A state health official says North Dakota's flat landscape and its relatively small population help hold down air pollution.

Jim Semerad is an air control division manager at the state Health Department. He says the air can be cleaner on the wind-swept prairie than in valleys and ravines that trap pollution. And he says fewer people means fewer cars, which are considered sources of cancer-causing pollution.

A recent Environmental Protection Agency report studied 80 cancer-causing substances released by cars, factories and other sources. It found North Dakota's air the second cleanest, behind Wyoming.

The study put North Dakotan's average excess cancer risk at 16.7. That refers to the added number of people per 1 million residents expected to develop cancer because of exposure to air pollution. The national average was 36.

- Associated Press