The process of nuclear testing releases negligible amounts of strontium (Sr) to the environment and, therefore, does not raise much concern. However, the little Sr released can be incorporated into food substances and converted into part of the food chain. The initiation of atmospheric examination of nuclear weapons in the mid-twentieth century led to the emission of significant amounts of Sr to the environment. This was evident from the substantial drop in quantities of Sr ingested from food substances after the deferment of the atmospheric testing of nuclear bludgeons. This phenomenon proved that the emitted strontium undeniably found its way into the food chain. However, the suspension of nuclear testing does not solve the entire problem of Sr emission to the environment because accidents can still occur at these nuclear plants and cause accidental leakage of highly toxic wastes. Apart from weapon facilities, plants that reprocess fuels also pose similar hazards. The utilization of nuclear fission as a radioactive process in research reactors and nuclear power plants causes the pollution of soils with Sr-90.
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Chernobyl was one of the nuclear reactants that had a history of nuclear accidents that led to the emission of radioactive Sr-90 to the environment. This accident saw the release of considerable amounts of Sr-90 that scattered globally as fall-out and made a large deposit in the countries belonging to the former Soviets Union.
The key route of entry of Sr into the human body is through the ingestion of polluted water and food substances. Contaminated air, also, has the same effects when air containing Sr particles adsorbed on dust particles is inhaled. However, the hazards associated with the ingestion of contaminated food surpass those related to the inhalation of contaminated air. Approximately twenty to thirty percent of the ingested Sr-90 is absorbed in the gut following the intake of Sr-contaminated food while the remaining fraction is excreted. Nearly all of the absorbed Sr-90 goes to the bones of the skeletal tissues. At the same time, the rest is disseminated to the peripheral tissues such as blood, pliable tissues, extracellular fluids, as well as the surface of the bones. The Sr-90 can stay permanently in these tissues. It can also be broken down and expelled in as urine or feces.
As earlier mentioned, Sr-90’s key route of entry into the human body is through its incorporation into the food chain. This occurs when grass grows on Sr-contaminated soil. Cows feed on this grass, and the strontium becomes absorbed into the cows’ gastrointestinal tract and later on passes into milk. The consumption of such milk, therefore, leads to the introduction of radioactive Sr into the human body. Strontium, a radioactive substance decays into 90Y, which is a transitory decay product. 90Y undergoes a beta decay releasing the energy of approximately 0.93 MeV that plays a vital role in the internal dose of Sr-90. Sr-90 is a structural analog of calcium, thereby making it a suitable candidate for absorption mechanisms that are similar to calcium.
Consequently, S-90 is absorbed, metabolized like calcium, and integrated into the plant and animal tissues. In lactating females, Sr can get into the milk and pose a great danger to breastfeeding children. Such children have extremely high chances of developing leukemia and cancer of the bones because of the partial substitution of calcium by strontium in their developing bones.
Urinalysis is the main method of establishing the levels of strontium in the body. However, the accuracy of the test is higher when taken immediately after the intake than when measured some time after the intake.
Different investigations carried out between the years 1961and 1988 give evidence of inadvertent exposure of workers to radiostrontium. These cases do not provide a clear quantitative account of the absorption of inhaled Sr in humans. Still, the discovery radiostrontium in feces and urine undoubtedly reveals that the inhalation of radiostrontium leads to their absorption in the body.
Animal studies, particularly in dogs, reveal that the chemical form of the inhaled strontium greatly influences the rate of absorption. Complexes with a high solubility such as SrCl2 undergo rapid clearance in the lungs. The nasopharyngeal section of the respiratory tract is responsible for the absorption of strontium. An experiment by Cuddihy and Ozog in 1973 shows that 67% of Sr administered as 85SrCl2 to the nasal tract of hamsters is absorbed within the first four hours. Another 63% is absorbed into the nasopharynx.
Recent studies reveal the fractional absorption of ingested strontium administered to healthy subjects as SrCl2 in food. This is through the quantification of strontium concentration-time profiles from the plasma of subjects who have ingested strontium and those who have had injections of strontium intravenously. This approach looks at the bioavailability of Sr in the body following different routes of administration. Another approach compares the amount of Sr ingested, and that excreted in feces. These studies show that about 20% of the ingested Sr is absorbed in the alimentary canal.
A difference in the absorption of Sr with changes in the age of the subjects is a phenomenon that is practical in animal studies (rats) but not in human studies. However, owing to the practicability of animal models to human biological processes, this suggests that there is an increase in Sr absorption during the neonatal period in humans. Giving 1.4 mg of Sr (SrCl2) as an oral dose to grown male rats and adult humans yield similar outcomes (an absorption of 19%) in a study carried out by Sips et al. in 1995.
Dermal exposure to compounds containing Sr does not give sufficient evidence of systemic toxicity. This implies that there is poor absorption of Sr from the surface of the skin. However, the integrity of the skin tissues determines the extent of absorption. An experiment by Ilyin et al. in 1975 shows that scratched skin allows more absorption than intact skin.
There is insufficient information regarding the dispersal of inhaled Sr in humans. Nevertheless, it can be assumed that inhaled strontium and ingested strontium are distributed similarly. Strontium can be transmitted across the placenta in pregnant female animal and human subjects. However, only one study demonstrates Sr transfer across the placenta following inhalation, where an intratracheal dose of 89Sr is given to rats at two weeks of gestation. There are insignificant differences in Sr absorption rates in the fetuses of control and experimental rodents.
The dispersion of orally ingested Sr mimics calcium absorption with nearly all of the ingested Sr ending up in the bones. This is evident from autopsy reports of human bone samples, which reveal that Sr absorption is half of the normal Ca absorption. Also, Sr gets into human mammary glands and can be conveyed to infants during lactation.
Dermal exposure to strontium as 85SrCl2 gets into the patella three hours after exposure. The same Sr takes about six hours to be detected in the forearm, suggesting that dermal exposure ultimately leads to absorption in the bones.