When NPR science reporter Richard Harris compared Denver's normal background radiation levels to the improving situation around Japan's earthquake-damaged nuclear power plants, we wanted to find out more.  Radiation levels are falling in northeastern Japan, and as Harris reported, "it appears it won't be long before the levels are equivalent to those in Denver."  It turns out that Denver and much of Colorado has background radiation higher than in many other places. Partly it's the altitude. There's less atmosphere shielding Colorado from cosmic rays.  And the ground is rich in uranium, which gives off radioactive radon gas.  Colorado's background radiation is higher, but not dangerous, says Dr. Jeff King, who teaches nuclear engineering at the Colorado School of Mines.  He speaks with Colorado Public Radio's Mike Lamp.