LOS ANGELES, Sept. 8 (Xinhua) -- Eight people who refused to be evacuated from their homes were reported missing as a wildfire swept a canyon in the foothills near Boulder, the U.S. state of Colorado, authorities said on Wednesday.
Initially 20 people were reported missing, and 12 of them have been accounted for, said Boulder Sheriff's Cmdr. Rich Brough.
Rescue workers continued their search for the remaining eight and checked homes in the area, according to Brough.
Since its outbreak, the fire has burned 6,168 acres (about 2, 498 hectares) and destroyed 53 homes.
About 3,500 people have been evacuated from about 1,000 homes. There have been no reports of injuries.
Governor Bill Ritter declared a state of emergency on Tuesday, saying the state was throwing 5 million dollars into the battle to save homes and forests in the Fourmile Canyon area west of Boulder.
Federal emergency authorities have also promised a federal firefighting grant.
"It is important right now for people who've been evacuated to just be patient," Ritter said after a survey of the burned area. " This is a very volatile situation."
The Colorado Department of Public Health and Environment has issued a health risk warning, saying smoke from wildfires can induce asthma and worsen chronic heart and lung diseases.
Residents are advised to close air conditioners' fresh air intake or find shelter elsewhere as the smoke contains a mixture of gases and fine particles from burning trees and other plant material.
Local hospitals have begun receiving patients complaining of respiratory ailments related to the smoke, and more are expected as the fire continues to spread.
The cause of the fire is still under investigation.