face cover，face cover，face cover，face cover，Around 39% of residents in Plumas County, California, are under evacuation orders as officials battle the second largest fire in state history, the Plumas County Sheriff's Office said on Sunday.Plumas is one of four counties -- along with Butte, Lassen and Tehama -- under the intense blaze of the Dixie Fire, which has been raging for 25 days and increased in activity Sunday amid hot, dry and windy conditions, according to Cal Fire."We have a deep responsibility to rebuild this community," California Gov. Gavin Newsom said while visiting the town of Greenville, which was ravaged by the fire last week. "But also address the intensity of what is occurring globally, but manifesting in smash-mouth ways here in California -- and that's climate change."Greenville, in Plumas County, was nearly leveled when the fire swept through it. Nearly every business in the city's commercial center appeared to have collapsed or been gutted, with flames still flickering in the debris, video recorded Wednesday by storm chaser Brandon Clement showed.As of Sunday, no deaths were reported in connection with the fire, but four people are unaccounted for, according to the Plumas County Sheriff's Office.The Dixie Fire is one of 11 wildfires burning across California. Throughout the country Sunday, there were 107 large wildfires burning in 15 states, while more than 2.2 million acres have burned, according to an update by the National Interagency Fire Center.Lingering heat could challenge firefighting effortsFor the 8,500 firefighters battling the flames that have burned over 489,000 acres, according to CalFire, conditions are not forecast to improve soon.A large area of the northwestern US and parts of Northern California are under an excessive heat watch. And though the areas impacted by the fire are not part of those warnings yet, they will likely see a marked warming trend over the next several days, CNN meteorologist Pedram Javaheri said."The heat is expected to linger through this weekend, making firefighting efforts that much more challenging," Javaheri said. "Some moisture could sneak into the region and prompt a slight chance for a few thunderstorms in the high Sierra. Unfortunately, these electric storms can bring more harm than help."The cause of the Dixie Fire is still under investigation, officials said. The blaze is currently 21% contained, according to CalFire.The Dixie Fire surpassed the Mendocino Complex Fire, which burned more than 450,000 acres across several counties in Northern California in 2018, to become second only to the August Complex Fire, which ravaged Northern California in August of 2020 and burned more than 1 million acres.In neighboring Oregon, there are 16 fires raging, while Montana and Idaho have the most of any US state -- with 25 and 20 respectively, according to NIFC.Due to heavy smoke from ranging wildfires, IQAir, a Swiss air quality technology company, determined that Denver's air was the most polluted of any city in the world early Saturday at an Air Quality Index of 167. The measurement is by US standards, where a higher number indicates poorer air quality.CNN's Claudia Dominguez, Amanda Watts and Aya Elamroussi contributed to this report.