KYIV, Ukraine (AP) — Authorities in Ukraine advised residents in the capital Kyiv to stay indoors Friday as air pollution, partly caused by fires in the region, blanketed the city.

Ukraine’s Ministry of Environmental Protection and Natural Resources said the pollution was a result of the burning of peatlands and other wildfires in the region combined with autumn temperature fluctuations.

The capital woke up to thick smog with the rancid smell of blazing fires in the air. Some people were spotted wearing masks.

The Ukrainian capital topped a list of the most polluted major cities early Friday in a real-time database by IQAir, a Swiss company that monitors air quality levels. Its air quality appeared to have improved somewhat since as the city came down in the ranking later in the day.

Kyiv’s Department of Environmental Protection and Climate Change said that “the likely cause of this is fires in the Kyiv region.”

Fires have been reported in the Vyshhorod district, around 20 kilometers (around 12 miles) north of the capital.

Officials warned about an increased concentration of suspended particles, such as dust, soot, and smoke, in the air. In some areas of the city, air pollution levels have reached the maximum of the 100-point scale.

While human-driven climate change does not directly cause fires, it can increase the risk of wildfire as warming temperatures and increasingly dry air, trees and soil can make it easier for fires to spread. Forest fires around the world have worsened in recent years, with almost twice as much tree cover burning in 2023 than 20 years ago, according to the World Resources Institute.

Wildfire smoke can cause air quality to deteriorate even many miles away from fires. The main concern from the wildfire smoke is fine particle pollution, known as PM2.5. Fine particle pollution can cause short-term problems like coughing, as well as long-term impacts on the lungs and heart.

Pollution is a major health concern — with one major study estimating that pollution kills around 9 million people globally a year.

Residents in Kyiv were advised to close their windows, limit time spent outdoors, drink plenty of water, and use an air purifier.

“Particular attention should be paid to these recommendations by people with respiratory and cardiovascular diseases,” said a statement from the Ecology Ministry.

Autumn temperature variations trap harmful substances in the air, worsening pollution and reducing air quality, the ministry added.