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EC number: 236-813-4 | CAS number: 13494-80-9
- Life Cycle description
- Uses advised against
- Endpoint summary
- Appearance / physical state / colour
- Melting point / freezing point
- Boiling point
- Density
- Particle size distribution (Granulometry)
- Vapour pressure
- Partition coefficient
- Water solubility
- Solubility in organic solvents / fat solubility
- Surface tension
- Flash point
- Auto flammability
- Flammability
- Explosiveness
- Oxidising properties
- Oxidation reduction potential
- Stability in organic solvents and identity of relevant degradation products
- Storage stability and reactivity towards container material
- Stability: thermal, sunlight, metals
- pH
- Dissociation constant
- Viscosity
- Additional physico-chemical information
- Additional physico-chemical properties of nanomaterials
- Nanomaterial agglomeration / aggregation
- Nanomaterial crystalline phase
- Nanomaterial crystallite and grain size
- Nanomaterial aspect ratio / shape
- Nanomaterial specific surface area
- Nanomaterial Zeta potential
- Nanomaterial surface chemistry
- Nanomaterial dustiness
- Nanomaterial porosity
- Nanomaterial pour density
- Nanomaterial photocatalytic activity
- Nanomaterial radical formation potential
- Nanomaterial catalytic activity
- Endpoint summary
- Stability
- Biodegradation
- Bioaccumulation
- Transport and distribution
- Environmental data
- Additional information on environmental fate and behaviour
- Ecotoxicological Summary
- Aquatic toxicity
- Endpoint summary
- Short-term toxicity to fish
- Long-term toxicity to fish
- Short-term toxicity to aquatic invertebrates
- Long-term toxicity to aquatic invertebrates
- Toxicity to aquatic algae and cyanobacteria
- Toxicity to aquatic plants other than algae
- Toxicity to microorganisms
- Endocrine disrupter testing in aquatic vertebrates – in vivo
- Toxicity to other aquatic organisms
- Sediment toxicity
- Terrestrial toxicity
- Biological effects monitoring
- Biotransformation and kinetics
- Additional ecotoxological information
- Toxicological Summary
- Toxicokinetics, metabolism and distribution
- Acute Toxicity
- Irritation / corrosion
- Sensitisation
- Repeated dose toxicity
- Genetic toxicity
- Carcinogenicity
- Toxicity to reproduction
- Specific investigations
- Exposure related observations in humans
- Toxic effects on livestock and pets
- Additional toxicological data
Endpoint summary
Administrative data
Link to relevant study record(s)
- Endpoint:
- basic toxicokinetics in vivo
- Type of information:
- experimental study
- Adequacy of study:
- supporting study
- Study period:
- no information
- Reliability:
- 2 (reliable with restrictions)
- Rationale for reliability incl. deficiencies:
- study well documented, meets generally accepted scientific principles, acceptable for assessment
- Objective of study:
- metabolism
- Qualifier:
- no guideline followed
- Principles of method if other than guideline:
- - Principle: The present review describes the role of speciation in considering the metabolism of tellurium from the viewpoint of toxicometallomics.
- Method: The investigation of the metabolic pathways of the metalloids to understand their toxicological or pharmacological effects is carried out using speciation (a hyphenated technique that combines HPLC with an inductively coupled plasma-mass specrometer (ICP-MS)). - GLP compliance:
- not specified
- Specific details on test material used for the study:
- Different Te species in biological samples detected (see any other information on materials and methods below)
- Radiolabelling:
- no
- Species:
- other: Rat and human samples
- Strain:
- not specified
- Details on species / strain selection:
- no details given
- Sex:
- not specified
- Details on test animals or test system and environmental conditions:
- no details given
- Route of administration:
- not specified
- Vehicle:
- not specified
- Details on exposure:
- no details given
- Duration and frequency of treatment / exposure:
- no details given
- No. of animals per sex per dose / concentration:
- no details given
- Control animals:
- not specified
- Positive control reference chemical:
- no information
- Details on study design:
- no details given
- Details on dosing and sampling:
- no details given
- Type:
- metabolism
- Results:
- Inorganic tellurium in the form of tellurite is reduced and simply methylated in the body. Trimethyltelluronium (TMTe) was identified as major metabolite.
