Photo: Steven Buss http://www.flickr.com/photos/steven-buss/349912280

If Los Angeles were ever any one thing, it would have to be apocalyptic. Bone-dry concrete rivers, air that looks and smells like tar, hills that burn bright orange, like the depths of hell, in late summer months, rain that restores mostly sewage to its rightful resting place in the coastal surf--they're signals of dysfunction that hit the gut hard on first contact.

And yet, it's easy to forget that an environment so far gone, an infrastructure so taxed, has anything to do with you and me and our lifestyle choices--a problem that seems to be especially true when it comes to our freeways, those atherosclerotic arteries of the city corpus, and the air pollution that spews from them.

(We rarely blame ourselves when we're stuck in bad traffic.)

In fact, the layer of haze that suffocates the Southland is all about you and me, about our driving habits and our health. Our dirty air, whose primary source is traffic, is estimated to kill 6,200 people in Southern California every year by tearing into our heart and lungs.

Now, an epidemiologist at the Southern California Environmental Health Sciences Center says particulate matter from cars, trucks and diesel exhaust is also infiltrating the brain.

Six years into analyzing air quality and cancer registry data, Dr. Roberta McKean-Cowdin and her colleagues at the University of Southern California have concluded that a child breathing average Los Angeles air is at least 10 percent more likely to develop brain cancer than a young person living in, for example, Cheyenne, Wyo.

The researchers acknowledge a 10 percent increase in risk is modest, and that the odds of developing brain cancer are small.

"It's still a rare event. Even if you double the frequency of a rare event, it's still a rare event," McKean-Cowdin said.

But their findings mean at least a handful of kids in Los Angeles right now have brain cancer that grew quite simply out of the air we breathe, out of the way we move around.

"If it's a cause, it's a cause for someone. And that's someone who didn't have to have it," she said.

***

Picture yourself stuck in heavy afternoon traffic on the I-405. It's 95 degrees outside. Your air conditioning is broken and the heat ripples up from the asphalt into a low-lying brown haze. You roll down your window and reach out to feel the air. The roar of idling engines is followed quickly by the thick taste of exhaust, which tickles the back of your throat. You cough, roll up the window and opt for the hothouse effect.

That cough is partly your body's reaction to fine soot, or what policy-makers call PM2.5 pollution: particulate matter smaller than 2.5 microns in diameter. (That's tiny. For reference, a human hair is about 75 microns in diameter.)

In Los Angeles, the annual average PM2.5 concentration is around 10 micrograms of fine particulate matter per cubic meter of air--the equivalent of two drops of sweat in an Olympic-sized swimming pool. That concentration might seem fairly light, but it's in fact one of the highest in the nation.

And it's this concentration (which, by the way, is within Environmental Protection Agency current standard for healthy air) that the USC researchers found to be associated with a 10 percent increased probability of juvenile brain cancer.

As McKean-Cowdin clarified, many parents should actually feel pretty good about those odds. Others who live in neighborhoods dissected by freeways and unmitigated by green space have more to worry about. Ambient air pollution levels tend to be the highest in the lowest-income, least educated areas, where factories and roadways abound.

And that's the trick of averages: turn them over and you're bound to find some ugly social inequities.

"There's a drop-off [in vehicle emissions] that's pretty significant over short distances, but even the drop-off results in concentrations that are pretty high," said Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University. "If I were in L.A., I wouldn't want to live six blocks from the freeway."

Diesel engines are especially bad to have as neighbors. They are heavy producers of ultrafine particles, the tiniest members of the PM2.5 family, which are the best at working their way deep into your body thanks to their composition and size.

But, Constantinos Sioutas, professor of civil and environmental engineering at USC, said diesel's not the only culprit.

"We like in science, and life in general, to follow simplistic categories, like you have a Dutch passport therefore you must be from the Netherlands, or you were born in New York, therefore you're called a New Yorker, but it's a lot more complicated than that," he said. "I've tested I-don't-know-how-many thousands of vehicles in my life, and every single one is different than the other. There are old gasoline vehicles that actually emit a lot more than newer diesel trucks."

Even emission control technologies, intended to curb pollution, are now thought to be increasing ultrafines--for which there is no regulatory standard--by cutting down the number of larger particles escaping vehicles, but allowing gases into the air that nucleate into new, smaller ones.

The distinctions can be confusing--between cars and trucks, old and new, fine and ultrafine--but the lesson is simple: Traffic is the No. 1 cause of air pollution in our region.

And while it might disproportionately affect the poorer among us, ultimately we're all victims.

"There are two types of pollution," Jacobson said. "There's the type that's emitted, and there's the type that forms in the atmosphere chemically, and you can't really escape that one."

***

If you find it hard to believe that air pollution can infiltrate the brain, you're not alone. Even McKean-Cowdin was skeptical when her mentors at the Keck School of Medicine, Susan Preston-Martin and John Peters, began pressuring her to investigate a link.

"We put together an application [for funding] because they kind of insisted that I help them," she said, "But it ended up being really interesting."

Interesting because researchers Lillian Calderón-Garcidueñas of the University of Montana and Günter Oberdörster at the University of Rochester only recently discovered the routes by which ambient pollutants might enter the brain.

