A toddler breathes from a nebulizer while sitting in a crib.
A two-year-old girl receives nebulizer treatment in Beijing in 2015. Jason Lee/Reuters

The landmark Children’s Health Study tracked thousands of children in California over many years—and transformed our understanding of air pollution’s harms.

The following is an excerpt from the new book Choked: Life and Breath in the Age of Air Pollution (University of Chicago Press, $27.50).

Across Southern California, in school gyms and libraries and lunchrooms, the children filed in, one by one, to put their lips around a plastic tube and blow with all their might. Thousands of them, year after year, in rich neighborhoods and poor ones, from the breezy towns along the Pacific coast to the hot, smoggy valley locals know as the Inland Empire.

Erika Fields was one of them, back in the 1990s, when she was in high school at Long Beach Poly, just outside Los Angeles. Even now, she’s the kind of person who raises her hand, who steps forward when volunteers are needed, and she liked being the only one called out of her class, walking down the hall to the quiet room where the breathing machine sat on a desk. She liked, too, the sense of being part of something bigger than herself, something that might really matter in the world.

In the empty classroom, the woman from the University of Southern California would hand her a sterile mouthpiece, attached by a tube to the spirometer ready to gauge the power of her lungs. Erika would give it a couple of practice puffs to get comfortable before the one that counted. “I remember her saying ‘Push, push, push. Blow all the air out.’ And then she would show me on her laptop, and I could see on a graph where I pushed the most,” and watch the line edge downward as her breath tailed off.

A nurse encourages a 15-year-old patient to exhale hard during a spirometry test of his lung capacity at the Venice Family Clinic in Venice, California. (David McNew/Getty Images)

After that, there was a survey to fill out, a couple of pages about her health and her family, about smoking in the home and pets and diet and exercise, and then Erika would walk back down the hall, back to her classmates and the ordinary rhythms of the school day.

She didn’t know it then, but those brief, once-a-year interruptions to her routine helped lay the foundation for insights that would ultimately change scientists’ understanding of what air pollution does to the human body. In the vast stacks of accumulating numbers—results from Erika Fields’s breath tests and thousands of others— a team of patient researchers would discern the outlines of a threat that had, until then, been hard to see.


Ed Avol was one of those scientists. He grew up breathing the foul air of 1960s L.A., and he remembers well the hacking coughs that filled the playgrounds of his childhood. An engineer by training, he worked early in his career on hospital-based studies that examined the effects of dirty air as researchers had for decades, by pumping pollution into small rooms and watching volunteers exercise inside.

The team he was part of wasn’t allowed to make conditions in their smog chambers any worse than what Angelenos would experience outdoors, but in the 1980s that still gave them plenty of latitude. The researchers would monitor subjects as they pedaled, measuring their heart rates and oxygen levels, making note of their coughing, their shortness of breath, and their red, watery eyes.

By that time, it was clear to scientists that ozone—the main ingredient in the smog that still plagues L.A. and so many other cities—had an immediate effect on those who breathed it. And the impact could be far more serious than the discomfort Avol saw so plainly: When ozone blankets a city, asthmatics wheeze, emergency room visits spike, and even in healthy people, the lungs can grow inflamed and struggle to do their job.

A pall of smog lies over the Los Angeles skyline in July 1978. The scientists who conducted the Children’s Health Study already knew that ozone was harmful to human health. But they discovered, to their surprise, that it was not what was doing the most damage to children’s lungs. (Nick Ut/AP)

But Avol had begun to ponder an even bigger question: If ozone’s immediate effects on the body were so clear, what was it doing over the long term to those who breathed it day after day, month after month, from one year to the next, over the course of a lifetime? Did its effects vanish when the air cleared, or had ozone—or, perhaps, some other, less familiar pollutant—wrought unseen damage that would accumulate slowly, lying in wait to bite unexpectedly decades later?

As it happened, John Peters and Duncan Thomas, researchers at USC’s medical school, were wondering the same thing. Concerned with the implications for public health, California’s Air Resources Board, the country’s most aggressive air pollution regulator, had asked them to design a long-term study that would definitively answer that very question. Avol signed on, and the team began, in the early 1990s, to set out the parameters that would guide their effort, to ensure that its long, slow time frame would be rewarded by results of rock-solid reliability.

