Hazardous Chemical Information System (HCIS)

Fumed silica

E/S first adopted in 1990

No standard should be applied without reference to Guidance on the interpretation of Workplace exposure standards for airborne contaminants.

Documentation notice: National Occupational Health and Safety Commission documentation available for these values.

Note:

(1) Fumed silica should not be confused with silica fume . Fumed silica is produced synthetically by a vapour phase hydrolysis of silicon tetrachloride . Silica fume is the byproduct of a high-temperature process when elemental silicon is produced by reacting coke and silica sand (crystalline) in an electric arc furnace.

(2) Introduction to Silica - Amorphous:

Silicon dioxide, SiO2, exists in two varieties, amorphous and crystalline . In crystalline forms, the structures are characterised by tetrahedral configuration of atoms within the crystals, whereas in the amorphous forms, the SiO4 (silicate) subunits show no regular lattice pattern in the structures . Crystalline silicas show discrete reflections in X-ray diffraction from the internal planes formed by the orderly pattern of atoms, while in amorphous silica, X-rays are scattered randomly and no discrete reflections are seen . All types of amorphous silica can be converted to crystalline forms when heated to a sufficiently high temperature.

The fibrogenic potential of crystalline free silica, according to one theory, can be attributed to its tetrahedral configuration of atoms (1) . However, this hypothesis is still a matter of debate . Bye et al (2) , using the enzyme LDH as an indicator of cytotoxicity, showed that the cytotoxicity of a series of diatomaceous earth products towards macrophages in vitro was not dependent on the presence of a crystalline silica component . The researchers suggested that crystallinity may not be the only determinant of fibrogenicity .

It is apparent that whether silica particles can cause biological damage is determined by their surface characteristics (3) . Iler (4) postulated that the bonding of bio-organic molecules on silica particles caused denaturation of proteins and rupture of cell membranes . Denaturation of proteins could cause immunological reactions and rupture of membranes could cause cell death . This bonding can be attributed to three forces: ionic attraction, hydrogen bonding and hydrophobic bonding . Freshly cleaned quartz, and amorphous silica which have been subjected to high temperatures, have surfaces that have a predominance of siloxane groups that can give rise to hydrophobic bonding to organic molecules (4) .

There are several naturally-occurring and synthetic amorphous silicas , namely diatomaceous earth, precipitated silica, silica gel, fumed silica and silica fume (thermally generated) . Synthetic amorphous silica is usually prepared by vapour-phase hydrolysis, precipitation or other processes which ensure the absence of crystalline free silica . Three forms of synthetic amorphous silica have come on the market in recent years, according to their method of preparation: silica gel (silica G), precipitated silica (silica P) and fumed silica (silica F) . Fumed silica is derived from vapour-phase hydrolysis of a silicon-bearing halide, such as silicon tetrachloride. 

2. CHEMICAL AND PHYSICAL PROPERTIES

The molecular weight of fumed silica is 69.02 . Its density varies around 2g/ml.

Given below are some of the chemical and physical properties of fumed silica, in comparison with other synthetic amorphous forms (5) :

3. MAJOR INDUSTRIAL USES

Fumed silica has a wide range of applications in the industries of rubber, paper, chemicals and paints . In the rubber industry, the use of fumed silica enhances tensile strength, abrasion resistance, stiffness and gives better colour . In the paper industry, the qualities of increased opacity and brightness allow better printability and improved smoothness . In its application in paints, varnishes and protective coatings, it provides higher binding power, efficient extension of prime colours, improved thixotropy and protection from corrosion.

4. ANIMAL STUDIES

Jahr (7) reviewed the toxicity of amorphous silica and concluded that amorphous silica caused some tissue reaction but there was no progression to collagen formation.

Schepers et al (8) exposed rats to fumed silica at an airborne concentration of 50mg/m3 . The majority of rats died from pulmonary obstruction and emphysema after three to five months . Upon cessation of further exposure, the surviving animals recovered quickly and the cellular nodules and emphysema were almost completely resolved.

