Dr. Saima Khan, a pediatrician in California’s Imperial Valley, finds herself dreading the wind.

On gusty days, when sand swirls across the interstate and the sun disappears behind a white scrim of dust, she knows her examination rooms will be filled with young children with asthma, many still in diapers and struggling for breath.

Asthma is more than just a nuisance for Khan’s kids. It can be deadly.

Khan is the associate medical director at Clinicas de Salud del Pueblo, a nonprofit network of health clinics serving low-income families. She typically confers with patients in Spanish, wearing a white doctor’s jacket and a simple, crisp headscarf. Her patients often are in denial or unaware of their own symptoms.

“That scares the hell out of me,” she said. “They are at very high risk of their lungs shutting down.”

The wind in this desolate region along the U.S.-Mexico border, one of the state’s poorest and most polluted places, is never benign: Some of the country’s most dangerous air – rich with microscopic particles that can burrow deep in the lungs and affect the heart – is carried on its invisible wings.

The ingredients of this rank, airborne witches’ brew make the Imperial Valley an especially precarious place to be a child. School-aged children are rushed to emergency rooms for asthma at a rate that’s nearly double the state average. Likewise, Imperial County holds the state record for asthma hospitalizations, with children younger than 5 being the most frequently admitted.

The bad air here long has been blamed on agricultural burns and so-called “fugitive dust” from the desert, both of which are factors. However, a study commissioned by Reveal has found that diesel and gas exhaust are the most significant contributors to the region’s polluted air, most likely a combination of traffic and, especially, the snaking lines of idling vehicles waiting at the border.

The study ties the chronic traffic congestion – a confluence of post-9/11 security, aging infrastructure and the North American Free Trade Agreement – to the health crisis that grips this binational region, where a burgeoning population now regularly goes back and forth across the border in an effort to make a living.

The study, by a San Diego State University environmental health professor, found that diesel and gasoline from Mexicali, Mexico, most likely from idling vehicles at the border crossing, is the No. 1 source of particulate matter in Calexico, California, increasing in intensity when the wind blows into the Imperial Valley from the south. The pollution from traffic – long linked by scientists to compromised breathing and smaller lung capacity in children – is exacerbated by industrial emissions, another major source of dangerous particulates, according to the study.

Northbound commuters typically wait between 45 minutes and two hours to cross – distracted by newspaper vendors, cellphone charger sellers, even a cheerleading squad from a Mexicali high school doing fundraising gymnastics routines in the median.

For Khan, the fight against asthma has become both professional and personal.

She was born in Pakistan, grew up in Germany and did her residency in Alabama. As a foreign-born physician, Khan received a special visa waiver that allowed her to live in the United States in exchange for three years of practice in a medically underserved rural community.

But she has stayed voluntarily in the Imperial Valley – eight years and counting – leaving only briefly at her husband’s urging for a private practice in an affluent community near San Diego.

She missed her patients from Calexico, Brawley and El Centro, “who need me more and are more grateful,” she said. So she returned, even though she, husband Amir and daughters Maleeha, 11, and Mysha, 9, have developed asthma while living here. At home, she keeps several sets of stethoscopes in her bathroom drawer.

At the clinic, Khan instructs parents to avoid anything that might gather dust – even stuffed animals. Like those parents, she has been on the receiving end of distressing phone calls from school.

“They called me when Mysha was 6, saying that she had chest pain and a cough after recess,” Khan recalled. “I heard that, and I thought, ‘Oh, shoot.’ ”

She is compassionate but no-nonsense, attempting to be nonjudgmental with the revolving door of patients who forget to take their medications or whose parents are unwilling to admit that their child has a chronic disease. Many parents come in distraught. “Try telling a 3-year-old they can’t run,” Khan said.

Some days, the strain and exhaustion of administering nonstop breathing treatments to bright T-shirted boys and girls with sparkly barrettes are etched on Khan’s face.

She will lay her head in her hands on her keyboard, fighting off her own shortness of breath, which has escalated since the family bought a house in a short sale that had sat vacant, gathering dust.

