Silicic acid, calcium salt

Endpoint summary

Additional information

1. General conclusion

Due to its physico-chemical properties, inhalation of dust of synthetic amorphous calcium silicate (CS) may be of importance for human health. CS is closely related to silica, as SiO₂ forms the parent material of silicates.

Therefore, it is assumed that the physico-chemical properties of synthetic amorphous CS are largely represented by those of synthetic amorphous silica (SAS). Hence, it is reasonable to draw conclusions about key issues arising from potential inhalation by read-across and (quantitative) structure-analogy relationship (QSAR). This also implies that the particle size and morphology rather than the particle composition of the synthetic amorphous calcium silicate are assumed to be the determinant of tissue responses in the lung.

Note: In accordance with the OECD SIDS¹synthetic amorphous silicate and silica are classified as a class of substances with low priority for further work, because the exposure to humans to respirable dust is anticipated to be low.

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Reference:

¹UNEP Publications 2004: OECD SIDS, Synthetic amorphous silica and silicates, SIAM 19 [http://www.chem.unep.ch/irptc/sids/OECDSIDS/Silicates.pdf]

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2. Particle properties and relevance for specific hazards

Due to limited data about particle properties of calcium silicate itself, the data of SAS as structure-analogous substance are used.

Toxicological inhalation studies were performed by using a homogeneous atmosphere of silica dust with MMAD of approx. 3 µm which - compared to particle size distributions of bulk material - only represents 1% of the material. As a consequence the inflammatory response in the lung will be determined by particle size and morphology rather than by particle composition. Under comparable testing conditions, we expect that synthetic amorphous calcium silicate shows the same behaviour.

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 4.5: Stintz 2003). Free settling SAS tends to form aggregates and agglomerates in a size range of > 100 μm which are not respirable or even not inhalable.

This means that in experimental studies due to the high shear forces applied, the toxicologically relevant particle fraction is assumed to be at least at a factor of 50 higher than under workplace conditions.

A comparative overview of measured particle-size distributions obtained under either condition is presented in the tables below:

Particle size profile under normal use conditions (low shear force)

Test material	SAS^*type	MMAD  [µm]	Respirable fraction [wt%]	Chapter IUC5 / Doc-name	Reference
Cab-O-Sil M5	amorphous, pyrogenic,  CAS-No. 112945-52-5	615	=< 1.0 (=< 10 µm) (low energy dispenser, LD)	4.5 / READACROSS_Stintz 2003(1)_CabM5_Particle_LaserD	Stintz 2003
Syloid 74	amorphous, silica gel,  CAS-No. 112926-00-8	480	< 0.05 (=< 10 µm) (low energy dispenser, LD)	4.5 / READACROSS_Stintz 2003(2)_Syloid74_Particle_LaserD	Stintz 2003
Zeosil 45	amorphous, precipitated,  CAS-No. 112926-00-8	211	=< 1.0 (=< 10 µm) (low energy dispenser, LD)	4.5 / READACROSS_Stintz 2003(3)_Zeosil45_Particle_LaserD	Stintz 2003

Particle size profiles under experimental inhalation conditions (high shear force)

Test material	SAS^*type	MMAD / GSD* [µm]	Respirable fraction [wt%]	Chapter IUC5 / Doc-name	Reference
Microcal ET	amorphous, calcium silicate, CAS No. 1344-95-2	8 - 9	~60	4.5 / PQ Corp 2009_ CS_particle,key,RL1	PQ Corp. 2009
Shieldex AC-5	amorphous, calcium silicate, CAS No. 1344-95-2	~4	~90	4.5 / Grace 2009_ CS_particle,RL1	Grace 2009
Sipernat 880	amorphous, calcium silicate, CAS No. 1344-95-2	10 - 11	~50	4.5 / Evonik 2007_ CS_particle,RL2	Evonik Degussa 2007
Sipernat 22 S	amorphous, precipitated, CAS-No. 112926-00-8	~0.6	~65 (=< 6 µm)	7.2.2 / READACROSS_Degussa 83-0062-DGT_Sip22S_ inhal,rat,RL2	Appelman and Reuzel/ TNO 1983
Cab-O-Sil M5	amorphous, pyrogenic, CAS-No. 112945-52-5	1.2 – 1.3 and 2.2 – 3.5 (depending on the technical device)	No data	7.5.3 / READACROSS_ ASASP2003_CABM5, inhal, 5 d,rat,RL1	Arts, Muijser, Kuper and Junker/ TNO 2003
Zeosil 45	amorphous, precipitated, CAS-No. 112926-00-8	2.8 – 3.2	No data	7.5.3 / READACROSS_ ASASP2003_ZEO45,inhal, 5 d,rat,RL1	Arts, Muijser, Kuper and Junker/ TNO 2003
Syloid 74	amorphous, silica gel, CAS-No.: 112926-00-8	1.6 – 1.7	No data	7.5.3 / READACROSS_ ASASP2003_SYL74,inhal, 5 d,rat,RL1	Arts, Muijser, Kuper and Junker/ TNO 2003

^*SAS = synthetic amorphous silica/silicate; GSD = geometric standard deviation