- Type:
- distribution
- Results:
- Rat red blood cells accumulate tellurium in the form of dimethylated tellurium
- Type:
- excretion
- Results:
- Excretion in urine takes place via trimethyltelluronium
- Details on distribution in tissues:
- Inorganic tellurium in the form of tellurite is reduced and simply methylated in the body.
The other characteristic of Te metabolism is the accumulation of Te in red blood cells (RBCs) of rats. It was reported that Te is slightly accumulated in RBCs. Speciation studies by HPLC-ICP-MS and ESI-MS-MS revealed that the chemical form of accumulated Te is the dimethylated Te species, although the precise chemical form has not yet been identified. Dimethylated Te is specifically bound to hemoglobin-like DMAs. Thus, these results suggest that the mechanisms underlying Te accumulation in RBCs may be similar to that underlying arsenic accumulation. Because tellurite is not methylated in RBCs in vitro, dimethylated Te is not formed in RBCs. The final methylation product of arsenic is DMAs, whereas that of Te is TMTe, suggesting that the metabolic intermediate in the Te methylation may be formed in organs and is accumulated in RBCs. - Details on excretion:
- Excretion in urine takes place via trimethyltelluronium
- Metabolites identified:
- yes
- Details on metabolites:
- The major urinary Te metabolite in rat, an experimental animal, was identified by HPLC-ICP-MS and ESI-MS-MS after tellurite, an inorganic Te compound, was ingested. The Te metabolite was trimethyltelluronium (TMTe). However, no peaks corresponding to Te-containing sugar, i.e., tellurosugars, were detected on HPLC-ICP-MS equipped with a gel filtration column or a cation-exchange column. These results based on speciation suggested that Te is discretely metabolized from Se; namely the enzyme(s) catalyzing the methylation of metalloids cannot distinguish Te from Se, the enzyme(s) catalyzing the transfer of the sugar moiety to Se can discriminate Te from Se. At present, there is no reasonable explanation as to why Se is excreted in two forms, i.e., selenosugar and TMSe, while TMTe is the only urinary metabolite of Te.
The final methylation product of arsenic is DMAs, whereas that of Te is TMTe, suggesting that the metabolic intermediate in the Te methylation may be formed in organs and is accumulated in RBCs. - Conclusions:
- Inorganic tellurium in the form of tellurite is reduced and simply methylated in the body. Rat red blood cells accumulate tellurium in the form of dimethylated tellurium, and tellurium is excreted into urine as trimethyltelluronium.
Proposed metabolic pathway of Te: TeO4^2- (tellurate) reduced to > TeO3^2- (tellurite) reduced to > Te(-II) (telluride) methylated to > CH3TeH (MMTe) methylated to > (CH3)2Te (DMTe) methylated to > (CH3)3Te+ (TMTe). - Executive summary:
The present review describes the role of speciation in considering the metabolism of tellurium from the viewpoint of toxicometallomics. The investigation of the metabolic pathways of the metalloids to understand their toxicological or pharmacological effects is carried out using speciation (a hyphenated technique that combines HPLC with an inductively coupled plasma-mass specrometer (ICP-MS)).
Inorganic tellurium in the form of tellurite is reduced and simply methylated in the body. Rat red blood cells accumulate tellurium in the form of dimethylated tellurium, and tellurium is excreted into urine as trimethyltelluronium.
Proposed metabolic pathway of Te: TeO4^2- (tellurate) reduced to > TeO3^2- (tellurite) reduced to > Te(-II) (telluride) methylated to > CH3TeH (MMTe) methylated to > (CH3)2Te (DMTe) methylated to > (CH3)3Te+ (TMTe).
- Endpoint:
- basic toxicokinetics in vivo
- Type of information:
- experimental study
- Adequacy of study:
- supporting study
- Reliability:
- 4 (not assignable)
- Rationale for reliability incl. deficiencies:
- other: supporting information; not a relevant standard method
- Executive summary:
The following information was considered relevant and is cited from the publication:
Toxicokinetics:
Absorption
"The mean (± SD) gastrointestinal absorption in healthy volunteers ingesting between 15 and 57 µg has been estimated as ten per cent (± 4 per cent) for elemental tellurium, 23 per cent (± 9 per cent) for tellurate and 21.5 per cent (no SD given) for tellurite (Kron et al, 1991).
