The radioactivity of all radioactive waste weakens with time. The dangers of ionizing radiation due to radioactivity and X-rays were not immediately recognized. The dangers of radioactivity and radiation were not immediately recognized. Radon is a significant contributor to environmental radioactivity. Large releases of radioactivity from nuclear reactors are extremely rare. DU used in US munitions has 60% of the radioactivity of natural uranium. Standard atomic weight requires the sources be terrestrial, natural, and stable with regard to radioactivity. See the article on Airborne particulate radioactivity monitoring for more information. They have mutated due to generations of exposure to heightened radioactivity. Snails have been observed with unusual levels of radioactivity. There are environmental concerns about radioactivity released from Hanford. It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. In 1905, Thomson discovered the natural radioactivity of potassium. Much of the radioactivity had decayed by 1980. In May 2012, TEPCO released their estimate of cumulative radioactivity releases.