What is the difference between strontium-86 and strontium-90?
For all practical purposes, Sr90 is chemically identical to Sr86. However, the extra neutrons crammed into Sr90 make it radioactive. 

As mentioned in the comments above, alkaline earth metals (e.g. Ca, Sr, Ba) behave chemically in a similar fashion, and tend to deposit in bone like calcium does. This keeps the radioactive Sr90 in contact with the rapidly-growing hematopoietic material in bone, increasing the risk of cancers such as leukemia.