Residents hit by the deadly California wildfires have been warned to beware of extremely hazardous air quality.
At least 1,800 homes and other buildings have been destroyed
Residents in five southern counties were urged to stay indoors due to pollution levels that are three times higher than normal.
Officials confirmed 640,000 people had fled homes, the biggest mass evacuation in California's history. At least 14 people died as a result of the fires.
Governor Arnold Schwarzenegger vowed to hunt down any arsonists responsible.
Arsonists are thought to have started at least two of the fires.
Damage in San Diego county alone is estimated at about $1bn (£487m), with nearly 800 sq miles (2,072 sq km) of land scorched.
At least 1,800 homes and other buildings have been destroyed.
The California Environmental Protection Agency warned that the elderly, children and people suffering from heart and respiratory conditions were particularly vulnerable from the pollution levels.
The central San Bernardino Mountains, parts of the San Bernardino Valley and areas in Orange and Riverside Counties are badly affected.
Satellite photographs revealed thick smoke over much of southern California.
Patricia Rey, of the Environmental Protection Agency, told Agence France-Presse news agency the fires had dumped into the atmosphere the equivalent greenhouse gas emissions of 440,000 cars over a one year period.
About 23,000 homes are still at risk from five major fires across three counties but many blazes have now been contained.
Hunt for arsonists
Cooler weather had helped the emergency efforts, fire officials said.
But Chip Prather, chief of the Orange County Fire Authority, said: "This is still a very fluid situation that's going to go on for a number of days."
San Diego County spokeswoman Nicole Ossola confirmed 640,000 people had fled their homes and 19,000 remained under evacuation orders.
A total of 3,000 are still being housed in temporary shelters but many have been venturing home to see if their properties are still standing.
Governor Schwarzenegger told a news conference a large reward was being offered for information leading to the arrest of any arsonists.
"We will hunt down the people that are responsible for that, and we will arrest them, and we will prosecute them to the full extent of the law," he said.
"If I were one of those people who started the fires, I would not sleep soundly right now, I tell you, because we are right behind you."
President George W Bush visited the devastated areas on Thursday and said the federal government would provide every help to victims of the fires.