In the past six years, three Imperial Valley children have died of asthma, according to the Coroner Division of the Imperial County Sheriff’s Office – the most recent death, in 2014, was an 8-year-old girl.

An unemployment capital

On clear mornings, Khan can make out the rumpled, copper-colored folds of Mount Signal, which straddles the border between Calexico (population: 39,000) and Mexicali, a Mexican city of over 1 million that, due largely to NAFTA, has become one of the most toxic metropolises in North America.

From a lone dirt road up the mountain – called “El Centinela” on the Mexican side – Imperial County’s past and present come into sharp relief. To the east is a barren desert with stinging sandpaper wind, parched washes and miles of cracked, dusty earth.

To the west is the extreme makeover masterminded by speculators and engineers at the turn of the 20th century, who diverted the Lower Colorado River to “reclaim” the desert. Here in America’s drylands, which get less than 3 inches of rain a year, the Imperial Irrigation District soon would force-feed the desert with water – digging 1,700 miles of canals that keep hundreds of thousands of acres of alfalfa, sugar beets, onions and other crops, including tropical aloe vera, on artificial life support.

The developers christened the land Imperial, implying wealth. Yet nearly half of the valley’s families, the vast majority of them Latino, live below the federal poverty level in one of the unemployment capitals of the United States. They inhabit a landscape where angry coils of brown-orange smoke erupt from fields, the result of agricultural burning, and a pleasant Sunday trip to Wal-Mart can turn ominous as dirty black clouds from trash fires envelop the parking lot.

It is a place of strange perfumes – the whiff of a feedlot, flecks of hay suspended in air; the dank, at times ghastly, smell of a filthy river spanning the border that bears pesticide-laced Imperial Valley farm runoff and municipal and industrial waste from Mexicali.

In this out-of-sight, out-of-mind swath of California, school principals entrusted with their students’ health must be ever-vigilant for hazards few educators encounter: crop dusters illegally spraying too close to school right before the 8 a.m. bell, or – equally insidious – vehicles waiting to cross the U.S. port of entry in downtown Calexico.

Five blocks from the border, students at Jefferson Elementary School in Calexico are used to the constant parade of cars flickering through the steel security bars of the border fence. Exposure to exhaust fumes can weaken a child’s lung development and is a major risk factor for asthma.

Gwyneth Rodriguez, a precocious 11-year-old with long black hair and the confidence to correct her teachers, was first hospitalized for asthma at 5. Since then, she has been rushed to the emergency room “countless times,” her mother says. As with many children here, Gwyneth’s asthma was misdiagnosed – first it was a cold, then bronchitis, then, in her mother’s words, “you name it.”

“It feels like my throat is closing in,” Gwyneth said of her bad days. “My chest hurts, and then I panic and my body gets crazy and my asthma gets worse.”

For the valley’s most vulnerable residents, including many of Khan’s patients, the combination of poverty and poor air quality can be a potentially lethal mix.

In 2009, Marie Dugan, a 16-year-old sophomore in El Centro, collapsed in her bedroom after frantically scrambling for her nebulizer. It was too late. “You’re a teenager,” her aunt, Carmen Bravo, still grieving, said of her niece. “You don’t think you’re going to die.”

For decades, sparsely populated Imperial County – home to 178,000 people spread out over more than 4,000 square miles – has violated federal and state clean air standards. Residents breathe dangerously elevated levels of ozone and particulate matter – solid particles or liquid droplets that can be 25 to 100 times thinner than a human hair.

Coarser particles are generated largely by so-called fugitive dust – blowing off fields, ditch bank roads along canals, sand dunes churned up by all-terrain vehicles, even U.S. Border Patrol “drag roads” along the border fence used to track footprints of people entering the country illegally. Microparticles from vehicle exhaust, soot and smoke are even stealthier adversaries, with the capacity to kindle all manner of human miseries, including strokes, heart attacks and chronic obstructive pulmonary disease.

