Each year, more than 27 million American and Canadian kids get to school by bus. Most of those buses run on diesel（柴油）fuel, which give out pollution that riders can take in. Pollution levels can be several times higher inside a diesel school bus than outside it. Why? Pollution can leak in from the floor or blow in through windows.

An American government program started in 2012 offered schools money spent on cleaner school buses, but not all schools could get it. Five years later, emissions from buses in the winning districts fell. And a year after getting new buses, student attendance had improved in those districts. For an average district of 10,000 students, about six more students attended school each day in the winning districts, compared to the losing districts. Winning schools with higher rates of bus-riding kids had an average of 14 more students in class each day. And winning districts that replaced the oldest school buses? They had an average of 45 more students at school each day.

Those numbers may sound small, but they add up. And school attendance matters for students' achievements. Almost 3 million American kids ride school buses more than 20 years old. If American school districts had replaced all of those older buses, there would have been 1.3 million fewer student absences each year.

There might be other reasons for better attendance. For example, maybe kids preferred new buses. However, the most likely reason for fewer student absences was better health. A study done on adults showed that a brief exposure to diesel emissions reduced "network connectivity" in the brain. In other studies, such network-connectivity changes have been linked to worsened memory and mental tasks. A Washington State program upgraded pollution controls in old diesel buses. Afterward, fewer kids were hospitalized in those districts than in the schools without bus upgrades.

Almost all American school districts can apply for the program. But schools in low-income, tribal and rural areas will get priority. Kids in these areas tend to face the most health risks from older buses.