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.”

Video above: California wildfires force thousands to flee

At nearly every community meeting on firefighting efforts in the U.S. West, residents want to know why crews don't simply put out the flames to save their homes and the valuable forests surrounding them.

It's not that simple, wildfire managers say, and the reasons are many, some of them decades in the making and tied to climate change. The cumulative result has been an increase in gigantic wildfires with extreme and unpredictable behavior threatening communities that in some instances didn't exist a few decades ago.

"How do we balance that risk to allow firefighters to be successful without transferring too much of that risk to the public?" said Evans Kuo, a "Type 1" incident commander assigned to the nation's biggest and most dangerous wildfires. "I wish it wasn't the case, but it's a zero-sum game."

More than 20,000 wildland firefighters are battling some 100 large wildfires in the U.S West. Their goal is "containment," meaning a fuel break has been built around the entire fire using natural barriers or manmade lines, often created with bulldozers or ground crews with hand tools.

Estimated containment dates for some wildfires now burning aren't until October or November.

WHY SO LONG?

A big concern is safety. Kuo said residents sometimes plead with him to send firefighters into areas where he knows they could get killed.

"That's a deal-breaker," he said on a day off after 18 straight days of 5 a.m. to 10 p.m. shifts on a wildfire in Washington state. "I'm not putting people at risk."

Actually putting out these large fires, or labeling them "controlled," will require cold weather combined with rain or snow, weeks away for many states.

"I'd say pray for rain because that's the only thing that's going to get us out of this fire season," Idaho's state forester, Craig Foss, told Republican Gov. Brad Little and other state officials this week during a discussion of the wildfire season.

HAVE WILDFIRES CHANGED?

Kuo has been fighting wildfires for 30 years with the U.S. Forest Service, spending the first part of his career as a frontline firefighter with groundcrews, the backbone of any effort to stop a wildfire. At the time, wildfires of 150 square miles (390 square kilometers) were uncommon. Now blazes reach fives times that size and more, getting large enough to create their own weather.

"That's kind of redefining what the new normal is," said Kuo. "We get these megafires."

IS WILDFIRE SUPPRESSION IN THE PAST PLAYING A ROLE NOW?

For much of the last century, firefighters had been mostly successful at suppressing wildfires in ecosystems that evolved to rely on wildfire. Early on, firefighters benefitted from forests that had already been periodically cleared of brush and debris by wildfires that could move through every couple of decades. But with fire suppression, experts say, that brush and debris accumulated to where now, wildfires can ladder up into the branches and into the crowns of large trees, creating the giant wildfires that kill entire swatches of a forest.

HOW HAS DROUGHT IMPACTED WILDFIRE SUPPRESSION?

On top of fire suppression have been several decades of drought that studies link to human-caused climate change. That's exacerbated by this year's hot and dry weather, leading to historically low moisture contents in forests that have become tinder-dry.

"Our protection districts are seeing far warmer and dryer than normal conditions creating historically dry fuels," said Dustin Miller, director of the Idaho Department of Lands.

Those dry fuels allow wildfires to spread more quickly. On big fires, embers can shoot out to start spot fires on the other sides of natural barriers such as rivers. Sometimes spot fires can put firefighters at risk of being trapped by flames in front and behind them.

Miller said the state is likely facing $100 million in costs to fight fires this year on land the state is responsible for protecting, which is mostly state forests but also includes some federal and private forests.

WHAT ABOUT DISEASE AND INSECT INFESTATION?

Disease and bug infestations in trees whose defenses have been weakened by drought have led to forest-wide epidemics that have killed millions of trees in the U.S. West. Those dead trees, called snags, become fuel for wildfires while at the same time posing an increased danger to firefighters who can be hit by falling branches or the unstable trees themselves.

ARE MORE HOMES IN WILD AREAS AN ISSUE?

Homes built in what firefighters call the wildland-urban interface pose special problems for firefighters, typically tying up many firefighters on structure protection rather than have them actively engaging a wildfire.

"We base our strategy and tactics on protecting values at risk," Kuo said. "Homes, subdivisions, communications towers, gas pipelines, railways and roadways, transmission lines."

He said homes built with defensible space helps. More people in forested areas, as well as people recreating, has led to more human-caused wildfires. The National Interagency Fire Center in Boise says humans cause about 87% of all wildfires each year.

Related video above: California camp spared from wildfire's destruction

ARE THERE ENOUGH FIREFIGHTERS?

The nation has just more than 20 Type 1 response teams to handle the nation's biggest wildfires fires, and Kuo and his colleagues on those teams, like just about every other firefighting position this year, are in short supply.