Groth et al (6) reported an animal inhalation study . Rats, guinea pigs and monkeys were exposed to fumed silica, silica gel, or precipitated silica, for 5.5-6 hours/day, 5 days/week, for up to 18 months at 15mg/m3 (total dust) concentration (or 6.9-9.9mg/m3 respirable dust) . Few or no silica-containing macrophages were found in the lungs and lymph nodes of the guinea pigs and rats . The most significant finding was that fumed silica induced early nodular fibrosis in the lungs of the monkeys . However, the toxic potential of fumed silica might not have been fully shown in this study, as an exposure duration of 10-18 months might be relatively brief for monkeys . The researchers attributed the fibrogenic action of fumed silica to a combination of factors: greater surface area, greater solubility and higher content of aluminium and iron compounds than the other amorphous silicas. 

5. HUMAN STUDIES

The American Society for Testing and Materials (ASTM) has reviewed (9) three studies which involved a total of 353 workers exposed for up to 32 years to fumed silica at concentrations 1.6-53mg/m3 . No pulmonary dysfunction was observed except in smokers.

Fumed silica and silica fume (thermally generated) exhibit entirely different toxicities . (Note: the toxicity of silica fume is still under review by the Exposure Standards Working Group. ) Silica fume has been shown to have a more significant pneumoconiotic effect than the synthetic fumed silica (1) .

6. CONCLUSION

One animal study indicated that fumed silica is more toxic than precipitated silica and silica gel . At high concentrations, it is fibrogenic to animals, but its fibrogenic potential appears to be far less than that of crystalline silica.

Fumed silica has only been found in industry in recent time . The available limited information has not demonstrated a significant health effect in humans through occupational exposure. 

7. RECOMMENDATION FOR EXPOSURE STANDARD

To protect most workers from pulmonary dysfunction, the Exposure Standards Working Group recommends a time-weighted average exposure standard of 2mg/m3 (respirable dust) .

REFERENCES

1. American Conference of Government Industrial Hygienists ( ACGIH ), Documentation of the Threshold Limit values and Biological Exposure Indices, 5th Edition, Ohio , 1986

2. Bye E et al, "In vitro cytotoxicity and quantitative silica analysis of diatomaceous earth products", Brit J Ind Med, 41, 228-234, 1984

3. Baumann H, "Characterization of Silicon Dioxide surface by successive Determination of the Solution Rate", In: Health Effect of Synthetic Silica Particulates, ASTM STP 732, DD Dunnom Ed., American Society for Testing and Materials, pp.30-47, 1981

4. Iler RK , "The Surface Chemistry of Amorphous Synthetic Silica - Interaction with Organic Molecules in an Aqueous Medium", In: Health Effects of Synthetic Silica Particulates, ASTM STP 732, DD Dunnora Ed., American Society for Testing and Materials, pp.3-29, 1981

5. Stockinger HE, The Halogens and Nonmetals Boron and Silicon, In: Patty's Industrial Hygiene and Toxicology, edited by Clayton GD & Clayton FE, 3rd rev. ed , Vol 2B, pp 3011-3014, 1981

6. Groth DH et al, "Chronic effects of inhaled amorphous silicas in animals", In: Health Effects of Synthetic Silica Particulates, ASTM STP 732, DD Dunnom Ed., American Society for Testing and Materials, pp.118-143, 1981

7. Jahr J, "Possible Health Hazards from Different Types of Amorphous Silicas ", In: Health Effects of Synthetic Silica Particulates, ASTM STP 732, DD Dunnom Ed., American Society for Testing and Materials, pp.199-213, 1981

8. Schepers GWH et al, "The biological action of Degussa Submicron amorphous silica dust (Dow Corning Silica)", Am Med Assoc Archives of Ind Health, 16, 125-146, 1957

9. American Society for Testing and Materials, Health Requirements for Occupational Exposure to Synthetic Amorphous Silica, ASTM Standard E1156-87, Philadelphia , 1987

Footnotes:
Documentation notice:

Entries carrying a notice for National Occupational Health and Safety Commission documentation indicate that these substances have been reviewed in detail by the Exposure Standards Expert Working Group and that documentation supporting the adopted national values is available in the National Commission's Documentation of the Exposure Standards [NOHSC:10003(1997)].