SAN DIEGO, Calif. — Wildfires across the West are sending toxic plumes of smoke into the air, affecting cities thousands of miles away.

And as fires grow more frequent and destructive, doctors are raising awareness about the possible risks for pregnant women and their unborn babies.

"Clinicians now are realizing that climate change is impacting our patients today because the events are happening so frequently," said Dr. Marya Zlatnik, OBGYN and maternal-fetal medicine expert at the University of California San Francisco.

Dr. Zlatnik says historically, medical education for pregnant women has focused more on individual choices impacting health, like smoking.

“There’s sort of this whole, broad category of risks we haven’t studied yet. But they’re things we have lots of reason to suspect shouldn’t be ingested by pregnant women or kids," said Dr. Zlatnik.

One year ago, the Bay Area woke to an apocalyptic, smoke-choked sky as fires burned throughout Northern California. Today, fires are consuming the state at the same pace.

The toxic particles in wildfire smoke are small enough to enter the lungs and bloodstream. And Dr. Zlatnik says researchers are now finding soot from wildfires in placentas.

“Anything that damages the placenta or causes inflammation in the placenta can potentially sort of directly harm the baby or lead to problems with pregnancy, like preterm birth, or the baby not growing as well.”

A study published last month found there may have been as many as 7,000 additional premature births in California attributable to wildfire smoke exposure between 2007 and 2012.

“That can have a lifelong impact for that baby. Prematurity is probably the number one cause of neural developmental problems and is certainly very expensive and very scary for parents," said Dr. Zlatnik. "Anything we can do to avoid unnecessary inflammation is something that could potentially really have long-term beneficial health impacts.”

Authors of the new study note that premature births cost the U.S. healthcare system an estimated $25 billion per year, and that even modest reductions in preterm birth risk could greatly benefit society.

Expecting her first child in October, Laura Canton says she's hyper-aware of the changing climate as she thinks about her daughter's future.

“When we walk on the beach, we see bottle caps, plastic bottles, Legos, just everywhere on the beach. It’s sad to know Ocean is going to grow up and may not get to go to Yosemite or the forest because it's going to be gone," said Canton.

Living in San Diego, she's experienced the hazy fog of wildfires.

“The clouds come in, and you can see it’s a different haze. The sunsets are really orange," said Canton. "It’s very dry here. So, if one house lights up, our houses are so close together, it could take down the whole city, it is scary.”

Dr. Zlatnik says there are steps pregnant women can take to protect themselves and their unborn children.

She recommends patients monitor air quality, checking the news, or using phone apps like AirVisual and Plume Air Report.

"The best way to improve air is to use either a home fan, air conditioning system that has a high-quality MERVE 13 filter that's going to filter out the PM2.5 material or a portable air cleaner, air purifier, that’s filtering out that particulate matter.”

A more affordable option, making one at home with a box fan and a furnace filter.

“I didn’t use to advise this for the women I care for in pregnancy, that they needed to invest in one of these things," said Dr. Zlatnik. "But that’s one of the ways that people who are pregnant can protect themselves.”

EL DORADO COUNTY, Calif. —

Thousands of people rushed to get out of South Lake Tahoe as the entire tourist resort city came under evacuation orders and wildfire raced toward the large freshwater lake of Lake Tahoe, which straddles California and Nevada.

Evacuation warnings issued for the resort city of 22,000 on Sunday turned into orders Monday. Vehicles loaded with bikes and camping gear and hauling boats snaked through thick, brown air that smelled of a campfire. Police and other emergency vehicles whizzed by.

"This is a systematic evacuation, one neighborhood at a time," South Lake Tahoe police Lt. Travis Cabral said on social media. "I am asking you as our community to please remain calm."

The new orders come a day after communities several miles south of the lake were abruptly ordered evacuated as the Caldor Fire raged nearby.

Watch above sister station KCRA's live, ongoing coverage of the Caldor Fire

South Lake Tahoe's main medical facility, Barton Memorial Hospital, proactively evacuated 36 patients needing skilled nursing and 16 in acute care beds Sunday, sending them to regional facilities far from the fire, public information officer Mindi Befu said. The rest of the hospital was evacuating following Monday's expanded orders.

The entire Lake Tahoe area in the Sierra Nevada mountains is a recreational paradise for San Francisco Bay Area locals looking for a weekend getaway, as well as a national destination. The area offers beaches, water sports, hiking, ski resorts and golfing.

South Lake Tahoe, at the southern end of the lake, bustles with outdoor activities, with casinos available in bordering Stateline, Nevada.

South Lake Tahoe Mayor Tamara Wallace prepared to leave with her husband, youngest child, dogs and items given to them from their deceased parent — objects that can't be replaced.

