The air in this steel town was once so polluted with coal and coke soot that streetlights were sometimes turned on at high noon. Now, much of Pittsburgh's pollution comes from Ohio, West Virginia and further west, according to Neil Donahue, who studies transport pollution at Carnegie Mellon University.
While coal-burning power plants strung for hundreds of miles along the Ohio River are still a problem, many plants can switch back and forth between coal and natural gas. So when gas prices drop, as they did last year, the plants rely more heavily on that cleaner-burning fuel, which accounts for some of the city's improved air quality.
With its deep valleys, however, Pittsburgh's topography works against it, especially during the hazy days of summer, according to Donahue. That's when warm blankets of air bottle up pollution in the city's deep river valleys for days at a time.
