High-energy cosmic rays and solar particles react with the upper atmosphere generating high-energy protons and neutrons that shower to the ground. Neutrons are particularly troublesome as they can penetrate most man-made construction (a neutron can easily pass through five feet of concrete). This effect varies with both latitude and altitude. In
London, the effect is two times worse than on the equator. In Denver, with its high altitude, the effect is three times
worse than at sea-level San Francisco. In a commercial airplane, the effect can be 100-800 times worse than at sea-level.