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It all started in 2001 with a complaint from a concerned kindergarten teacher. "She came to us because she wondered what her kids were being exposed to as they lined up by the buses that idled outside their school," says Diane Bailey, a former NRDC scientist in the Health & Environment program. Although it was common to see tailpipes churning out black smoke or to notice the unpleasant smell of exhaust in and around school buses (the vast majority of which run on diesel fuel), no one had studied the actual risks posed to the 25 million children in the United States who rode them on a daily basis.

So researchers from NRDC, the Coalition for Clean Air, and the University of California, Berkeley decided to be the ones to dig into this. They rented school buses, equipped them with high-tech air-monitoring devices, and traveled along elementary-school bus routes in the Los Angeles area. They took measurements both inside and outside the buses, as well as in passenger cars traveling ahead of them.

The findings, published in the resulting report, No Breathing in the Aisles, were alarming. Levels of diesel exhaust inside the school buses were up to four times higher than those found in passenger cars just ahead of them, and more than eight times what you'd find in the average sample of California air. Scariest of all: The authors estimated that 23 to 46 of every one million children may eventually develop cancer from the diesel exhaust they inhale just while traveling to and from school.

"Children are vulnerable," says Melissa Lin Perrella, senior attorney and director of the Western Air Quality and Environmental Justice programs at NRDC. "Not only are their bodies more susceptible to getting sick from air pollution, but they often cannot advocate for themselves."

Most of the buses used for this study weren't spewing out black smoke—proving that diesel pollution doesn't have to be visible in order to be harmful. "The tiniest particles, the ones that you can't see, are the most hazardous," Bailey says. "They're small enough to get lodged deeply into your lungs and transfer to your blood, which can contribute to many long-term health problems."

The study's findings set off a chain reaction. Researchers at Berkeley did a second, independent study, verifying NRDC's initial results. Soon after, Yale University published its own research on school buses in Connecticut, with similar conclusions. And in 2013, the U.S. Environmental Protection Agency launched Clean School Bus USA—a program that provides funding and resources to school districts and bus companies looking to upgrade their fleets. In 2015 alone, it allocated more than $7 million to 85 school districts (out of 530 applicants) across the country.

Since then, several states—including Washington, California, New Jersey, and Texas—have launched their own incentive funding programs. Many cities and school districts and several states have developed anti-idling regulations for school buses, and some have instituted mandatory regulations for retiring old buses or retrofitting them with soot-removing filters.

Cleaner fuels and stricter emissions standards have helped, too. Buses purchased in 2007 or later are 60 times cleaner than those built before 1990, and those changes have already made a noticeable difference in kids' health. A 2015 study by scientists at the University of Michigan and the University of Washington found that children had improved lung function and were absent less often after their schools adopted cleaner fuels and technologies.

Certainly, much has improved. But experts estimate there are still about 250,000 older, dirtier school buses in service. In a 2016 New York Times op-ed, Johns Hopkins University researchers calculated that replacing just 10 percent of these buses with newer models would cut about 5,000 tons of pollution in the first year alone. With old buses built to last several decades and new ones costing $80,000 apiece, however, progress has been slow.

NRDC is driving further reductions in diesel pollution by targeting other exhaust sources, such as municipal trash trucks and street sweepers, public transit vehicles, and shipping and industrial equipment. But Perrella says protecting America's children remains as important today as it's ever been. "Given what we know about the dangers of diesel exhaust, and for how long we've known it," she says, "cleaning up dirty school buses should be a no-brainer."