Ashes (residues), cenospheres

Endpoint summary

Additional information

There were no studies available in which the toxicokinetic properties of ashes (residues), cenospheres were investigated. The toxicokinetic behaviour was assessed taking into account the available information on physicochemical and toxicological characteristics according to Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2008).

Ashes (residues), cenospheres are a glass-ceramic mixture of metal and metalloid oxides in variable proportions. Based on mass percent ranges, the major constituents of ashes (residues), cenospheres are: Silicon dioxide (SiO₂), aluminium oxide (Al₂O₃) and iron oxide (Fe₂O₃). Therefore, available information on these compounds was also taken into account for assessment of the toxicokinetic behaviour.

 

Physicochemical properties relevant for assessment of toxicokinetics

- Physical state and granulometry:

Ashes (residues), cenospheres are solid hollow microspheres mostly composed of glassy aluminosilicate. The available data on particle size distribution indicate that only < 1.5% of the particles are contained in the respirable (alveolar) fraction of particles with MMAD < 5 µm. Therefore, ashes (residues), cenospheres have in general a low exposure potential to the alveolar region of the lung and most of the inhaled material would presumably intercept in the naso-pharyngeal region.

- Water solubility:

The glass-ceramic (aluminosilicate) component as well as the individual main constituents of ashes (residues), cenospheres (SiO₂, Al₂O₃, Fe₂O₃) are not soluble in water; the water soluble fraction (0.17-0.23% (w/w) of the total mass) contains mostly calcium, potassium, sodium, magnesium compounds. Thus, the available data on water solubility anticipates a negligible level of systemic absorption of ashes (residues), cenospheres as a whole via any route of exposure.

 

Toxicological data relevant for assessment of toxicokinetics

There are no substance specific toxicological data available for ashes (residues), cenospheres.

Ashes (residues), cenospheres and ashes (residues), coal share a common production process as substances derived from coal combustion. Ashes (residues), cenospheres represent a fraction of ashes (residues), coal separated by physical means. Both substances exhibit similarities in physicochemical properties and chemical composition. The main differences consist in a much lower content of water soluble matter and the particle size distribution of ashes (residues), cenospheres (MMAD > 70 µm; < 1.5% of respirable particles with MMAD < 5 µm).

Studies available for ashes (residues), coal are therefore taken into account by read-across following an analogue approach, the results of these studies being considered a worst case for ashes (residues), cenospheres.

The results of these studies can be found in the corresponding sections: Acute toxicity, Irritation, Sensitisation, Repeated dose toxicity and Mutagenicity. In summary, ashes (residues), coal are not toxic by the oral route. Short- and long-term inhalation exposure to high airborne concentrations may lead to inflammatory responses (local effects) in the lung comparable to those induced by nuisance dust. Furthermore, ashes (residues), coal are not skin and eye irritating, skin sensitising or genotoxic. Based on the analogue approach mentioned above, conclusions drawn on the toxicological properties of ashes (residues), coal are considered to apply to ashes (residues), cenospheres.

The available toxicological data support the notion that ashes (residues), cenospheres are not significantly absorbed and distributed in the organism as described in more detail below.

Assessment of toxicokinetics 

Absorption and distribution

Ashes (residues), cenospheres mainly consist of water insoluble components. Silicon dioxide and aluminium oxide are slightly soluble in body fluids (e.g. in the stomach) leading to the formation of silicic acid and aluminium chloride, respectively, which both show low levels of absorption and rapid clearance via the kidneys (IARC, 1997; McEvoy, 1990). Moreover, there is evidence suggesting that the presence of silicon-containing compounds may reduce aluminium absorption and facilitate aluminium excretion (Krewski et al., 2007). Therefore, a low level of absorption of ashes (residues), cenospheres via ingestion or after dermal contact and no relevant systemic bioavailability are expected. This is supported by studies on the acute oral toxicity as well as on the skin and eye irritation potential of ashes (residues), coal in all of which no mortalities occurred and no signs of systemic toxicity were observed. Furthermore, in a 3-year feeding study in cattle, the chemical analyses of milk, blood, urine and faeces indicated that no systemic absorption occurred after oral administration of 1850 mg/kg/day of fly ash (Herrmann, 1955).