It was that design that, a few years later, drew Jim Gauderman to the project, known by then as the Children’s Health Study. Gauderman is a biostatistician, schooled in the complex methodologies that sift human truths from vast haystacks of numbers. It’s not just his training that suits him to his work. He speaks in measured tones, weighs his words thoughtfully, has the calm, steady bearing of a man who knows painstaking diligence is sometimes the only route to scientific clarity.

He could see the meticulous groundwork Avol and the others had laid would one day bring a clear answer to a question that ate at him too. Gauderman was also from L.A., and had wondered, since his days as a young cross-country runner, what hidden scars all those training sessions along traffic-clogged roads might have left on him. He and Avol, junior partners at first on a project others had conceived, would eventually become its leaders.

There were many metrics the scientists could have chosen to use, but the one they settled on carried a unique power— the power, in some sense, of life itself. They’d test the strength of children’s lungs, over and over again, year after year. It was a measurement that, at least implicitly, carried a prediction, one that would follow a child forever.

For there are few facts about a human body that correlate more closely with its health, with the very length of its life, than the development of its lungs. Weaker lungs mean illness and frailty, and fewer years; stronger ones allow for vitality and longevity. So what Avol and Gauderman were measuring, very literally, were the invisible boundaries being drawn around their subjects’ futures.

For the children, of course, those stories were yet to be written. The team recruited 4,000 of them, kids between 10 and 16, in a dozen towns and neighborhoods as diverse as California itself. Later, they would add more cohorts, bringing the total number of subjects to more than 11,000, the youngest of whom joined as kindergartners and finally aged out of the study when they finished high school in 2015.

It was Gauderman who sifted, day after day, through the data that began pouring in. In addition to the annual lung tests, there were height and weight checks for every participant, and air quality measurements from monitors in the children’s neighborhoods. To a man who knew how to read them, the numbers sketched the outlines of the children’s lives and health, of their bodies and their lungs and the air that filled them.

It was still too soon, though, to find answers to the questions driving this work. While the years went by and the children’s lungs grew, Gauderman would have to watch and wait. Early on, he was mostly looking for errors that might skew the results. Did the paperwork list a child’s height as four feet when he stood last year at five? “We would go to the field staff and say, ‘What happened? This kid shrunk a foot, which value is right?’”

Eventually, he began searching for patterns. They weren’t easy to make out. Because, of course, all those young lungs were growing, and gaining power, no matter how dirty the air they breathed. Gauderman’s task was to determine whether some were developing more slowly than others. If they were, there could be any number of causes, from genetic predisposition to poor diet or a parent who smoked.

Even in the most polluted towns, there would be many children with strong lungs, and in the cleanest places, some would struggle for breath. He would have to use his most sophisticated statistical techniques, and sometimes invent new ones, to decode the message buried in the numbers, to disentangle what mattered from the irrelevant noise. As one millennium ended and another began, as new children joined the study and the older ones trudged down school corridors again and again, as their lungs matured and the data piled higher, the signal grew stronger. Until, eventually, it became undeniable.

There was no eureka moment, no sudden, made-for-TV “a-ha.” It wasn’t that kind of science, and Gauderman was not that kind of scientist. The message peering out at him wasn’t what he had expected, so he checked and rechecked, looked at it from every angle, until he was certain what he was seeing was real. In 2004, the answer to the question the team had posed more than a decade earlier appeared, at last, on the pages of the publication reserved for the most consequential of health breakthroughs: the New England Journal of Medicine.

No one would have been surprised, of course, to discover pollution’s effects were long-lasting; they wouldn’t have been looking if they hadn’t had an inkling that might be the case. What shocked these careful men was the scale, the sheer force, of the impact. It turned out that, year by year, as their trove of data had grown, the damage was accumulating, too, creating a gap likely to forever haunt the children caught on the wrong side.

