Denver experienced the worst air quality of any city in the world for several hours Saturday as heavy smoke from raging western wildfires polluted the city.

IQAir, a Swiss air quality technology company, determined that Denver’s air was the most polluted early Saturday at an Air Quality Index of 167. The measurement is by US standards, where a higher number indicates poorer air quality.

The company tracks the AQI for 94 major world cities. Later on Saturday, the city of Johannesburg, South Africa, moved to the top spot with a US AQI of 178.

The polluted air is a symptom of the 107 wildfires that are currently burning in the US and which have scorched 2.1 million acres as of Saturday, according to the National Interagency Fire Center.

And it’s likely that the drought conditions fueling the fires will get worse before they get better.

“The fire outlook continues to reflect warmer and drier conditions leading to the high potential for severe wildfire activity throughout the western United States through the rest of summer and into the fall. Widespread high temperatures observed across areas in the West and with periods of lightning activity continue to exacerbate the wildfire situation,” the NIFC said on its website.

The wildfire season has been devastating for California, where the Dixie Fire — the nation’s largest active fire — continues to grow and destroy anything in its path.

The fire, which is burning in four California counties, swelled to 447,723 acres Saturday and remains 21% contained, according to Cal Fire.

Five people who were missing after the fire tore through the historic town of Greenville, California, have been found safe, the Plumas County Sheriff’s Office announced Saturday. A total of 21 unaccounted-for people are now confirmed safe, while authorities are still searching for five others, the office said.

Greenville was nearly leveled when the fire swept through it. In the community’s commercial center at Main Street and State Route 89, nearly every business appeared to have collapsed or been gutted, with flames still flickering in the debris up and down the sides of both streets, video recorded Wednesday by storm chaser Brandon Clement showed.

Fires across the region prompted California’s Placer County Public Health and the Placer County Air Pollution Control District to issue an air quality advisory Thursday, forecasting hazardous smoke impacts and warning residents to stay indoors.

“Sad to say, the prediction is correct,” Ann Hobbs, associate planner at the Placer County Air Pollution Control District, told CNN. Hobbs said officials had anticipated a change in the wind would bring poor air quality and smoke to residents living in the area.

“Avoid all unnecessary outdoor activity,” said Hobbs, adding that the air quality is the worst the area has seen all year.

Meanwhile, some areas within British Columbia are under evacuation orders or alerts as the White Rock Lake wildfire continues to spread, according to the British Columbia Wildfire Service.

The fire, burning near Vernon, has scorched about 135,900 acres. The city expanded its evacuation alert Friday night to include most of the city, according to a news release.

The expanded evacuation alert was made “out of an abundance of caution based on the advice of BC Wildfire due to increased fire activity and increased ember debris,” the City of Vernon tweeted.

Denver experienced the worst air quality of any major city in the world due to smoke from western wildfires – CNN