Ingestion of 0.5 µg tellurium oxide produced a garlic breath odour within 75 minutes which lasted for 30 hours (Reisert, 1884).
Tellurium dusts and fumes can be absorbed via the lung. Workers exposed to tellurium concentrations up to 0.1 mg/m3 had urine tellurium concentrations of up to 0.06 mg/L (Steinberg et al, 1942).
Organometallic complexes of tellurium and soluble tellurium salts can be absorbed through the skin (Blackadder and Manderson, 1975)."
Distribution
"Tellurium is distributed widely with high concentrations particularly in kidneys, liver, bone, brain and testes (Meditext, 1997)."
Excretion
"Excretion is mainly renal although small amounts of tellurium are exhaled as dimethyl telluride which has a distinctive garlic odour which may persist for many days; Reisert (1884) reported garlic breath odour for 237 days following ingestion of 15 mg tellurium oxide.
The susceptibility to this effect varies considerably between individuals and is exacerbated by alcohol consumption (Cerwenka and Cooper, 1961).
The whole body retention time of tetravalent tellurium has been estimated as more than two months (Kron et al, 1991)."
- Endpoint:
- basic toxicokinetics in vivo
- Type of information:
- read-across from supporting substance (structural analogue or surrogate)
- Adequacy of study:
- supporting study
- Reliability:
- 4 (not assignable)
- Rationale for reliability incl. deficiencies:
- other: supporting information; not a relevant standard method
- Justification for type of information:
- see section 13.1 for read-across justification
- Reason / purpose for cross-reference:
- read-across source
- Radiolabelling:
- yes
- Remarks:
- Tellurous acid labelled with 127 mTe
- Executive summary:
The following information was considered relevant and is cited from the publication:
"Hydrocephalus has been produced in approx. 50 % of offspring of rats fed 3,300 ppm of metallic Tellurium in their diets during pregnancy. The distribution of Tellurous acid labelled with 127 mTe was studied in maternal and fetal tissues after its intravenous injection in pregnant rats fed control and metallic Tellurium diets. The labelled Tellurium freely permeated the placental barrier as well as maternal and fetal blood-brain barriers, giving the following relative distributions 4 hours after intravenous administration: Maternal tissues: kidney > liver > blood > muscle > CNS tissues > CSF; fetal tissues: blood > livery > kidney > whole brain. In addition to the induction of hydrocephalus in the fetus, the feeding of metallic Tellurium resulted in The following information was considered relevant and is cited from the publication:a significant increase in the uptake of radiotellurium by maternal brain but not by non-nervous tissues. Appreciable binding of the labelled Tellurium to plasma proteins was observed. The persistency of radioactivity in fetal and maternal tissues for 1 week after injection indicated that prolonged binding of the isotope must also occur in tissues. The possibility of a direct teratogenic action of Tellurium on the fetus was considered."
- Endpoint:
- basic toxicokinetics in vitro / ex vivo
- Type of information:
- experimental study
- Adequacy of study:
- key study
- Study period:
- 2012-09-24 to 2012-12-27
- Reliability:
- 1 (reliable without restriction)
- Rationale for reliability incl. deficiencies:
- guideline study
- Objective of study:
- bioaccessibility (or bioavailability)
- Qualifier:
- according to guideline
- Guideline:
- other: OECD Series on Testing and Assessment No. 29 (23-Jul-2001): Guidance document on transformation/dissolution of metals and metal compounds in aqueous media
- GLP compliance:
- yes (incl. QA statement)
- Radiolabelling:
- no
- Details on study design:
- The objective of this study is to assess the dissolution of chemical compounds in artificial alveolar fluid.