She did not think the Caldor Fire would come so close. Fires in the past did not spread so rapidly near the resort city.

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Video above: KCRA's Mike TeSelle provides an on-the-ground update of the Caldor Fire

"It's just yet another example of how wildfires have changed over the years," she said. "It's just a culmination of 14 to 18 more years of dead trees, the droughts we’ve had since then, those kinds of things."

The region faces a warning from the National Weather Service about critical fire weather Monday through Tuesday.

The fire destroyed multiple homes Sunday along Highway 50, one of the main routes to the south end of the lake. It also roared through the Sierra-at-Tahoe ski resort, demolishing some buildings but leaving the main buildings at the base intact.

Flames churned through mountains just a few miles southwest of the Tahoe Basin, where thick smoke sent tourists packing at a time when summer vacations would usually be in full swing ahead of the Labor Day weekend.

Video above: Cal Fire chief talks about Caldor Fire's potential threat to Tahoe Basin

Cal Fire Director Thom Porter said during a news conference last week that the Caldor Fire is now, "the No. 1 priority in the nation of fires to get additional and new resources that are becoming available. It is that important."

He added that the fire "is knocking on the door to the Lake Tahoe Basin."

"We have all efforts in place to keep it out of the basin, but we do need to also be aware that that is a possibility based on the way the fires have been burning and the concerns that we have been living in all of these other fires and their growth," Porter said.

"The weather has outstripped and Mother Nature has taken over and taken fires like the Dixie to places that I never thought was possible," he said.

There were reports of cabins burned in the unincorporated community of Echo Lake, where Tom Fashinell has operated Echo Chalet with his wife since 1984. The summer-only resort offers cabin rentals but was ordered to close early for the season by the U.S. Forest Service due to ongoing wildfires.

Fashinell said he was glued to local television. "We're watching to see whether the building survives," he said.

The last major blaze in the area took South Lake Tahoe by surprise after blowing up from an illegal campfire in the summer of 2007. The Angora Fire burned less than 5 square miles but destroyed 254 homes, injured three people and forced 2,000 people to flee.

The Caldor Fire has scorched 277 square miles since breaking out Aug. 14. After the weekend's fierce burning, containment dropped from 19% to 14%. More than 600 structures have been destroyed, and at least 20,000 more were threatened.

Video above: A look at the Grizzly Flats neighborhood damaged in El Dorado County by the Caldor Fire

It's among nearly 90 large blazes in the U.S. Many are in the West, burning trees and brush sucked dry by drought. Climate change has made the region warmer and drier in the past 30 years and will continue to make the weather more extreme and wildfires more destructive, according to scientists.

In California alone, more than a dozen large fires are being fought by more than 15,200 firefighters. Flames have destroyed about 2,000 buildings and forced thousands to evacuate this year while blanketing large swaths of the West in unhealthy smoke.

In Southern California, a section of Interstate 15 closed Sunday after winds pushed a new blaze, the Railroad Fire, across lanes in the Cajon Pass northeast of Los Angeles.

Farther south, evacuation orders and warnings were in place for remote communities after a wildfire ignited and spread quickly through the Cleveland National Forest on Saturday. A firefighter received minor injuries, and two structures were destroyed in the 2.3-square-mile Chaparral Fire burning along the border of San Diego and Riverside counties, according to Cal Fire. It was 13% contained Monday.

Meanwhile, the Dixie Fire, the second-largest in state history at 1,205 square miles, was nearly halfway contained about 65 miles north of the Lake Tahoe-area blaze. Nearly 700 homes were among almost 1,300 buildings that have been destroyed since the Dixie Fire began in early July.

Containment increased to 26% on the French Fire, which covered nearly 40 square miles (104 square kilometers) in the southern Sierra Nevada. Crews protected forest homes on the west side of Lake Isabella, a popular recreation area northeast of Bakersfield.

The U.S. Department of Defense is sending 200 Army soldiers from Washington state to help firefighters in Northern California, the U.S. Army North said in a statement Saturday. Eight Air National Guard and Air Force Reserve C-130 aircraft capable of dropping thousands of gallons of fire retardant also have been sent to fight wildfires in the West.

El Dorado County officials launched a structure damage map so people can see the status of the homes in the area. The map is updated daily at 9 a.m. Check out the map below.

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Hearst station KCRA and Associated Press writer Janie Har contributed from San Francisco.

POLLOCK PINES, Calif. —

Record-setting blazes raging across Northern California are wiping out forests that are central to plans to reduce carbon emissions and are testing projects designed to protect communities from wildfires, the state's top fire official said Wednesday, hours before a fast-moving new blaze erupted.

