The hazards of ionizing radiation have aroused concern since a short time after the discovery of x-rays and natural radioactivity in the 1890s. Misuse of x-rays and radium prompted efforts to encourage radiation safety and to set limits on exposure, culminating in the first recommended "tolerance doses" in 1934. After World War II, the problems of radiation protection became more complex because of the growing number of people subjected to radiation injury and the creation of radioactive elements that had never existed before the achievement of atomic fission. Judging the hazards of radiation became a matter of spirited controversy. Major public debates over the dangers of radioactive fallout from atmospheric bomb testing in the 1950s and early 1960s and the risks of nuclear power generation in later periods focused attention on the uncertainties about the consequences of exposure to low-level radiation and the difficulties of resolving them.