Officials warned that smoke-filled air would blanket the New York City area on Sunday, creating unhealthy conditions for some, as soot and ash from Canadian wildfires drifted across the border.

The air quality health advisory, issued on Saturday, will expire on Sunday night, the National Weather Service said.

On Saturday, parts of the city reached 136 on the Air Quality Index, according to IQAir, a Swiss air quality monitoring company, briefly making it one of the cities in the United States with the worst air quality. Parts of Massachusetts and New Jersey were also under air quality advisories on Saturday.

The index in New York City was predicted to reach a maximum of 120 on Sunday, putting it in the classification of “unhealthy for sensitive groups,” according to the New York State Department of Environmental Conservation. The index could reach 135 in the Upper Hudson Valley and 120 on Long Island.

The Weather Service advised vulnerable groups, which includes young children and those who have respiratory ailments, to take precautions like limiting strenuous outdoor activity.

An index reading of below 50 is considered good. A reading of above 150 means the air is considered “unhealthy,” and the ill effects might be felt more widely by healthy people.

For weeks, smoke from wildfires in Canada has been drifting across the border to the United States during its annual fire season, which usually runs from March to October.

Wildfires are a common cause of extended periods of unhealthy air, and fine particles of soot, ash and dust can billow high in the air and be blown for hundreds of miles by prevailing winds.

The drifting smoke from Canada’s fires has become a growing issue in the United States in recent years. In 2023, wildfires burned more land in Canada than ever before, and created smoke so intense that it turned the skies over New York City a frightening shade of orange. Experts have warned that climate change was turning environments like Canada’s forests into a tinderbox.

This year, smoke from Canada’s wildfires has again drifted south across the U.S. border, inflaming diplomatic relations. This month, six Republican lawmakers wrote to Canada’s ambassador to the United States, demanding to know the Canadian government’s plan for tackling the wildfires and accusing it of lax forest management.

The premier of Manitoba Province in Western Canada, where wildfires have been particularly bad, blasted the letter, accusing the U.S. lawmakers of “trying to trivialize and make hay out of a wildfire season where we’ve lost lives in our province.”