The test media is selected to simulate relevant human-chemical interactions (as far as practical), i.e. entering the human body by inhalation. The amount dissolved of the test item is specified by the mass concentration of the substance in the test media under the applied test conditions. The total amount dissolved will be determined by measuring the total concentrations of dissolved tellurium. - Bioaccessibility (or Bioavailability) testing results:
- Dissolved mean tellurium concentrations in alveolar fluid at a loading of 20 mg/L were measured to be:
172.8 ± 2.8 mg/L after 2 hours
209.6 ± 6 mg/L after 5 hours
197.2 ± 4.3 mg/L after 24 hours
238.4 ± 16.1 mg/L after 72 hours - Conclusions:
- The following dissolved mean tellurium concentrations were measured in artificial alveolar fluid at a loading of 20 g/L:
2 hours 172.8 ± 2.8 mg/L
5 hours 209.6 ± 6 mg/L
24 hours 197.2 ± 4.3 mg/L
72 hours 238.4 ± 16.1 mg/L - Executive summary:
A study was performed to assess the dissolution of tellurium powder in artificial alveolar fluid. The test media is selected to simulate relevant human-chemical interactions (as far as practical), i.e. entering the human body by inhalation.
The Test was performed on the basis of the guidance for OECD-Series on testing and assessment Number 29 and on Stopford et al., 2004.
Test conditions: Artificial alveolar fluid,one single loading of the test substance,measurements of dissolved tellurium after 2, 5, 24 and 72 hours of agitation at 37°C.
The test was performed in duplicate vessels with one single loading of the test substance, i.e. 20 g/L (10.00 g / 500 mL in vessel 1 and 2, respectively). The vessels were agitated in an incubation cabinet at 100 rpm at 37 ± 2 °C. During the test an increase of pH was observed, values were in a range between 8.5 and 9.3.
Solved tellurium was quantified by ICP-OES.
The following dissolved mean tellurium concentrations were measured in artificial alveolar fluid:
2 hours 172.8 ± 2.8 mg/L
5 hours 209.6 ± 6 mg/L
24 hours 197.2 ± 4.3 mg/L
72 hours 238.4 ± 16.1 mg/L
The maximum percentage after 72 hours is calculated to be 1.2 % dissolved tellurium in relation to loading.
- Endpoint:
- basic toxicokinetics in vitro / ex vivo
- Type of information:
- experimental study
- Adequacy of study:
- key study
- Study period:
- 2012-10-25 to 2012-12-13
- Reliability:
- 1 (reliable without restriction)
- Rationale for reliability incl. deficiencies:
- guideline study
- Objective of study:
- bioaccessibility (or bioavailability)
- Qualifier:
- according to guideline
- Guideline:
- other: OECD Series on Testing and Assessment No. 29 (23-Jul-2001): Guidance document on transformation/dissolution of metals and metal compounds in aqueous media
- GLP compliance:
- yes (incl. QA statement)
- Radiolabelling:
- no
- Details on study design:
- The objective of this study is to assess the dissolution of chemical compounds in artificial gastrointestinal fluids.
The test media were selected to simulate relevant human-chemical interactions (as far as practical), i.e. entering the human body by ingestion. The amount dissolved of the test item is specified by mass concentration of the substance in the test media under the applied test conditions. The total amount dissolved will be determined by measuring the total concentrations of dissolved tellurium. - Conclusions:
- The dissolved mean tellurium concentration of 56.68 ± 1.46 mg Te/L was measured in gastrointestinal fluids at a loading rate of 2 g/L.
The bioaccessibility in gastrointestinal fluids is calculated to be 0.65 ± 0.02 %, as percentage dissolved tellurium in relation to loading. - Executive summary:
A study was performed to assess the dissolution of Tellurium in artificial gastrointestinal fluids. The test media is selected to simulate relevant human-chemical interactions (as far as practical), i.e. entering the human body by ingestion.
The test was on the basis of DIN 19738 and OECD Series No. 29.
2 g of the test substance were weighted in a vessel and 30 mL of artificial salvia was added. After 0.5 hours stirring, 70 mL artificial gastric juice and 10 g whole milk powder was added. The pH was corrected to pH 2.0 with HCL. After another 2 hours stirring at pH 2.0, 100 mL of artificial intestinal fluid was added. The pH was corrected to pH 7.5 with NaHCO3.
