Tailings are produced when metallic ores are ground into a fine powder to free the metal-bearing mineral. Its maximum particle size of about 0.08 in (2 mm) is small enough to retain water and support plant growth. Fine dust is one potential hazard, especially if trapped in the lungs of miners. The greatest revegetation difficulties come from metals in lethal concentrations and deficiencies of critical nutrients, especially nitrogen and phosphorus. Some old tailings have remained barren for many years, with consequent high erosion rates. However, some metalophytes are able to survive on these wastes. Curiously, some plants have populations which can also survive on these tailings.