Silicic acid, calcium salt

Endpoint summary

Additional information

1. Oral exposure

In a 2-year feeding study on rats receiving various high levels of calcium silicate, terminal (24-months), Si tissue-levels were significantly increased in kidney and liver at dietary levels of 5 % [= NOEL] and higher, while at 1 % a positive trend was only distinguishable in the kidneys of male rats. No interim measurements had been performed, neither are there data about excretion [Columbia 1956].

Intestinal absorption of structure-analoguous SAS* appears to be insignificant as compared to the high doses applied in animals and humans: In the acute human oral ingestion test, the small apparent increases in the urine output of human volunteers – if any at all - were remarkably low as compared with the high dose of 2500 mg SiO₂ applied (0.5 % within 4 days p.a.) [Lang 1966: see Degussa 1966].

2. Inhalation exposure

No experimental data about particle distribution and inhalation of calcium silicate is available. Based on structure-analogy, it is concluded that - under normal handling and intended use - particulate calcium silicate in air is hardly respirable and that inhaled particle will efficiently be cleared from the lung. Experimental information about SAS is summarised below:

2.1 Particle size distribution of the aerosols used in inhalation studies as compared with dusts under technical application:

The respirable fractions of the experimental SAS aerosols that consisted of particles with aerodynamic diameters of =<5 µm represented >=50 wt%, those consisting of particles with aerodynamic diameters of =<10 µm represented >80 wt%.

In the commercial products, that fraction of particles in the whole-size range of air-borne particles according to EN/DIN 481 that is potentially able to reach the thoracic and alveolar site is below 1 vol% (= wt%) (see: 4. Physical chemical properties_Particle characteristics; Chapter 4.5).

2.2. Deposition in and elimination from lung tissue / NOAEC

Analytical data on the kinetics of silica deposition in the lung of experimental animals during and after prolonged exposure to respirable silica are largely consistent.

At respirable exposure concentrations of up to 30 mg/m³ for prolonged time, the initial uptake phase is characterized by relatively high deposition followed by a phase of low increase; there is no evidence of Si accumulation in the lung tissue.

Synthetic amorphous silicas are rapidly eliminated from the lung tissue: after an exposure period of 13 weeks (rat, 35 mg/m³), the elimination half-life from lung tissue can be estimated to be equal or less than 7 weeks [Degussa 1987].

On the other hand, crystalline silica - although inhaled at a more than 10fold lower concentration - persisted in the lung with no substantial decrease post-exposure [Johnston et al. 2000].

At the experimental NOAECof 1 mg SAS/m³(rat, 13 wk), 6/20 rats, about 30 %, showed a slight, but measurable increase in Si in the lung tissue 1 week post-exposure [see Degussa 1987]. With the elimination characteristics in mind, no accumulation during chronic exposure is supposed to occur.

Note: The efficient clearance may also be explained by the limited, but significant water solubility of amorphous silicas [see 4.8: READACROSS_Vogelsberger 2003/2004_SAS_solub,37°C,RL1]. The clearance of SAS by dissolution is estimated to be at least 20 times faster than particulate clearance by macrophages [see ECETOC 2006, Chapter 7, p. 79]. This can be assumed to apply for calcium silicate, too, which also shows significant but limited solubility in water.

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Reference:

ECETOC (2006): Synthetic Amorphous Silica (CAS No. 7631-86-9). JACC Report No. 51, European Centre for Ecotoxicology and Toxicology of Chemicals, Brussels, September 2006