The highest levels of the most destructive particulates are in Calexico, near the border crossing. Developed in unison by the Imperial Land Co., the Imperial and Mexicali valleys now form a vast binational region with a common airshed. With a booming population seeking work in Mexicali’s “maquiladoras,” manufacturing operations that have proliferated in the free-trade zone under NAFTA, and with increased congestion, both valleys have been plagued by deteriorating air quality that has spurred a public health crisis on both sides of the border.

Some 7.6 million trucks and passenger vehicles commute through Calexico’s two ports of entry every year, ferrying everyone from barbers to washing-machine repairmen and swap-meet dealers driving pickups loaded with appliances.

In the valley’s geography of extremes, springtime gusts form white caps on irrigation canals, and searing summer averages of 104 to 115 degrees – second only to Death Valley – can heat up a can of soup inside a car. The winter months are the most perilous: It is then that the valley, a sunken bowl that mostly lies below sea level, is beset by thick inversion layers that hold particulates, soot from fuel and diesel exhaust and other pollutants in its chilly grip. Soot released during burning is a dark chariot for a variety of toxic and carcinogenic chemicals that are harmful to the human body.

Poor air quality does not cause asthma, but it is a significant trigger, along with dust mites, mold, cockroaches, cigarette smoke and – especially for kids – strong emotions, including laughter. A classmate sneezing and passing gas at the same time led one Calexico boy to the ER.

In the most severe episodes, the airways constrict suddenly, which can progress rapidly to asphyxiation if not immediately treated. The exact number of fatalities is difficult to determine because the disease can be listed as a secondary cause of death and not even appear on a death certificate.

The state Department of Public Health’s California Breathing survey does not have complete data for all 58 counties, including Imperial, on asthma prevalence among children and teenagers. (Merced County tops those for which the department has statistics.) But the 2012 survey found that children in Imperial County visited emergency rooms for asthma at a rate nearly two times higher than the state average.

The reasons for these disturbing statistics are not fully understood and have prompted an ambitious new wave of research on this issue.

What is known is that there are profound correlations between poverty and health.

Four out of every 5 Imperial County residents are Latino. And according to an American Lung Association report, Latinos in the U.S. face a number of specific challenges that can contribute to poor respiratory health: rundown housing near busy roads, poor English proficiency, no health insurance and agriculture and construction jobs that expose them to pesticides and other hazards.

The valley’s 39 percent rate of childhood obesity also plays a role. Being overweight potentially alters the body’s immune defenses and increases the severity of asthma symptoms. In addition, there is evidence that kids prone to asthma from lower socioeconomic backgrounds grow up with stressors that can heighten the body’s inflammatory responses. In that sense, the environment literally gets under their skin.

In a 2009 Department of Public Health survey of asthma and allergies along the Imperial County border, 1 in 5 children had been diagnosed with asthma at some point in their life. More significantly, though, a large number of children who never had been diagnosed reported asthmatic symptoms. Researchers concluded that it was likely that many children in Imperial County – anywhere from 2 to 23 percent – had undiagnosed and untreated asthma.

“The differences between Imperial County and the rest of the state are pretty dramatic,” said Dr. Rick Kreutzer, chief of the department’s Division of Environmental and Occupational Disease Control. Poor air quality and a lack of access to primary care are two likely reasons, he said.

Many parents don’t have a clear understanding of asthma, said Aide Fulton, who runs a child asthma program affiliated with El Centro Regional Medical Center. “They usually go to the ER just because they’re afraid,” she said. “In reality, they could have prevented the attacks if they had followed a treatment plan.”

At the same time, many health care providers are not well-versed in comprehensive asthma treatment, she said. And there is only one doctor for every 4,537 patients in Imperial County, according to a study funded by the Robert Wood Johnson Foundation, a ratio that is three times worse than the state average.

Thus, the annual memorial walk held in El Centro in honor of Marie Dugan, the 16-year-old who died of asthma, always has a bittersweet edge. It is an event that should not be necessary.

“These are preventable deaths,” Fulton said.

Cauldron of chemicals comes with free trade

The industrialization spurred by the maquiladora program and NAFTA, which allowed foreign companies to set up shop in Mexico, drove millions of migrants from rural Mexico to cities along the U.S. border, straining Mexicali’s fledgling infrastructure.

