OAKLAND, Calif. - The five-day air quality forecast shows a sea of orange and red.
Those are not colors anyone wants to see, as they indicate unhealthy levels of pollutants in the air.
The numbers are slightly better than on Thursday, where cities such as Livermore soared into the low 200s. But the district said that the heavy wildfire smoke from the Camp Fire in Butte County, still less than half contained, would produce an air quality advisory through Tuesday, two days before Thanksgiving.
As a result of the smoky skies, schools were closed and sports were canceled.
Research suggests children, the elderly and those with existing health problems are most at risk.
Short-term exposure to wildfire smoke can worsen existing asthma and lung disease, leading to emergency room treatment or hospitalization, studies have shown.
Some studies also have found increases in ER visits for heart attacks and strokes in people with existing heart disease on heavy smoke days during previous California wildfires, echoing research on potential risks from urban air pollution.
For most healthy people, exposure to wildfire smoke is just an annoyance, causing burning eyes, scratchy throats or chest discomfort that all disappear when the smoke clears.
But doctors, scientists and public health officials are concerned that the changing face of wildfires will pose a much broader health hazard.
"Wildfire season used to be June to late September. Now it seems to be happening all year round. We need to be adapting to that," Dr. Wayne Cascio, a U.S. Environmental Protection Agency cardiologist, said this week.
In an overview published earlier this year, Cascio wrote that the increasing frequency of large wildland fires, urban expansion into wooded areas and an aging population are all increasing the number of people at risk for health problems from fires.
Wood smoke contains some of the same toxic chemicals as urban air pollution, along with tiny particles of vapor and soot 30 times thinner than a human hair. These can infiltrate the bloodstream, potentially causing inflammation and blood vessel damage even in healthy people, research on urban air pollution has shown. Studies have linked heart attacks and cancer with long-term exposure to air pollution.
Whether exposure to wildfire smoke carries the same risks is uncertain, and determining harm from smog versus wildfire smoke can be tricky, especially with wind-swept California wildfires spreading thick smoke hundreds of miles away into smoggy big cities.
"That is the big question," said Dr. John Balmes, a University of California, San Francisco, professor of medicine who studies air pollution.
"Very little is known about the long-term effects of wildfire smoke because it's hard to study populations years after a wildfire," Balmes said.
Decreased lung function has been found in healthy firefighters during fire season. They tend to recover but federal legislation signed this year will establish a U.S. registry tracking firefighters and potential risks for various cancers, including lung cancer. Some previous studies suggested a risk.
Regular folks breathing in all that smoke worry about the risks too.
"It's kind of freaky to see your whole town wearing air masks and trying to get out of smoke," said freshman Mason West, 18. "You can see the particles. Obviously, it's probably not good to be breathing that stuff in."
West returned home this week to Santa Rosa, hard hit by last year's wine country fires, only to find it shrouded in smoke from the Paradise fire 100 miles away. West's family had to evacuate last year for a week, but their home was spared.
"It's as bad here as it was in Chico," West said. "It almost feels like you just can't get away from it."
Smoke has been so thick in Santa Rosa that researchers postponed a door-to-door survey there for a study of health effects of last year's fire.
"We didn't feel we could justify our volunteer interns going knocking on doors when all the air quality alerts were saying stay indoors," said Irva Hertz-Picciotto, a public health researcher at the University of California, Davis. The study includes an online survey of households affected by last year's fire, with responses from about 6,000 people so far.
AP Medical Writer Lindsey Tanner contributed to this report.