By the time the oldest subjects finished high school—when growth was winding down and the consequences had become irreversible—dirty air had left its mark on the lungs of the children who breathed it. Those lungs failed to reach their full power, were weaker than they ought to have been, the very air that had sustained them also holding them back. Pollution had planted a hidden seed of vulnerability in these children, an unseen frailty that would set their futures on a different trajectory than their peers’, would make their bodies less robust and could even, eventually, shorten their lives.

Lungs that are 20 percent weaker than normal are a medical red flag, a warning that will prompt a doctor to hunt for a cause, and hope, especially when the patient is young, to diagnose something treatable.

What the Children’s Health Study found was that kids breathing the dirtiest air were nearly five times as likely to experience that level of lost function— almost 8 percent of them in the most polluted places, compared with 1.6 percent in the cleanest ones. For every 100 children who grew up with the worst pollution, in other words, at least six would be burdened as a result with a lifelong health problem. Many more, of course, would have a lesser degree of impairment, a smaller wound that would, meaningfully if less dramatically, drain their vigor too. “That,” Ed Avol recalls, “was a wake-up call.”

The finding, Avol says now, shattered scientists’ naive assumption that if air pollution was doing serious, long-term harm, they would have known about it already. In fact, he explained to me, while the impact was profound, its slow, steady accumulation made it hard to spot. “That percent, percent-and-a-half per year” of lost development, “unless you make careful measurements and have careful monitoring information and do some fairly sophisticated data analysis, it doesn’t just jump out at you.” Once they did so, pollution’s power was plain to see.

It was the first glimpse of a frightening truth, one whose repercussions went far beyond Southern California. For the unsettling reality the team had uncovered was that, across the country and around the world, the fuels we have built our lives on— the gasoline, the diesel, the coal we burn to travel from place to place, to power our light bulbs and laptops, to stay warm in winter— are changing our children’s bodies.

There was another surprise in the data too. From the outset, ozone had looked like the prime suspect. Its immediate impact was so clear that if air pollution was doing any long-term damage, the scientists figured, the familiar noxious gas would be to blame. But while its harm was real, ozone was not what was dampening the power of those young lungs. Tiny airborne particles known as PM2.5, so small they are thought to enter the bloodstream and penetrate vital organs, including the brain, were a far more potent danger. Nitrogen dioxide, one of a family of gases known as NOx, also had a powerful effect. In fact, it poured out of cars, trucks, and ships in such close synchronicity with PM2.5 that even Jim Gauderman’s statistical models couldn’t disentangle the two pollutants’ effects.

That wasn’t all. In what may have been their most worrisome discovery, the team found the pollutants were wreaking harm even at levels long assumed to be safe. In the years to come, the implications of that uncomfortable finding would be felt far beyond the pages of prestigious scientific journals.


Long Beach, where Erika Fields grew up, and where she gave birth to a baby after high school, wasn’t the most polluted of the Children’s Health Study communities, but it was certainly on the wrong end of the scale. Around the time her son was born, she started getting bad colds that would last for days, then clear up in a matter of hours. Eventually, she saw a doctor, who diagnosed allergies and sinus problems and said poor air quality was probably to blame.

Today, Fields lives with her family in the suburbs east of Los Angeles, an area known for its terrible air, where the city’s smog sits against the Santa Ana Mountains. The baby she had as a teenager is now a young man, and his sinuses act up just like his mom’s. Her youngest son, an energetic grade schooler who climbs on the back of her chair as we speak on Skype, has asthma. The attacks don’t happen often, but they’re frightening when they do.

It started suddenly, when he was a toddler. “He just kept vomiting and vomiting and got very lethargic, and panting like a puppy, and I could not understand what was going on,” she tells me. It’s never been quite that bad again, but “it’s like he can’t get a full breath of air in, so in trying to breathe in he’s coughing.” Even now that he’s older, the struggle still causes him to throw up.

All these years later, the emails from USC are few and far between, but Fields replies to every one. Until we spoke, she hadn’t heard about the study’s findings, the breakthroughs in which she’d played a small role. She’s still glad she raised her hand and walked down that hallway, though. “I felt,” she tells me, “like I was just doing my part.” With the help of Erika Fields and many others like her, anonymous but essential, the Children’s Health Study had upended the modern understanding of air pollution, and set researchers around the world on a new path, one whose farther reaches they are still charting today.

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