unprotected outdoor humans in the open out to 900 meters, with death occurring in one or two days. The median lethal dose (LD50) of 6 Gray would extend to between 1350 and 1400 meters for those unprotected and outdoors, where approximately half of those exposed would die of radiation sickness after several weeks. A human residing within, or simply shielded by, at least one concrete building with walls and ceilings thick, or alternatively of damp soil 24 inches (60 cm) thick, would receive a neutron radiation exposure reduced by a factor of 10. Even near ground zero, basement sheltering or buildings with similar radiation shielding characteristics would drastically reduce the radiation dose. Furthermore, the neutron absorption spectrum of air is disputed by some authorities, and depends in part on absorption by hydrogen from water vapor. Thus, absorption might vary exponentially with humidity, making neutron bombs far more deadly in desert climates than in humid ones. Effectiveness in modern anti-tank role The questionable effectiveness of ER weapons against modern tanks is cited as one of the main reasons that these weapons are no longer fielded or stockpiled. With the increase in average tank armor thickness since the first ER weapons were fielded, it was argued in the March 13, 1986, New Scientist magazine that tank armor protection was approaching the level where tank crews would be almost fully protected from radiation effects. Thus, for an ER weapon to incapacitate a modern tank crew through irradiation, the weapon must be detonated at such proximity to the tank that the nuclear explosion's blast would now be equally effective at incapacitating it and its crew. However although the author did note that effective neutron absorbers and neutron poisons such as boron carbide can be incorporated into conventional armor and strap-on neutron moderating hydrogenous material (substances containing hydrogen atoms), such as explosive reactive armor, can both increase the protection factor, the author holds that in practice combined with neutron scattering, the actual average total tank area protection factor is rarely higher than 15.5 to 35. According to the Federation of American Scientists, the neutron protection factor of a "tank" can be as low as 2, without qualifying whether the statement implies a light tank, medium tank, or main battle tank. A composite high density concrete, or alternatively, a laminated graded-Z shield, 24 units thick of which 16 units are iron and 8 units are polyethylene