The most relevant route of exposure for ashes (residues) is by inhalation, and in fact, toxicological concern may arise from the accumulation potential of deposited particles in the lung after prolonged and/or repeated exposure rather than from absorption via the respiratory tract. In turn, particle deposition largely depends on particle size (aerodynamic diameter). In humans, inhalation of “respirable” particles involves exposure to the particles in a mineral dust that are able to penetrate into the alveolar spaces of the lungs. It is generally considered that respirable particles have an aerodynamic diameter of <3 -4 µm, while most particles larger than 5μm may be deposited in the tracheobronchial airways and thus not reach the alveolar region (IARC, 1997). Particles deposited in the respiratory bronchioles and proximal alveoli are cleared more slowly.

The information available on SiO₂, Al₂O₃and Fe₂O₃ indicate that inhaled particles of these compounds deposit along the respiratory tract according to their aerodynamic diameter. Thus, small particles (< 5 µm) can reach the alveolar region. Following deposition on the surface of the lung, there is either rapid mucociliary clearance if deposition is in the upper airways or phagocytosis by alveolar macrophages and slower clearance if deposition is in the lung periphery. Clearance by mucociliary mechanisms is generally considered to be efficient; clearance from the lung periphery is slow and incomplete and some of the particles may be carried by macrophages into the pulmonary interstitium and lymphoid tissues (ECHA, 2008; Friberg et al., 1986; IARC, 1972, 1997).

Based on the available data on particle size distribution, most of the ashes (residues), cenospheres particles (ca. 98.5%) are > 5 µm and hence not able to reach the alveolar region. Inhaled particles are thus expected to deposit mainly in the upper airways and to be removed from the lung by the above mentioned clearance mechanisms.

 

Metabolism

As indicated above, ashes (residues), cenospheres as a whole and their main components are unlikely to be absorbed and systemically distributed to a relevant extent. Moreover, none of these compounds is prone to undergo metabolic transformation.

 

Excretion

From the available data on absorption and distribution described above it follows that ashes (residues) will mainly be excreted in the faeces after oral exposure. Inhaled particles cleared from the lung as a result of mucocilliary mechanisms will likely be swallowed and excreted via the gastrointestinal tract as well. Soluble material leaching from the primary particles and eventually being absorbed will most likely be excreted in the urine, as described for SiO₂and Al₂O₃(Friberg et al., 1986; IARC 1997; Krewski et al., 2007).

 

References (not in IUCLID):

 

ECHA (2008)Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance.

Friberg, L., Nordberg, G.F., Kessler, E. and Vouk, V.B. (eds.) (1986) Handbook of the Toxicology of Metals. 2nd ed. Vols I, II.:: Elsevier Science Publishers B.V.

IARC (1972)Monographs on the Evaluation of Carcinogenic Risks to Humans. Volume 1Some Inorganic Substances, Chlorinated Hydrocarbons, Aromatic Amines,N-Nitroso Compounds and Natural Products.: World Health Organization, International Agency for Research on Cancer.

IARC (1997) Monographs on the Evaluation of the Carcinogenic Risks to Humans. Volume 68 Silica, Some Silicates, Coal Dust and para-Aramid Fibrils.: World Health Organization, International Agency for Research on Cancer.

Krewski et al. (2007). Human health risk assessment for aluminium, aluminium oxide, and aluminium hydroxide.J Toxicol Environ Health B Crit Rev. 2007;10 Suppl 1:1-269.

McEvoy, G.K. (ed.) (1990) AHFS Drug Information 90.,: American Society of Hospital Pharmacists, Inc.