Strontium is chemically similar to calcium and can replace calcium in bones. The radiation from ${\mathrm{Sr}}^{90}$ can damage bone marrow where blood cells are produced, and lead to serious health problems. The time taken for 99.99% of a sample of ${\mathrm{Sr}}^{90}$to decay is $\left(\lambda \text{ for }S{r}^{90}=0.23y{r}^{-1}\right)$