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Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
20
Absorption rate - dermal (%):
0.1

Additional information

Grouping and read across

Strontium metal completely dissolves upon contact and during the reaction with water under a strong evolution of gas and an immediate precipitation of a white crystalline solid, presumably strontium hydroxide (Sr(OH)2). The water solubility test of strontium (OECD TG 105) indicates a high dissolution from strontium metal (6.74 g/L at 20°C, determined as dissolved strontium, separated by filtration from undissolved test item and precipitates), a rapid formation of Sr2+ + 2OH- + H2 (g) and a corresponding increasing solution pH to a pH > 13. Strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015; Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, it may be assumed that systemic toxicological effects (not local) are related to the strontium ion.

 

Therefore, the assessment of the systemic toxicity of strontium is based on elemental strontium concentrations. Read-across of systemic toxicity data available for soluble strontium substances is applied since the strontium ions determine the toxicological potential of strontium.

Absorption

Oral

The fractional absorption of ingested strontium has been estimated in healthy human subjects or hospital patients who received an oral dose of strontium chloride (SrCl2) or ingested strontium in the diet. Absorption was quantified in these studies from measurements of plasma strontium concentration-time profiles for ingested and intravenously injected strontium (bioavailability), or from measurements of the difference between the amount ingested and excreted in feces (balance). Collectively, the results of these studies indicate that approximately 20% (range, 11–28%) of ingested strontium is absorbed from the gastrointestinal tract. Balance measurements can be expected to yield underestimates of absorption as a result of excretion of absorbed strontium in the feces; nevertheless, the two methods have yielded similar estimates of absorption.” (ATSDR, April 2004)

 

Dermal

Strontium is corrosive to skin (pH ≥ 11.5). In the absence of measured data on dermal absorption, current guidance suggests the assignment of either 10% or 100% default dermal absorption rates. In contrast, the currently available scientific evidence on dermal absorption of metals (predominantly based on the experience from previous EU risk assessments) yields substantially lower figures, which can be summarised briefly as follows:

Measured dermal absorption values for metals or metal compounds in studies corresponding to the most recent OECD test guidelines are typically 1 % or even less. Therefore, the use of a 10 % default absorption factor is not scientifically supported for metals. This is corroborated by conclusions from previous EU risk assessments (Ni, Cd, Zn), which have derived dermal absorption rates of 2 % or far less (but with considerable methodical deviations from existing OECD methods) fromliquidmedia.

However, considering that under industrial circumstances many applications involve handling of dry powders, substances and materials, and since dissolution is a key prerequisite for any percutaneous absorption, a factor 10 lower default absorption factor may be assigned to such “dry” scenarios where handling of the product does not entail use of aqueous or other liquid media. This approach was taken in the in the EU RA on zinc. A reasoning for this is described in detail elsewhere (Cherrie and Robertson, 1995), based on the argument that dermal uptake is dependent on the concentration of the material on the skin surface rather than its mass.

The following default dermal absorption factors for metal cations are therefore proposed (reflective of full-shift exposure, i.e. 8 hours):

From exposure to liquid/wet media: 1.0

From dry (dust) exposure: 0.1 %

This approach is consistent with the methodology proposed in HERAG guidance for metals

(HERAG fact sheet - assessment of occupational dermal exposure and dermal absorption for metals and inorganic metal compounds; EBRC Consulting GmbH / Hannover /Germany; August 2007)

Inhalation

Studies conducted in animals have shown that the rate of absorption depends on the chemical form of the inhaled strontium aerosol. Compounds of greater solubility are, in general, more rapidly cleared from the lung. For example, strontium is rapidly cleared from the lung after inhalation of SrCl2. In dogs that received a 2–22-minute nose-only exposure to an aerosol of 85SrCl2 (activity median aerodynamic diameter [AMAD] 1.4–2.7 μm, geometric standard deviation [GSD] 2.0), <1% of the initial lung burden remained in the lung 12 hours after the exposure; 37% of the body burden was distributed to the skeleton within 12 hours after the exposure, and 84% was in the skeleton 4 days after the exposure. In contrast to the relatively rapid absorption of inhaled SrCl2, after exposures to strontium in particles of fused clay, absorption is much slower. In dogs that received a nose-only exposure to 90Sr in fused montmorillonite clay particles (AMAD 2.2 μm, GSD 1.7), the average halftime of elimination of strontium from the lung was 490 days. Thus, strontium compounds of lower solubility are more slowly absorbed from the lung. Support for this also comes from studies in which the rates of absorption of various compounds of strontium were compared in rats. Rats were exposed to aerosols of 85Sr carbonate, phosphate, fluoride, oxide, or titanate (particle sizes and doses not specified). Greater than 99% of the initial lung burden of 85Sr was cleared from the lung 5 days after inhalation of the carbonate, phosphate, fluoride, or oxide, whereas 60% of the 85Sr remained in the lung after inhalation of the more insoluble strontium titanate.” (ATSDR, April 2004)