He and his crew agreed to work longer than their 14-day shift on the Washington fire to make sure another Type 1 crew would be available.

Another problem is lengthening wildfire seasons mean many seasonal firefighters leave for school well before wildfire season ends.

Josh Harvey, fire management bureau chief for the Lands Department, said about 30% of the state's firefighters head back to school. Overall, Harvey said there have been widespread shortages of firefighters, fire engines and logistical support, and the state can no longer rely on help from neighboring states or federal partners.

There have even been occasional shortages of jet fuel for retardant bombers in some states.

"We've never seen anything like it before," Harvey said. "We are living and making fire history right now."

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.

Firefighters working in searing weather struggled to contain a Northern California wildfire that continued to grow Sunday and forced the closure of a major highway, one of several large blazes burning across the U.S. West amid another heat wave that shattered records and strained power grids.

Two firefighters died Saturday in Arizona after a plane they were in crashed during a survey of a small wildfire in rural Mohave County. The aircraft was helping perform reconnaissance over the Cedar Basin Fire near Wikieup, a tiny community of about 100, when it went down around noon.

The two firefighters were the only people on board, officials said. The National Transportation Safety Board is investigating the crash.

In California, officials asked all residents to reduce power consumption quickly after a major wildfire in southern Oregon knocked out interstate power lines, preventing up to 4,000 megawatts of electricity from flowing into the state.

The California Independent System Operator, which runs the state's power grid, said Saturday the Bootleg Fire took three transmission lines off-line, straining electricity supplies as temperatures in the area soared.

"The Bootleg Fire will see the potential for extreme growth today," the National Weather Service in Medford, Oregon, tweeted Sunday.

Pushed by strong winds, the Bootleg Fire exploded in size to 224 square miles (580 square kilometers) as it raced through heavy timber in Oregon's Fremont-Winema National Forest near the Klamath County town of Sprague River.

To the southeast, the largest wildfire of the year in California was raging along the border with Nevada. The Beckwourth Complex Fire — a combination of two lightning-caused blazes burning 45 miles (72 kilometers) north of Lake Tahoe — showed no sign of slowing its rush northeast from the Sierra Nevada forest region after doubling in size between Friday and Saturday.

Late Saturday, flames jumped a major highway, U.S. 395, which was closed near the small town of Doyle in California's Lassen County. Flames threatened rural properties and forced evacuations in Nevada's Washoe County. "Take immediate steps to protect large animals and livestock," the Truckee Meadows Fire Protection District tweeted.

The blaze, which was only 9% contained, increased to 131 square miles (339 square kilometers). Temperatures in the area could top 100 degrees (37 Celsius) again Sunday.

It was one of several fires threatening homes across Western states that were expected to see triple-digit heat through the weekend as a high-pressure zone blankets the region.

Death Valley in southeastern California's Mojave Desert reached 128 degrees Fahrenheit (53 Celsius) on Saturday, according to the National Weather Service's reading at Furnace Creek. The shockingly high temperature was actually lower than the previous day, when the location reached 130 F (54 C).

If confirmed as accurate, the 130-degree reading would be the hottest high recorded there since July 1913, when Furnace Creek desert hit 134 F (57 C), considered the highest measured temperature on Earth.

The National Weather Service warned the dangerous conditions could cause heat-related illnesses.

Palm Springs in Southern California hit a record high temperature of 120 F (49 C) Saturday. It was the fourth time temperatures have reached 120 degrees so far this year, the Desert Sun reported.

In California's agricultural Central Valley, 100-degree temperatures blanketed the region, with Fresno reaching 111 degrees F (44 C), just one degree short of the all-time high for the date.

Las Vegas late Saturday afternoon tied the all-time record high of 117 F (47 C), the National Weather Service said. The city has recorded that record-high temperature four other times, most recently in June 2017.

NV Energy, Nevada's largest power provider, also urged customers to conserve electricity Saturday and Sunday evenings because of the heat wave and wildfires affecting transmission lines throughout the region.

In Southern California, a brush fire sparked by a burning big rig in eastern San Diego County forced evacuations of two Native American reservations Saturday.

In north-central Arizona, Yavapai County on Saturday lifted an evacuation warning for Black Canyon City, an unincorporated town 43 miles (66 kilometers) north of Phoenix, after a fire in nearby mountains no longer posed a threat.

A wildfire in southeast Washington grew to almost 60 square miles (155 square kilometers) as it blackened grass and timber while it moved into the Umatilla National Forest.

In Idaho, Gov. Brad Little declared a wildfire emergency Friday and mobilized the state's National Guard to help fight fires sparked after lightning storms swept across the drought-stricken region.