As the city boomed with the influx of U.S. investment, a cauldron of chemicals has filled the air. There are many culprits: petroleum facilities, paper manufacturers and power plants, including those built by U.S. companies. Open-air burning is a way of life, from garbage fires along the road to celebratory New Year’s bonfires that darken the air with skyline-obliterating soot.

On weekends, dense plumes of smoke from agricultural burning dissipate over the Mexicali Valley, virtually unregulated as they catch the wind. Then there is the illegal burning believed to occur at night, including of tires, which release benzene, formaldehyde, carbon monoxide, nitrogen oxide, polycyclic aromatic hydrocarbons and a host of other toxins.

Nearly a quarter of the metropolitan area’s streets are unpaved, adding relentless dust to the equation.

“When you come out of your house to go to work,” said Margarito Quintero Nuñez, a senior researcher on energy and an environment professor at the Universidad Autónoma de Baja California, “there is usually a strange smell in the air.”

Across the border, Calexico is Imperial County’s epicenter for the smallest and most pernicious pollution particles – known as PM2.5 and considered “respirable” by scientists for their ability to infiltrate the deepest and most sensitive areas of the lungs. The Reveal study, which analyzed particle concentrations, wind patterns and other data in the county for 2010, found that cross-border traffic plays a major role, with diesel and gas vehicle exhaust – likely from idling vehicles – being the biggest offender, followed by industrial sources from Mexicali.

The levels increase significantly when wind blows from the south or southeast, its most common direction. Environmental health professor Zohir Chowdhury of San Diego State University and his colleagues found that desert and road dust and agricultural burning also can be virulent sources of pollution.

The region’s traffic woes have been exacerbated by border delays at the downtown port of entry in Calexico. Today, it is an obsolete relic, built 20 years before 1994’s NAFTA, a remnant of an era when Mexicali was a sleepy backwater with a licentious reputation.

Now, it is the third-busiest land port in California: a traffic chokehold in which some 22,000 northbound vehicles come to a standstill on Mexicali streets each day. At peak travel times, vehicles form a 1 1/4-mile or more chain down Avenida Cristóbal Colón, idling for up to 3 1/2 hours, traffic experts say. Most crossers are residents of Mexico, with many driving antiquated vehicles that lack catalytic converters.

The border crossing’s aging infrastructure, which suffered cracks during a 2010 magnitude 7.2 earthquake, is no match for transborder traffic, though the numbers ebbed during the recent recession.

The majority of those crossing cannot get expedited passes, which cost $122.25 and require heavy background checks.

Since Sept. 11, 2001, it has taken border agents an average of a minute and a half to process every vehicle, delays that Mark Baza, executive director of the Imperial County Transportation Commission, says cost the county $384 million and the state $620 million a year in lost revenue and job creation.

Then there is the queue of 12,000 or so pedestrians a day who face triple-digit heat without proper shade during the summertime. An alternative port of entry opened about 7 miles east of downtown in 1996 to handle some 18,600 trucks a day. It is already at capacity, Baza said.

But a fix for Calexico appears to be on the way: The federal government has approved $98 million for the first phase of a long-planned new downtown border crossing.

It follows on the heels of a major overhaul at the San Ysidro port of entry, which processes 150,000 daily commuters from Tijuana to San Diego. The new crossing has 46 northbound vehicle inspection booths, which has slashed wait times to as little as 15 minutes from as long as four hours.

In contrast to the existing Calexico port, which is about as welcoming as a concrete bunker, the new downtown crossing has been designed as a “dignified passage between friends,” with lots of glass, said U.S. Rep. Juan Vargas, a Democrat whose district covers much of the state’s border area, including Calexico.

The first phase of the project is scheduled to be completed in 2018. Eventually, the port would expand from 10 to 16 northbound lanes and will redirect traffic to reduce backups.

Equally important, it will include drinking fountains and shade canopies for pedestrians and automobiles. Unlike the existing port, it will be equipped with license-plate readers, radiation detectors, X-ray equipment and other technology to speed up inspections.