Distribution

The distribution of absorbed strontium in the human body is similar to that of calcium, with approximately 99% of the total body burden in the skeleton (ICRP 1993). The skeletal burden of stable strontium has been estimated from analyses of bone samples from human autopsies. Skeletal burden was estimated in Japanese adult males to be approximately 440 mg compared to 850 g of calcium.” (ATSDR, April 2004)

 

Elimination and Excretion

Inhalation

Whole body elimination times have been measured in dogs and rats that received inhalation exposures to SrCl2. In dogs that were exposed to aerosols of 85SrCl2 (AMAD 1.4–2.7 μm, GSD 2.0), elimination halftimes were 0.6 (59%), 9 (12%), and 300 days (29%). The rapid early phase of elimination reflects the mechanical clearance of strontium deposited in the tracheobronchial region of the respiratory tract and transfer to the gastrointestinal tract and feces, whereas the slower elimination component reflects the elimination from the skeleton. A similar pattern of elimination has been observed in rats. In rats that were exposed to tracer levels of 85Sr or a mixture of 85Sr and 90Sr aerosols (AMAD 1.8–2.8), the long-term whole-body elimination half-time, measured 5– 230 days after exposure, was 330 days.”(ATSDR, April 2004)

 

Oral

The long-term (decades) elimination of strontium has been studied in people who were exposed to strontium in the Techa River area of Russia after fission products from a plutonium production process were released in the area. Whole-body elimination half-times were estimated in a study population of 361 males and 356 females to be 28 years in males and 16 years in females (Tolstykh et al. 1997). Most of the difference in the elimination rate estimated for males and females resulted from a pronounced increase in the elimination rate in females after age 50 years. The increase most likely reflects the increase in bone resorption that tends to occur in females after menopause. Müller et al. (1966) estimated a similar value, 25 years, for the long-term elimination half-time of strontium in 56 radium dial painters. In two dial painters, long-term elimination half-times were estimated to be 9 years. Estimates of the long-term elimination half-times of strontium reflect primarily the storage and release of strontium in bone. Over shorter time periods after exposure, faster elimination rates are observed that reflect soft-tissue elimination as well as elimination from a more rapidly exchangeable pool of strontium in bone. When whole-body elimination of a tracer dose of 85Sr was measured for periods of 42–108 days in nine subjects, the mean elimination half-time was 91 days (±32, SD). In three healthy subjects that received a single oral dose of SrCl2, the estimated average whole-body elimination half-times, estimated over 13 days, were 2 (30%) and 59 days (70%). Similar short-term rates of elimination have been observed within days to a few weeks after an intravenous injection of SrCl2.

Strontium that has been absorbed from the gastrointestinal tract is excreted primarily in urine and feces. In two dial painters, rates of urinary and fecal excretion of radium approximately 10 years after the exposure were approximately 0.03 and 0.01% of the body burden per 24 hours, respectively. The urine:fecal excretion ratio of 3 that was observed in the radium dial workers is consistent with ratios of 2–6 observed several days to weeks after subjects received an intravenous injection of SrCl2. Thus, urine appears to be the major route of excretion of absorbed strontium. The observation of fecal excretion of radioactive strontium weeks to decades after an oral exposure or over shorter time periods after an intravenous exposure suggests the existence of a mechanism for transfer of absorbed strontium into gastrointestinal tract, either from the bile or directly from the plasma. Evidence for direct secretion of strontium from the plasma into the intestine is provided by studies in animals. The available information does not address the extent to which biliary excretion may also contribute to fecal excretion of strontium.”(ATSDR, April 2004)