A few years ago I stood in a cramped trailer beside the busy 110 freeway in Los Angeles as researchers at the University of Southern California gathered soot thrown off by vehicles pounding by just a few yards from their instruments, which rattled whenever a heavy truck passed. I was there to learn about how scientists were beginning to link air pollution—from power plants, motor vehicles, forest fires, you name it—to one of the least understood and most frightening of illnesses: dementia.