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.