Because uranium minerals emit radon gas, and their harmful and highly radioactive decay products, uranium mining is considerably more dangerous than other (already dangerous) hard rock mining, requiring adequate ventilation systems if the mines are not open pit. In the 1950s, a significant number of American uranium miners were Navajo, as many uranium deposits were discovered on Navajo reservations. A statistically significant proportion of these miners later developed small-cell lung cancer, a type of cancer usually not associated with smoking, after exposure to uranium ore and radon-222, a natural decay product of uranium. The cancer causing agent has been shown to be the radon which is produced by the uranium, and not the uranium itself. Some survivors and their descendants received compensation under the Radiation Exposure Compensation Act in 1990.