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Description of key information

The dusts of synthetic amorphous silicas are considered as acutely non-toxic by all routes of exposure.

The acute inhalation of dust may cause discomfort and stress as well as signs of local irritation to nasal, bronchiolar and ocular mucous membranes.

The innocuous nature of synthetic amorphous magnesium silicate is established based on study results and by read across to data for the structurally related synthetic amorphous silica.

The median lethal doses by oral dermal or inhalation routes all exceed the regulatory threshold or limit dose levels.

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
LD50
Value:
5 000 mg/kg bw

Acute toxicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
LC50
Value:
20 000 mg/m³

Acute toxicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
LD50
Value:
2 000 mg/kg bw

Additional information

The acute oral and dermal toxicity studies completed with synthetic amorphous magnesium silicate in rats and rabbits respectively showed no signs of adverse reaction to exposure at the limit dose levels in each case. Magnesium silicate was shown to be innocuous or toxicologically inert in acute investigations. Acute oral administration of Magnesol (trade name for a synthetic amorphous magnesium silicate) to rats, resulted in no adverse toxic effects at 5000 mg/kg bw.

Magnesium silicate was administered in a second oral toxicity study as a 25% suspension in water and elicited no toxic effects at 5000 mg/kg bw.

Results are presented for read across to three studies with forms of silicon dioxide. In all three studies using silicon dioxide, the median lethal dose was calculated to be greater than 5000 mg/kg bw in rats.

Acute dermal toxicity of magnesium silicate was investigated in rabbits dosed at 2000 mg/kg bw. No clinical signs of reaction to dermal application were observed.

The dermal toxicity of 4 products containing silicon dioxide were investigated and results are presented for read across purposes. Local irritation was evident in some cases but there was no evidence of systemic toxicity at the limit dose of 5000 mg/kg bw. The dermal median lethal dose for silicon dioxide was also confirmed to exceed 2000 mg/kg bw.

All acute inhalation studies performed with dry dust were hampered by the technical problem that the high adhesive forces caused rapid precipitation onto equipment walls. This resulted in failure to achieve the highest test concentration of 5 mg/L as recommended in the OECD and EC-Guideline. Therefore, the maximum attainable chamber concentrations were distinctly lower than envisaged for forms of synthetic amorphous silicas.  

Studies with Magnesol, a synthetic amorphous magnesium silicate, are presented in the dossier. Inhalation of Magnesol at a nominal concentration of at least 20 mg/L of air in the test atmosphere produced no mortality in any of the ten test subjects indicating that the LC50 of the test article is greater than a minimum nominal concentration of 20 mg/L of air for a one hour period.

Information indicates five grades of Magnesol (a synthetic amorphous magnesium silicate).  Four of the samples had circa 10% of the particles within the respirable range. The respirable range is generally <10 micron. For the PQ grades of magnesium silicate, the respirable range is approximately 10% of the particles and therefore is comparable with the Magnesol. It can therefore be concluded that the PQ grades of magnesium silicates included in Grades 1, 2a, 2b and 3 are also not toxic by inhalation.

Read across information is also presented for synthetic amorphous silicon dioxide due to the similarity in . Rats exposed by whole body exposure to a nominal concentration of 58.8 mg/L, analytical concentration of 2.08 mg/L air, the maximum practical atmosphere concentration, resulted in no deaths and no evidence of systemic toxicity. In a second study, synthetic amorphous

silicon dioxide was administered by nose only exposure at the maximum achievable test concentration of 0.69 mg/L. (Due to substance-inherent properties resulting in sedimentation and adsorption to the equipment, the technical maximum attainable aerosol concentration in the chamber ranged  from 650 to 725 mg/m3, while the nominal concentration was 36.7 g/m3). About 65 % of the aerosol comprised particles with an aerodynamic diameter of <6 µm (part of respirable fraction).

None of the available data for synthetic amorphous magnesium silicate or read-across analogues, such as synthetic amorphous silica, gave any indications of adverse toxic responses following acute exposure via oral, dermal or inhalation routes.

Justification for read-across is therefore warranted given the similarities in toxicity profile and physico-chemical properties for the synthetic amorphous forms of silicon dioxide and magnesium silicate.

Justification for classification or non-classification

No classification for acute human health hazards is required: synthetic amorphous magnesium silicate or read-across analogues, such as synthetic amorphous silicon dioxide are non-toxic by all routes of exposure. Aerosol levels that are technically achievable under experimental conditions are acutely non-toxic and clearly sub-lethal.