The lines of idling vehicles at the current port “make you feel like you’re going to die,” Vargas said. It is also a public health nightmare. “The wind and the asthma do not know the border is there,” he said.

Dispelling myths about asthma

In the small Imperial Valley city of Holtville, where photographer Dorothea Lange documented Mexican pea pickers and field labor camps during the Great Depression, community health workers Graciela Ruiz and Lourdes Salazar serve as ground troops against an elusive enemy.

They are “promotoras” with the El Centro child asthma program’s Healthy Homes Equals Healthy People initiative, making house calls on people like Brenda Villa, a 37-year-old mother of two. Her 11-month-old son, Jose, had just returned from the hospital, his shortness of breath “like a fish out of water gasping,” Villa said.

Together, Ruiz, 43, and Salazar, 35, assess triggers in the home that might be contributing to the child’s asthma symptoms. Although Villa is a diligent housekeeper, the family cannot afford to replace the worn wall-to-wall carpeting in the living room, which can harbor dust and residue from the pesticides her husband sprays in the fields.

The two dispelled some common myths with Villa: that asthma isn’t an emotional disease; that children don’t always outgrow asthma; that if a child sleeps with a Chihuahua, it won’t transfer to the dog. They reviewed the litany of no’s – no bleach, no aerosols, no candles, no hanging laundry outside.

In a decrepit mobile home park around the corner from a withered grass soccer field, Ruiz and Salazar recently met with a mother who barely scraped by selling used goods at a local flea market, her severely asthmatic son constantly in and out of the hospital. Mold infused the walls of the family trailer, the broken windows stuffed with towels or boarded up with plywood. The floors were uneven and squishy with moisture. Outside, the battered metal looked as though it were hanging on for dear life.

Eventually, Ruiz and Salazar persuaded a city official and the park manager to move the family to a different trailer. Still, what they had seen was hard to shake. “This is the United States,” said Ruiz, who grew up in Guanajuato, Mexico. “Sometimes you just want to cry. You don’t want to come back for a second visit.”

Guadalupe X. Ayala, a clinical psychologist and professor at San Diego State University’s Graduate School of Public Health, grew up as an asthmatic child in Calexico. She remembers the constant ER visits, the hope that the disease would magically go away. As a teenager, she remembers hiding her medication, only to tell her parents, “I took it.”

Now, Ayala’s work is part of a bevy of new scientific efforts seeking to better understand the social, economic and environmental forces contributing to the region’s poor health, especially asthma.

With $2 million from the National Institute of Environmental Health Sciences, for instance, the nonprofit Comite Civico del Valle is partnering with the California Environmental Health Tracking Program to set up 40 air monitors across the county to provide a more fine-grained portrait of pollution hot spots, such as proximity to agricultural burning, hay-packing plants and field tilling.

Comite Civico also is collaborating with Dr. Saima Khan’s clinic and Ayala’s team at San Diego State on a new $4 million Imperial County asthma research project to examine the nitty-gritty of prevention and treatment on the ground. Khan also has a grant from the Vesper Society, a faith-based organization, to improve pediatric asthma care.

At the clinic, Khan explains to parents the goal of getting control of their child’s disease: to give them the freedom to “do everything like a normal kid who doesn’t have asthma.”

She would like to better understand the dynamics of denial. Maybe some think you don’t have asthma until you have a major attack. Maybe doctors are missing diagnoses and confusing patients, or maybe there’s a communication gap. Perhaps there’s something about hopping between the two countries for medical care. If the reasons were better grasped and the air quality improved, Khan believes she could do more for her patients.

“Sometimes you want to save the world and the world won’t let you,” she said.

Clarification: This story has been updated to specify the program that is partnering with Comite Civico del Valle on its air monitor project.

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Reporter Bernice Yeung contributed to this story. It was edited by Andrew Donohue and Robert Rosenthal and copy edited by Sheela Kamath and Nikki Frick. Patricia Leigh Brown can be reached at pattinyt@gmail.com.