After further 6.0 hours stirring at pH 7.5 the mixture was sampled, centrifuged and filtered. Dissolved tellurium was quantified by ICP-OES.
The test was performed in duplicate vessels with one single loading of the test substance, i.e. 2 g/L (2001 mg and 2000 mg in vessel A and B, respectively). The vessels were agitated in an incubation cabinet at 200 rpm at 37 ± 2 °C. During the test pH values were monitored and adjusted if needed according to DIN 19738.
The dissolved mean tellurium concentration of 56.68 ± 1.46 mg Te/L was measured in gastrointestinal fluids at a loading rate of 2 g/L.
The bioaccessibility in gastrointestinal fluids is calculated to be 0.65 ± 0.02 %, as percentage dissolved tellurium in relation to loading.
Referenceopen allclose all
Animal experiments were conducted using extremely high doses of Te. The experiments showed Te toxicosis in the animals, and the animals gave off a garlic-like odor that originated from dimethyltelluride (DMTe) formed in the body.
Although Te is a non-essential metalloid, it is expected to be metabolized in the same pathway as that of an excess amount of Se, an essential metalloid. Speciation has contributed to depicting the metabolic chart of Te and highlighting the mechanisms to discriminate the metabolic pathways of Te and Se in animals.
Dissolved Te concentrations in test vessels and method blanks
sample | total Te | mean Te | mean Te | LOD | LOQ |
2h 1a | 170.8 | 172.8 ± 2.8 | 177.2 ± 5.4 | 0.11 | 0.38 |
2h 1b | 174.7 | ||||
2h 2a | 180.4 | 181.6 ± 1.6 | |||
2h 2b | 182.7 | ||||
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|
5h 1a | 202.7 | 204.6 ± 2.7 | 209.6 ± 6.0 | 0.11 | 0.38 |
5h 1b | 206.5 | ||||
5h 2a | 213.3 | 214.6 ± 1.8 | |||
5h 2b | 215.8 | ||||
|
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|
|
|
|
24h 1a | 192.7 | 193.5 ± 1.1 | 197.2 ± 4.3 | 0.65 | 2.16 |
24h 1b | 194.2 | ||||
24h 2a | 200.7 | 200.9 ± 0.2 | |||
24h 2b | 201.0 | ||||
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|
72h 1a | 226.8 | 225.3 ± 2.1 | 238.4 ± 16.1 | 0.65 | 2.16 |
72h 1b | 223.8 | ||||
72h 2a | 245.0 | 251.5 ± 9.2 | |||
72h 2b | 258.0 | ||||
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|
2h BW 1a | < LOD |
|
| 0.11 | 0.38 |
2h BW 1b | < LOD | ||||
2h BW 2a | < LOD |
| |||
2h BW 2b | < LOD | ||||
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|
5h BW 1a | < LOD |
|
| 0.11 | 0.38 |
5h BW 1b | < LOD | ||||
5h BW 2a | < LOD |
| |||
5h BW 2b | < LOD | ||||
|
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|
|
|
|
24h BW 1a | < LOD |
|
| 0.65 | 2.16 |
24h BW 1b | < LOD | ||||
24h BW 2a | < LOD |
| |||
24h BW 2b | < LOD | ||||
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|
72h BW 1a | < LOD |
|
| 0.65 | 2.16 |
72h BW 1b | < LOD | ||||
72h BW 2a | < LOD |
| |||
72h BW 2b | < LOD |
LODs (limit of detection) and LOQs (limit of quantification) depend on calibration range. A calibration with an optimal concentration range for samples was performed before each measurement series. Therefore theses limits vary. Three measurement series were performed to quantify Te concentrations.
In the vessels the pHs were not stable over the course of the test. At the start of the study a pH of 8.5 and 8.6 were measured in the two test vessles. After 72 hours the pH value was approx. 9.3.
