Diesel exhaust is bad for child and planetary health, but change is on the way. We talk with the transportation director of Twin Rivers Unified School District and meet the people transforming the nation's biggest public transit program, including clean air advocates, electric bus builders, parent activists, and a guy called the Electric Bus Evangelist. We talk about everything from the economics to what's under the hood, and how states can use funds from Volkswagen's legal settlement to speed the transition to clean buses.