Strontium is a metalic chemical element (atomic number 38). It is highly reactive and never exists in its metallic state in nature. It has few practical uses, but due to its chemical properties can be used in optical lenses and magnets.
In medicine, strontium ramelate is used in the treatment of osteoporosis, and it can be used as a tracer when studying neurotransmitters. It is also common in nutritional supplements that claim to have a benefit on bone health, although this is controversial. However, it is agreed that non-radioactive strontium is not very toxic and normal exposures pose no risk to human health.
Because it is chemically very similar to Calcium, strontium can take the place of calcium in the structure of bones. This was a concern during the 1950s and early 1960s when atomic weapons were tested in the atmosphere - one of the fallout products was Strontium-90, a radioactive isotope which could be passed along in the food chain in milk from cows who had eaten grass contaminated with fallout. This resulted in an increased risk of bone cancer. This risk was one of the reasons why an atmospheric test ban was agreed to in the early 1960s.