Their work, published in separate papers in 2002, details a process not unlike the one we face as daily commuters. Particles simply enter the body through some anatomical onramp, cruise right up an anatomical freeway, and park themselves in anatomical headquarters. Some are inhaled into the sinus cavities, where they jump onto an olfactory nerve. Some smaller particles and soluble metals barge through the alveolar wall of the lungs and head into systemic circulation. And some don't drive to work at all but sort of telecommute; they cause a systemic inflammatory response from the lungs, which triggers the formation of reactive oxygen species in brain tissue.

Each of these three passageways opens the brain to DNA damage and chronic inflammation, both of which are tied to increased cancer risk.

But Calderón-Garcidueñas and Oberdörster, whose findings were based on animal and experimental data, only established mechanisms, or blueprints of how transport and damage might occur. McKean-Cowdin, and others interested in how the research might apply to Parkinson's and Alzheimer's, wanted to know whether these mechanisms were actually playing out in humans. And so they turned to population studies.

Children, it turns out, make wonderful study subjects. Their short lives offer small, precise windows of exposure. They move little distances--to the swings, to preschool, to a friend's house down the block--instead of roving widely as adults do and their actively growing brains are highly sensitive to agents that might cause mutations or growth abnormalities.

After examining the effect of air pollution on adult brains and finding no association--it was difficult to accurately link exposure levels to morbidity because grown people move around so much--McKean-Cowdin turned to a data set her mentor Preston-Martin had already assembled: It included all brain cancer cases in Southern California in children 5 years of age and younger from 1984 to 1991.

This set, combined with cancer registry data from 1991 through 2002 yielded an 18-year record that could be tied to relevant information: Where did each of the study subjects live? How far were their homes from roadways? To what extent were they exposed to traffic-related pollution?

It was this record that yielded a positive association--a breakthrough for McKean-Cowdin.

"People are waiting for information. They really want information," she said. "If you've ever had to deal with disease in your own life, you know it's really frustrating when you go to the doctor and they don't have anything to tell you."

The study investigators are in the final stages of revision before they submit their research to environmental health journals, meaning it has yet to be published or peer reviewed. But McKean-Cowdin said their findings fit well with studies looking at air pollution and cardiovascular outcomes, and they add new light to a body of work on cancer and traffic density, by Peggy Reynolds and others, that has so far been inconclusive.

McKean-Cowdin is equal parts caution--"I'm careful about what I put out there"--confidence--"We've redone things enough times and we've tried a variety of models. I think it's solid"--and optimism.

"Other than ionizing radiation (from medical X-rays) there are really no environmental identified causes of brain tumors that you can change. So if you could find something, that would be amazing."

***

You can lead a horse to water but you can't make it drink.

McKean-Cowdin's research is only a drop in an ocean of evidence that our dirty air is killing us. We know by now that Southern California's pollution ranks among the worst in the nation. We know by now that vehicle emissions are the No. 1 cause of our dirty air. Yet Los Angeles continues to feature the highest rate of per-capita driving in the nation for a city its size and density. (Think Steve Martin's Wacky Weatherman in "L.A. Story," who drives half a block rather than walking.)

"People [here] are married to their vehicles. They'd rather die than give them up," said Sioutas of USC. "People on average work and drive [to] what, by European standards, would be another country."

"Not a lot of people in Europe think about living in Switzerland and working in Germany--you know, commuting every day."

It's easy to find reasons to drive in Los Angeles: the city is too spread out, maybe your car gets good gas mileage, there's no public transportation. But the excuses fall flat in the face of reality. In fact, Los Angeles is the densest metropolitan region in the country, the gains of efficient cars are often offset because their owners feel entitled to drive farther and our region has some of the best public transportation in the nation.

Of the 14 largest metropolitan areas, we rank second in total bus service miles, first in bus service miles per square mile and fifth in total rail transit track miles, said Paul Sorensen, associate director of the Transportation, Space and Technology Program at the RAND Corporation.

So what explains our driving habit? Laziness? Complacency? Culture? Maybe a feeling that the problem is too big for any one of us to have an effect on it?

"I think the ultimate issue is freedom and the way people define freedom," said Jennifer Regan, manager of sustainability programs at AEG Worldwide. "I think in L.A., people who can't get out of their cars are defining freedom as the power of moving themselves between distant locations."

Regan, a fair-skinned redhead with fiery eyes and old-fashioned grace, is 24 years old. A Los Angeles native, she gave up her car years ago and commutes by bus every day between Santa Monica and downtown. That's 170 miles each week, 9,000 miles per year, 3,600 gallons of gasoline saved every decade.

"As a carless L.A. resident, I define freedom as the ability to sleep while I'm being transported, or to learn a new language on my iPod, or to read a book, or to talk on the phone and not have to pay attention to the road," Regan said.

While her choice to ride public transportation is personally rewarding, it can be a frustrating road, going against the behavioral flow.

"I get aggravated sitting in my beautiful buses and watching the single drivers heading to downtown, and thinking to myself--this is the sick part--I see the same cars next to my bus every morning...I recognize drivers in the cars..."

About 17.6 million of us share the air of the Greater Los Angeles Area, meaning any individual bears only a tiny fraction of the blame for its degradation. But each of us--each heart, each brain, each pair of lungs--bears the collective brunt, the sum total of 17.6 million lifestyle choices.

Making change, it seems, means taking ownership--a commitment to cleaning up our air one mile, one micron, one microgram at a time.