Each weekday for most of the year, hundreds of thousands of school buses criss-cross their way through America. Every school day, the hulking monoliths transport nearly 26 million children, or about 55% of the student population, and travel over 4 billion miles annually. This makes the national school bus fleet the largest form of mass transportation in the U.S. — bigger than that of commercial buses, trains and airplanes combined. 
The yellow school bus might be one of the most iconic and ubiquitous symbols of childhood, but intertwined with that nostalgia is an ugly reality. For almost as long as school buses have been around, the children inside of them have been breathing in toxic fumes that can have dire consequences not just for their respiratory health, but for their brain development as well. 
In a new study, researchers from Georgia State University compared the standardized test scores of kids who rode old, dirtier diesel buses to those who commuted in buses with engines that had been modified, or retrofitted, to filter out up to 95% of harmful pollutants. Looking at test results from 2007 to 2015, they found a significant increase in English scores and smaller, but notable, gains in math scores  among bus-riding kids whose districts retrofitted their bus engines. 
For cash-strapped school districts, buying newer buses that adhere to the government’s stricter diesel regulations, enacted in 2007, can be out of reach, even with the medley of rebates and grants awarded each year under the Diesel Emissions Reduction Act and through the Environmental Protection Agency’s Clean School Bus program
But there is another solution that is often overlooked, say the researchers — a fix that not only protects young brains but is also a cost-effective way of decreasing absentee rates and improving test scores, which in turn increases lifetime earnings.
Diesel retrofits are engine modifications that can filter out up to 95% of harmful pollutants. At an average cost of $8,000 per engine, it’s a much cheaper option than buying newer buses that burn cleaner, ultra-low sulfur diesel, which can run a school upward of $130,000 apiece (the price jumps to about $360,000 for a propane-fueled or electric-powered bus). Currently, only an estimated 40% of all fleets run on the lower-emission diesel technology; the majority are still spewing known carcinogens. 


Though the respiratory dangers from emissions of diesel fuel have long been known, researchers are just beginning to understand the impact on the brain, which can have both short- and long-term effects. Particularly worrisome are the microscopic soot particles, known as particulate matter, that when inhaled can burrow deep inside a person’s lungs and enter the bloodstream. For kids, whose internal systems are much smaller and still developing, the effects are even more pronounced.
The evidence that soot and other toxins in high-sulfur diesel lead to lasting brain effects is building, said Jimmy O’Dea, a senior vehicles analyst for the Union of Concerned Scientists, a nonprofit research organization. 
“The scientific literature is really showing, with study after study, nearly every organ system in the body is at risk from higher exposure to particulate matter,” O’Dea told NationSwell. “Everything from the lung diseases that you might typically associate with bad air quality to heart and neurological diseases are being found to increase health risks from more exposure to these pollutants.”

school buses
Until 2007, school buses ran on older diesel technology laden with pollutants that have been shown to trigger asthma in children and affect their cognitive functioning.

Initially the Georgia State researchers set out to only look at students’ aerobic health using data from Georgia’s statewide fitness assessments. But at the time, Wes Austin, one of the study’s co-authors, had been sifting through other research on the effects of air pollution when a study linking it to dementia caught his eye. “I just happened to have a lot of state education data sitting around, so it didn’t seem like too much of a stretch to look at test scores too,” he said, adding, “but I didn’t think the study would go where it did.”
Austin described how the cocktail of toxins in diesel exhaust can cripple a young mind. “There’s a same-day effect, where carbon monoxide and other things that decrease your blood’s oxygen level can make you feel a little bit out of it, like a brain fog,” said Austin. “But in the long term, particulate matter PM2.5 is small enough that when you breathe it in, it passes through your nasal cavity and into your brain, and leads to white-matter lesions and inflammation.” 
The result, he said, “interferes with your neurons’ ability to communicate properly.”


Unlike other behemoth diesel-burners on the roads, such as commercial trucks and mass-transit buses, school buses have been slow to embrace new technology. Between 2012 and 2018, for example, the EPA awarded $39 million in rebates to replace nearly 2,000 buses across the country; this year, it’s on track to replace an additional 473 buses. Considering that there are more than 470,000 school buses on the road, that’s little more than a drop in the proverbial bucket.
“It’s a question of funding and school districts making it a priority,” said Allen Schaeffer, executive director of the Diesel Technology Forum, a nonprofit advocacy group. He noted that even as alternative-powered buses are getting more attention from the media and from the government — a new bill to replace old diesel buses with new electric ones was recently introduced in the Senate — 95% of school buses continue to burn diesel fuel. “It’s still the overwhelming dominant technology,” he said.
Though Americans by and large prefer cars that run on gasoline, the historical choice of diesel for school bus fleets made sense for two reasons. “First and foremost was safety,” Schaeffer said. “If a school bus gets rammed by a car, the risk of fire would be greater with gasoline than diesel because diesel is less likely to ignite under those kind of circumstances.”
Second is simple economics. When deciding where to put their dollars, school districts often don’t prioritize transportation, even as the proliferation of school choice and charter schools have caused bus routes to get longer and more children outside of districts are accommodated. And, as Schaeffer pointed out, applying for state and federal rebates to upgrade diesel buses is a competitive process.
With limited funds, schools often deprioritize transportation, he said. “They’re asking themselves, ‘Should I spend the dollars to get them to school in a fancier bus? Or should I spend the dollars in the classroom or to reduce the ratio of teachers to students?’ Those are the kind of questions that these districts are looking at.” 


Austin and his fellow Georgia State researchers looked at that question too. They found that paying for diesel-engine retrofits — in lieu of shelling out for brand-new buses — is a highly cost-effective way to preserve brain and lung health and improve academic achievement.
The research team carried out back-of-envelope calculations regarding the costs and benefits of bus retrofits. They looked at data from an earlier study that linked smaller class sizes to improvements in test scores and higher lifetime earnings. Reducing a classroom by seven students was found to cost about $870 per student. In contrast, the Georgia State researchers estimated that retrofitting a bus costs roughly $122 per student rider.
“Reducing class size by hiring more teachers is expensive,” said Austin. In fact, his study concluded that to see the same test-score gains, a district would need to spend anywhere from three to five times as much on class-size reductions than it would on bus retrofits. What’s more, the researchers found that “if a district retrofits its entire bus fleet, the effect on English test scores would be slightly larger than the effect of going from a rookie teacher to one with five years of experience.”
Diesel school buses were built to last a long time, as Schaeffer pointed out, and eventually the older, dirtier pre-2007 models still on the road will break down or be phased out. But until then, schools have a relatively cheap win-win available: the chance to improve overall student health and boost their chances for lifelong success.
More: The Diesel-Chugging Yellow School Bus Finally Goes Green