Which of these cities doesn't belong? Visalia and Modesto are both Central Valley California towns that suffer from the same congestion-created, hot-weather air pollution as their neighbors. But Hilo and Fairbanks are in Hawaii and Alaska two states most Americans would associate with fresh, clean air. However, Hilo on the Big Island experiences volcanic ash that can darken the sky, creating what's known as "vog." In Fairbanks, meanwhile, local air pollution is worsened by the use of outdoor wood-fired boilers called hydronic heaters. They may be on the way out, though the city council is considering outlawing them.