Environmental officials are considering a crackdown on diesel, a common and dangerous pollutant that can clog lungs, damage genes, and cause cancer. The California Air Resources Board last year declared that diesel particles are carcinogenic and is studying ways to reduce their health threat; the EPA is considering a similar move. In a series of articles, the Los Angeles Times explores diesel’s serious health threats, the high costs of weaning American society off its diesel dependence, and new, cleaner transportation alternatives.