"Duluth is located at the end of the largest, coldest and cleanest Great Lake," said David Thornton, assistant commissioner of the Minnesota Pollution Control Agency. "It has its own microclimate."
Winds sweep off the lake and clean air drifts in from nearby Canada, helping to dissipate any pollution.
There's also less heavy industry in Duluth, which has helped to clean up the air quality as well. What remains is cleaner too, thanks to state programs that encourage plants to install the latest cleaning systems.