Occupational exposure to crystalline
silica dust is associated with an increased risk for
pulmonary diseases such as
silicosis,
tuberculosis,
chronic bronchitis,
chronic obstructive pulmonary disease (
COPD) and
lung cancer. This review summarizes the current knowledge about the health effects of amorphous (non-crystalline) forms of
silica. The major problem in the assessment of health effects of amorphous
silica is its contamination with crystalline
silica. This applies particularly to well-documented
pneumoconiosis among
diatomaceous earth workers. Intentionally manufactured synthetic amorphous silicas are without contamination of crystalline
silica. These synthetic forms may be classified as (1) wet process
silica, (2) pyrogenic ("thermal" or "fumed")
silica, and (3) chemically or physically modified
silica. According to the different physicochemical properties, the major classes of synthetic amorphous
silica are used in a variety of products, e.g. as fillers in the rubber industry, in tyre compounds, as free-flow and anti-caking agents in
powder materials, and as liquid carriers, particularly in the manufacture of animal feed and
agrochemicals; other uses are found in
toothpaste additives, paints,
silicon rubber, insulation material, liquid systems in coatings, adhesives, printing inks, plastisol car undercoats, and
cosmetics. Animal inhalation studies with intentionally manufactured synthetic amorphous
silica showed at least partially reversible
inflammation,
granuloma formation and
emphysema, but no progressive
fibrosis of the lungs. Epidemiological studies do not support the hypothesis that amorphous silicas have any relevant potential to induce
fibrosis in workers with high occupational exposure to these substances, although one study disclosed four cases with
silicosis among subjects exposed to apparently non-contaminated amorphous
silica. Since the data have been limited, a risk of
chronic bronchitis,
COPD or
emphysema cannot be excluded. There is no study that allows the classification of amorphous
silica with regard to its carcinogenicity in humans. Further work is necessary in order to define the effects of amorphous
silica on morbidity and mortality of workers with exposure to these substances.