Silicic acid, calcium salt

Endpoint summary

Description of key information

In a 2-y feeding study, calcium silicate produced no significant adverse effects. 
Inhalation exposure to calcium silicate: Target organ is the lung. Based on results from synthetic amorphous silica (SAS), the parent substance, low exposure concentration of respirable dust particles provokes an inflammation response, which is reversible. No histopathological manifestations following exposure of 13 weeks to 1.3 mg/m3. An experimental NOAEC (acute to subchronic) of 1 mg/m3 (respirable) was established for SAS aerosol.    

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

3 000 mg/kg bw/day

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

1 mg/m³

Additional information

Summary/overview on target-organ effects

Dermal and oral exposure are not relevant toxicological issues, based on the inherent substance properties and experimental evidence. In a 2-y feeding study, calcium silicate produced no significant adverse effects in rats, the NOEL is considered to be approx. 3000 mg/kg bw [Columbia 1956].

No robust experimental data about inhalation toxicity is available for the target compound, calcium silicate (CS). The only long-term inhalation study in rats and guinea pigs provides evidence that no definite differences in effects exist between SAS and CS [Columbia 1966]. Yet, from this comparative study conducted only at one high exposure level of CS and SAS, a NOAEL cannot be derived.

Therefore, the analogy approach will be adopted, using data obtained with SAS:

The inhalation of respirable particles of synthetic amorphous silica (SAS) produces a time- and dose-related inflammation response of the lung tissue in animal studies. Progressive events following excess exposure are characterised as "interstitial fibrosis/early nodular fibrosis/incipient fibrosis”. However, a progression process of any lesion has not been observed like that seen after quartz exposure, i.e. all observations suggest reversibility. There are no signs of classical nodular silicosis or a lymphatic-type pneumoconiosis. On the other hand, crystalline silica produces persistent lung inflammation even at much lower exposure levels [Johnston et al. 2000].

Thirteen-weeks inhalation exposure to an average concentration of 1.3 mg/m³ of a pyrogenic SAS resulted in mild reversible pro-inflammatory cell proliferation rather than a pathologically relevant tissue change. Given the low-grade severity of this common lung-tissue response, 1 mg/m³ can be established as NOAEC and LOEC (sub-chronic, 13 weeks). The LOAEC was 5.9 mg/m³, the mid concentration, which produced clear signs of histopathological adverse effects (stimulation of collagen production, increase in lung weight, incipient interstitial fibrosis in the lung, slight focal atrophy in the olfactory epithelium). All these effects were reversible following discontinuation of exposure [Degussa 1987(a,b)].

No lung-tissue effects were observed following exposure of 5 days to 1 mg/m³ of the same silica [NOEC (short-term)]; the LOAEC (5 d) was 5.4 mg/m³ [ASASP 2003 a,b,c]. Measurements of the particle-size distributions under experimental conditions revealed, the exposure aerosol was practically fully respirable.

Based on the pathological relevance of effects after inhalation, 1 mg/m³(respirable) could be established as NOEC(short-term) and NOAEC(sub-chronic). This appears to be justified also in light of the fact that the sub-chronic study was conducted with a pyrogenic synthetic amorphous silica (SAS) which appears to induce more marked tissue responses than the precipitated SAS type.

The low exposure level did not provide any evidence of an accumulation of adverse effects over time. Therefore, the NOAEC of SAS of 1 mg/m³ is considered to apply also for prolonged/chronic exposure.

Based on the structural similarity as well as properties between synthetic amorphous silica (SAS) and synthetic amorphous silicates, the NOAEC of SAS is also adopted for calcium silicate.

Repeated dose toxicity: inhalation - systemic effects (target organ) respiratory: lung