1. Two men sit in foreground, barely visible Golden Gate Bridge in background
2. Dogwalker wears masks while six dogs on leashes follow along the beach
3. View of Golden Gate Bridge from smoky Crissy Field
4. Woman wearing mask holds small dog with bridge in background
5. Walking shot of man with dog on Crissy Field beach
6. SOUNDBITE (English) Chloe Allegret, San Francisco resident:
"It's pretty uncomfortable. It's definitely causing shortness of breath, pressure in the sinuses so we're taking it easy."
7. Food delivery workers loading up van while wearing masks
8. Postal worker delivers mail while wearing mask
9. UPS driver wears mask while making delivery
10. SOUNDBITE (English) Daniel Horn, San Francisco resident:
"It just feels very smoky and full of particulates. I'm definitely worried about my son who's breathing this stuff. We've got air filters at home and we're running them at full speed but I'm not sure it's enough to get rid of all the toxic particulates so I'm definitely worried about the health of my family and the city in general."
11. Man walks along city's Embarcadero wearing mask
12. Embarcadero cleaning crew rides by on bicycle while wearing mask
13. Two women sit on park bench wearing masks
14. SOUNDBITE (English) Kylie Garcia, San Francisco resident:
"It sucks for people who are allergic to this stuff. Even me, I'm walking out here and my god, now my throat is feeling raspy. I walk here everyday on my work lunch break. It's crazy. I can't believe it. It's really sad and hopefully they'll be able to stop it. We'll see how long it goes."
15. Fire boat floating on Bay in foreground, Bay Bridge in smog in background
Most schools in San Francisco, Sacramento, Oakland and Folsom will be closed Friday as a deadly wildfire in Northern California sends smoke into the Bay Area.
School districts announced the closures Thursday as the region experienced some of the worst air quality since the fire started a week ago about 180 miles (290 kilometers) north of San Francisco.
An Environmental Protection Agency website says the air quality in Sacramento is "hazardous" Thursday and San Francisco's is "very unhealthy." Many people walking around the cities wore face masks.
Several Northern California universities had announced closures earlier Thursday.
The wildfire that started a week ago has killed dozens of people and charred nearly 220 square miles (570 square kilometers).
Smoke masks. Eye drops. No outdoor exercise. This is how Californians are trying to cope with wildfires choking the state, but experts say an increase in serious health problems may be almost inevitable for vulnerable residents as the disasters become more commonplace.
"It just feels very smoky and full of particulates. I'm definitely worried about my son who's breathing this stuff. We've got air filters at home and we're running them at full speed but I'm not sure it's enough to get rid of all the toxic particulates so I'm definitely worried about the health of my family and the city in general," San Francisco resident Daniel Horn said.
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.
Increases in doctor visits or hospital treatment for respiratory infections, bronchitis and pneumonia in otherwise healthy people also have been found during and after wildfires.
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,
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.