tuesday because of the haze.

“The public is advised to limit prolonged outdoor activities, especially those sensitive to air pollution,” the country’s Environmental Management Bureau said on Facebook. It also urged residents to stay indoors and to wear masks outdoors.

Meanwhile, Malaysia’s Sarawak state on Borneo — the island that the country shares with Indonesia and Brunei — was poised to close offices and undertake cloud seeding in response to the worsening conditions. For several days, state capital Kuching has ranked as having the world’s worst air quality, according to IQAir.

ALSO READ: Southeast Asia haze risk rises as Indonesia’s fire hotspots jump

“Should an emergency be declared, all offices as well as public and private sectors in the affected areas will be closed, with only essential services allowed to operate,” Sarawak’s deputy premier, Douglas Uggah Embas, was quoted as saying by The Edge Malaysia.

Cloud seeding operations in the state — known for its palm plantations, hydroelectric plants, and oil and gas operations — were scheduled for Sept 3-5 to replenish reservoirs that were seeing low water levels due to prolonged dryness, Malaysian newspaper the New Straits Times reported.

Indonesia has deployed thousands of firefighters on the ground in Kalimantan, as well as aircraft to “waterbomb” the blazes from above, Rizal Irawan, deputy for environmental law enforcement at the Ministry of Environment, was quoted as saying by Antara news agency.

READ MORE: Indonesia’s forest fires surge as El Nino raises drought risks

Air pollution has also hit hazardous levels in the Indonesian part of Borneo as well as cities on Sumatra island, disrupting day-to-day activities and businesses. School classes have moved online, while residents are shortening time spent outdoors. Shops and ride-hailing drivers have also reported weaker demand as people stay at home, and dozens of flights have been canceled or delayed due to low visibility, according to local media reports.