Dissolved Te concentrations in test vessels and method blanks at test end
sample | dilution | total Te | mean Te | mean Te | LOD | LOQ |
1a | 100 | 55.40 | 56.48 ± 1.52 | 56.68 ± 1.46 | 0.007 | 0.021 |
1b | 100 | 57.55 | ||||
2a | 100 | 58.28 | 56.89 ± 1.97 | |||
2b | 100 | 55.50 | ||||
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|
BW 1a | 1 | 0.027 | 0.027 | 0.074 ± 0.042 | 0.019 | 0.057 |
BW 1b | 1 | < LOQ(0.016) | ||||
BW 2a | 1 | 0.108 | 0.098 ± 0.013 | |||
BW 2b | 1 | 0.089 |
Calculation of the mobilized amount and the resorption availability (bioaccessibility) following DIN 19738:
The mobilized amount of a compound is calculated by
Wi,mob= ρi*V/mE
Wi,mob: mobilized amount of compound i [mg/g]
ρi measured conc. of compound i [mg/L]
V total volume [L]
mE inweight of sample [g]
Wi,mob= 56.68 ± 1.46 mg Te /L * 0.23 L / 2 g Te = 6.518 ± 0.168 mg/g
The resorption availability (bioaccessibility) is calculated by:
Ri,=Wi,mob100 % /Wi,fest
Ri resorption availability of compound i [%]
Wi,mob as described above
mE inweight of sample [g]
Wi,fest total amount of the solid sample [mg/g]
=> 1000 mg pure tellurium
Ri= 6.518 ± 0.168 mg/g *100 % / 1000 mg/g = 0.65 ± 0.02 %
This value equals the percentage of dissolved tellurium.
Description of key information
State of the art toxicokinetic studies are not available.
Key value for chemical safety assessment
- Bioaccumulation potential:
- no bioaccumulation potential
Additional information
- Determination of relative bioavailabilities (see Table 2) allows to assume equitoxic potencies of elemental Tellurium versus Tellurium dioxide. This way, the read-across for tellurium based on Tellurium dioxide follows a worst-case approach.
- Because of utmost practical importance the dermal absorption of elemental Tellurium can be fixed due to a relatively robust estimation, i.e. 1 % of dermally applied dose.
- Once absorbed all inorganic Tellurium seems to be converted to dimethylated (and also trimethylated) Telluride/Telluronium because concentrations of “free” Tellurium are extremely low and also only the di- and trimethylated Tellurium moieties have been detected.
- This behavior is very much similar to the metabolic fate of Selenium with which it shares largely very similar physio-chemical properties.
- Due to this unique metabolic behavior elemental Tellurium and Tellurium dioxide seem to belong into the same class of toxicants with regard to adverse effects which do not differ by mode of action but only by their potency due to differences in bioavailability.
- Tellurium crosses the placenta barrier and fetuses may be therefore exposed to it.
Toxicokinetic profile of elemental Tellurium
No toxicokinetic studies are available for the source substance Tellurium. Publications with relevant information are used to evaluate the toxicokinetic profile of the target and the source substance, but they often do not distinguish between the different tellurium compounds. However, since Tellurium dioxide and Tellurium share the same metabolic pathway, the use of these data is considered acceptable.
Although daily uptake of Tellurium can be estimated with approximately 100 µg per person [37], its physiological role, if any, cannot be conclusively explained at the time being.
With respect to its chemical properties Tellurium is comparable to Selenium, but while Selenium is commonly accepted to be an essential element (physiologically needed micronutrient for living (mammalian) cells), Tellurium does not seem to be essential for humans.
It needs to be acknowledged that essentiality may have a specific influence on toxicokinetic behaviour because it is known that for essential elements specialized systems (like transport proteins) exist to allow for stable homeostatic conditions.
Absorption and bioavailability
All available data hint for a rather poor absorption of elemental Tellurium after oral uptake, i.e. systemic absorption was measured in human studies with elemental Tellurium, but also with tetra- or hexavalent Tellurium salts and is stated with 10 to 25 % of the applied dose [3].
In rats and rabbits absorption after oral exposure was determined with 10 to 40 % of applied dose.
The dermal absorption after application onto the skin is unknown [37] as is the case for human respiratory absorption [5].
For the determination of equitoxic potency of various Tellurium moieties (in this case elemental Tellurium and Tellurium dioxide) and hence definition of the NOAEL and further on derivation of DNELs or hazard classification the relative bioavailability is the most important figure.
