Tailings contain heavy metals and radioactive radium. Radium then decays over thousands of years and radioactive radon gas is produced. Tailings are kept in piles for long-term storage or disposal and need to be maintained and monitored for leaks over the long term.
If uranium tailings are stored aboveground and allowed to dry out, the radioactive sand can be carried great distances by the wind, entering the food chain and bodies of water. The danger posed by such sand dispersal is uncertain at best given the dilution effect of dispersal. The majority of tailing mass will be inert rock, just as it was in the raw ore before the extraction of the uranium, but physically altered, ground up, mixed with large amounts of water and exposed to atmospheric oxygen, which can substantially alter chemical behaviour.
An EPA estimate of risk based on uranium tailings deposits existing in the United States in 1983 gave the figure of 500 lung cancer deaths per century if no countermeasures are taken.