The calcium mistake, again
Strontium sits below calcium in group 2 and forms the same doubly charged ion. The two are close enough in size that no biological system separates them reliably.
So strontium follows the calcium pathway: absorbed in the gut, carried in blood, and built into the mineral lattice of bone and teeth.
Bone is not inert — it remodels throughout life — but the exchange is slow, and strontium 90 deposited in childhood is still there decades later. Its half-life of 28.8 years means it does not decay away either.
⚠️ What it emits is a beta particle, and then its daughter yttrium 90 emits a more energetic one. Both are delivered to the bone marrow, which is where blood cells are made and where leukaemia begins.
Caesium 137 is the other long-lived fission product, and the comparison is instructive: caesium follows potassium, spreads through soft tissue, and is excreted in about seventy days. Strontium follows calcium and stays.
Three hundred thousand teeth
By the mid-1950s atmospheric testing had put fallout into the whole northern hemisphere, and the argument about whether it mattered was conducted largely without data on what was reaching people.
In 1958 a group at Washington University in St Louis, working with the Greater St Louis Citizens’ Committee for Nuclear Information, began asking parents to send in their children’s milk teeth. Children who contributed received a button reading “I gave my tooth to science.”
They collected over 300,000.
The teeth were a near-perfect record. A milk tooth mineralises at a known age, so its strontium content captures the environment of a specific window in a specific child’s life.
⚠️ The results, published in Science in 1961, showed strontium 90 rising steadily with year of birth. Children born in the early 1950s carried many times more than those born before the war.
The findings entered the political debate directly. The Partial Test Ban Treaty, prohibiting atmospheric testing, was signed in 1963.
The route was milk, again
Strontium 90 fell on pasture. Cows ate the grass and passed it into milk, where it travelled with the calcium.
Children drink more milk than adults relative to their body mass, and they are building bone rather than maintaining it, so they incorporate a far larger share of what they take in.
This is the same pathway that carried iodine 131 after Chernobyl, and it recurs because it is short: grass to cow to child in about a day, with no processing step that would remove anything.
⚠️ Dairy restrictions are therefore the first countermeasure after any release, and the one that matters most for the youngest people exposed.
The lighthouses
Strontium 90 emits beta particles and almost no gamma, which makes it easy to shield, and its 28.8-year half-life means a source stays useful for decades.
The Soviet Union used it to power radioisotope thermoelectric generators for lighthouses and weather stations along the Arctic coast — places too remote to service, where a device had to run unattended for thirty years.
Several hundred were built. After 1991 many were forgotten.
⚠️ Some were found broken open by metal thieves who had no idea what they contained. In Georgia in 2001, three woodcutters found two warm metal cylinders in the forest, slept beside them for warmth, and suffered severe radiation burns. The cylinders were RTG cores from an abandoned installation.
Recovery programmes have located most of them. The lesson is the same as with cobalt sources: a source outlives the institution that deployed it.
What is left of the fallout
Roughly two half-lives have passed since the peak of atmospheric testing, so about a quarter of what was deposited remains — spread thinly enough across the northern hemisphere to be measurable rather than harmful.
The concentrated remainder is in three kinds of place: the Chernobyl and Fukushima zones, the ground beneath weapons production sites like Hanford and Sellafield, and the fuel ponds where spent fuel is stored.
⚠️ Strontium 90 is one of the two nuclides, with caesium 137, that governs how long high-level waste needs isolating before it stops being acutely dangerous. After about three hundred years both are essentially gone, and what remains is a long-lived, less intense hazard from the actinides.