For this purpose in vitro studies with elemental Tellurium (Te) and Tellurium dioxide (TeO2) were conducted [18, 19, 22, 23] to assess their behavior in a physiological environment with regard to any toxicokinetic differences.
Therefore, bioavailability simulating inhalation and oral uptake was measured by the substance`s solubility in artificial alveolar fluid and in artificial saliva and gastrointestinal fluid respectively.
The results indicate that Tellurium dioxide is of approximately three times higher solubility than elemental Tellurium (see following table for measured solubility data).
| Tellurium [mg/L] | Tellurium dioxide [mg/L] |
Mean solubility in artificial alveolar fluid after 72 hours | 238.4 ± 16.1
| 799.1 ± 7.3 (Te) 999.4 ± 9.1 (Tellurium dioxide) |
Factor | 3.35 (based on Te) | |
Mean solubility in artificial gastrointestinal fluid | 56.68 ± 1.46
| 156.7 ± 13.2 (Te) 196.03 ± 16.5 (Tellurium dioxide) |
Factor | 2.76 (based on Te) |
It is a well-accepted fact, that such in vitro studies with inorganic substances may underestimate the in vivo bioavailability, because living cells do possess active transport systems to control for homeostatic reasons the uptake of for example essential elements. Nevertheless the in vitro data do allow for a comparative insight into the bioavailability of different redox species of an element and are therefore suitable for comparing study results with different Tellurium compounds in particular since the element Tellurium is not thought to be essential.
Despite total lack of information, the relative absorption after dermal exposure compared with oral exposure may be estimated by using data from other metals:
The Health Risk Assessment Guidance for Metals (HERAG) proposes for dermal absorption after exposure to dust or other dry metal compounds a default value of 0.1 % [11].
From this it follows that the dermal systemic bioavailability of elemental Tellurium may be estimated to be 10 % of the oral bioavailability (based on 10 % oral uptake) which results in a systemic bioavailability of 1 % of the external dermal dose. This figure clearly exceeds the above mentioned 0.1 % as typical for other metals and can therefore be considered a very conservative approach.
The figure of 1 % of dermal uptake of elemental Tellurium will be used further in the exposure considerations.
Distribution
The majority (90 %) of Tellurium in the blood stream enters erythrocytes; the remainder is bound to plasma proteins [5]. Tellurium may therefore accumulate in red blood cells in the form of dimethylated Tellurium.
Tellurium can cross the placenta and blood-brain barrier as well as the fetal blood-brain barrier [1, 5].
Concentrations of Tellurium were < 5 µg/L in blood, < 1 µg/L in saliva and < 0.5 µg/L in urine; the body “load” for an adult person is 600 mg Tellurium.
Highest tissue concentrations have been found in the kidneys [5], but also in liver, bone, brain and testes [40].
The half-life time of Tellurium in humans is estimated to be 3 weeks [3].
Metabolism
Ingested Tellurium is transformed, methylated and then emitted into the blood stream where accumulation into red blood cells as dimethylated Tellurium takes place [28].
Trimethylated Tellurium was detected in blood serum and in urine as a main metabolite, but not in red blood cells.
Based on speciation studies it was found that all inorganic Tellurium is first reduced to Telluride (Te2-) and thereafter methylated [28, 40].
Since concentrations of “free” Tellurium seem to be extremely low in various media it is obvious that absorbed Tellurium is taken up and converted to the Telluride moiety in cells from which it is released into the blood stream as dimethylated Tellurium for further distribution and predominantly renal excretion.
Excretion
Loss of systemically available Tellurium for humans is approximately 80 % in urine, approximately 16 % in feces and approximately 2 % in exhaled air [3] as dimethylated Tellurium resulting in a typical garlic odour [40].
Excretion in urine seems to take place via Trimethyltelluronium [29]. But the excretion pattern seems also to depend on the chemical form and mode of administration because in rats the main route of excretion is via feces (60 to 80 %) [5].
In conclusion above short description of the toxicokinetic behavior allows for the following:
for further information and references please consult the "justification for read-across" in section 13
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