Singapore’s air quality deteriorated to “unhealthy” levels on Saturday for the first time in three years, data from the National Environment Agency (NEA) showed, threatening to deepen a regional dispute over forest fires.

The 24-hour Pollution Standards Index, which Singapore’s NEA uses as a benchmark, was in a range of 87-106 in the afternoon.

A reading above 100 is considered unhealthy. This is the first time the 24-hour benchmark has breached 100 since August 2016, data shows.

The NEA said a confluence of winds over the nearby region was blowing the smog from Sumatra towards Singapore.

On the streets, some people wore masks or covered their noses to avoid breathing in the smoky air.

Every dry season, smoke from fires to clear land for palm oil and pulp and paper plantations in Indonesia clouds the skies over much of the region, raising concerns about public health and worrying tourist operators and airlines.