Fires that are "exceedingly resistant to control" in drought-sapped vegetation are on pace to exceed the amount of land burned last year — the most in modern history — and having broader effects, said Thom Porter, chief of the California Department of Forestry and Fire Protection.

What started as a grass fire Wednesday afternoon swiftly blew up to threaten the city of Clearlake, spurred by winds up to 30 miles per hour (48 kilometers per hour). The sheriff warned there was an "IMMEDIATE threat to life and property" in the city about 80 miles north of San Francisco.

Television images showed rows of destroyed homes on at least two blocks as crews doused water on burning houses nearby. Children were rushed out of an elementary school as a field across the street burned. Authorities reported the fire at 1 p.m. and within an hour dozens of homes had burned.

Lake County Sheriff Brian Martin told KGO-TV that some people have been reluctant to leave.

"This isn't the fire to mess around with," he said.

The largest current fire in the West, known as the Dixie Fire, is the first to have burned from east to west across the spine of California, where the Sierra Nevada and Cascade mountains meet, the state's fire chief said.

It was also one of several massive fires that have destroyed areas of the timber belt the state was counting on to store carbon dioxide to meet its climate initiative goals.

"We are seeing generational destruction of forests because of what these fires are doing," Porter said. "This is going to take a long time to come back from."

Fires mostly across the northern part of the state continued to threaten thousands of homes, force new evacuations and led Pacific Gas & Electric to cut power to tens of thousands of customers to prevent utility equipment from sparking fires amid strong winds.

One of the most recent infernos, the Caldor Fire, continued to grow explosively southwest of Lake Tahoe, covering 84 square miles (217 square kilometers) after ravaging Grizzly Flats, a community of about 1,200.

Dozens of homes burned, according to officials, but tallies were incomplete. Those who viewed the aftermath saw few homes standing in the forest. Lone chimneys rose from the ashes, rows of chairs were all that remained of a church and burned out husks of cars littered the landscape.

Chris Sheean said the dream home he bought six weeks ago near the elementary school went up in smoke. He felt lucky he and his wife, cats and dog got out safely hours before the flames arrived.

"It's devastation. You know, there's really no way to explain the feeling, the loss," Sheean said. "Maybe next to losing a child, a baby, maybe. … Everything that we owned, everything that we've built is gone."

All 7,000 residents in the nearby community of Pollock Pines were ordered to evacuate Tuesday. A large fire menaced the town in 2014.

Time-lapse video from a U.S. Forest Service webcam captured the fire's extreme behavior as it grew beneath a massive gray cloud. A ceiling of dark smoke spread out from the main plume that began to glow and was then illuminated by flames shooting hundreds of feet in the sky.

John Battles, a professor of forest ecology at the University of California, Berkeley, said the fires are behaving in ways not seen in the past as flames churn through trees and brush desiccated by a megadrought in the West and exacerbated by climate change.

"These are reburning areas that have burned what we thought were big fires 10 years ago," Battles said. "They're reburning that landscape."

The wildfires, in large part, have been fueled by high temperatures, strong winds and dry weather. Climate change has made the U.S. West warmer and drier in the past 30 years and will continue to make the weather more extreme and wildfires more destructive, according to scientists.

Battles said the fires have created a vicious cycle for trying to reduce global warming. Burning increases carbon emissions while also destroying trees and other ground cover that can absorb greenhouse gas. Dead trees will continue to release carbon they once stored.

The fire is burning along the U.S. Route 50 corridor, one of two highways between Sacramento and Lake Tahoe. The highway through the canyon along the South Fork of the American River has been the focus of a decades-long effort to protect homes by preventing the spread of fires through a combination of fuel breaks, prescribed burns and logging.

"All of that is being tested as we speak," Porter said. "When fire is jumping outside of its perimeter, sometimes miles ... those fuel projects won't stop a fire. Sometimes they're just used to slow it enough to get people out of the way."

In the Sierra-Cascades region about 100 miles (161 kilometers) to the north, the month-old Dixie Fire expanded by thousands of acres to 993 square miles (2,572 square kilometers) — two weeks after the blaze gutted the Gold Rush-era town of Greenville. About 16,000 homes and buildings were threatened by the Dixie Fire, named for the road where it started.

"It's a pretty good size monster," Mark Brunton, a firefighting operations section chief, said in a briefing. "It's going to be a work in progress — eating the elephant one bite at a time kind of thing."

The Caldor and Dixie fires are among a dozen large wildfires in the northern half of California.

While most of the fires this year have hit the northern part of the state, Southern California has been largely spared and even experienced rare drizzle and light rain Wednesday.

Associated Press writers John Antczak in Los Angeles and Olga R. Rodriguez and Janie Har in San Francisco contributed to this report.