if it were its lighter congener calcium. Because the elements are chemically very similar, stable strontium isotopes do not pose a significant health threat. The average human has an intake of about two milligrams of strontium a day. In adults, strontium consumed tends to attach only to the surface of bones, but in children, strontium can replace calcium in the mineral of the growing bones and thus lead to bone growth problems. The biological half-life of strontium in humans has variously been reported as from 14 to 600 days, 1,000 days, 18 years, 30 years and, at an upper limit, 49 years. The wide-ranging published biological half-life figures are explained by strontium's complex metabolism within the body. However, by averaging all excretion paths, the overall biological half-life is estimated to be about 18 years. The elimination rate of strontium is strongly affected by age and sex, due to differences in bone metabolism. The drug strontium ranelate aids bone growth, increases bone density, and lessens the incidence of vertebral, peripheral, and hip fractures. However, strontium ranelate also increases the risk of venous thromboembolism, pulmonary embolism, and serious cardiovascular disorders, including myocardial infarction. Its use is therefore now restricted. Its beneficial effects are also questionable, since the increased bone density is partially caused by the increased density of strontium over the calcium which it replaces. Strontium also bioaccumulates in the body. Despite restrictions on strontium ranelate, strontium is still contained in some supplements. There is not much scientific evidence on risks of strontium chloride when taken by mouth. Those with a personal or family history of blood clotting disorders are advised to avoid strontium. Strontium has been shown to inhibit sensory irritation when applied topically to the skin. Topically applied, strontium has been shown to accelerate the recovery rate of the epidermal permeability barrier (skin barrier). Nuclear waste Strontium-90 is a radioactive fission product produced by nuclear reactors used in nuclear power. It is a major component of high level radioactivity of nuclear waste and spent nuclear fuel. Its 29-year half life is short enough that its decay heat has been used to power arctic lighthouses, but long enough that it can take hundreds of years to decay to safe levels. Exposure from contaminated water and food may increase the risk of leukemia, bone cancer and primary hyperparathyroidism. Remediation Algae has shown selectivity for strontium in studies, where most plants used in