Chronic Effects of Inhaled Amorphous Silicas in Animals

Abstract

Rats, guinea pigs, and monkeys were exposed by inhalation (5.5 to 6 h/day, 5 days/week) for up to 18 months at 15 mg/m3 concentrations of three types of synthetic amorphous silicas: fume, gel, and precipitated. Autopsies on rats were performed after 3, 6, and 12 months of exposure, and on guinea pigs and monkeys after 10 to 18 months of exposure. The most significant finding was the deposition of large quantities of amorphous silica in macrophages in the lungs and tracheal lymph nodes of exposed monkeys. Relatively few or no macrophages containing particles of amorphous silica were found in the lungs and lymph nodes of the guinea pigs and rats. It is also significant that fume silica induced early nodular fibrosis in the lungs of the monkeys. Correlating these histological findings, lung-function studies indicated statistically significant differences in lung volume and ventilatory mechanics measurements between those monkeys exposed to fume silica and the control group. In addition, monkeys exposed to precipitated silica demonstrated significantly lower lung volumes compared with controls, while monkeys exposed to silica gel had significant changes in ventilatory performance and mechanical properties.

Was there sufficient justification for the 10-fold increase in the TLV for silica fume? A critical review

“…However, he felt that the positive parenchymal effects reported in the human studies could have been clouded by the effects of unknown levels of crystalline silica. Without further analysis, Jahr gave his recommendation of 2.0 mg/m 3 as TLV for silica fume, the identical level he recommended for precipitated silica (PAS), a product which he himself stated, ''does not seem to cause silicosis or other lung impairment'' [Jahr, 1981], in agreement with other reports on this amorphous silica polymorph [Wilson et al, 1979;Volk, 1960;Schepers, 1981;Choudat, 1990;Groth et al, 1981].…”

“…However, within the influential review literature pertaining to both silica fume in particular, and amorphous silicas in general, such has not always been the practice [Mitchell, 1990;USEPA, 1996]. Mitchell's [1990] review of the health effects of occupational exposure to thermally generated amorphous silica fume was based on references supplied by the Amorphous Silica Working Party (Table III) studying the health effects not only of silica fume but also of fumed silica [Schepers et al, 1957;Groth et al, 1981], together with the innocuous amorphous silica polymorphs, precipitated silica, and silica gel [Wilson et al, 1979;Volk, 1960;Schepers, 1981;Choudat, 1990;Groth et al, 1981]. These latter papers were given equal emphasis with silica fume in both review and discussion, an unacceptable practice as noted by Stone [1990].…”

“…Gross [1981] has pointed out that ''One cannot and should not lump together all amorphous silicas.'' Such collectivization is exemplified in Table II where health effects of amorphous silica subchronic inhalation were assessed from studies on colloidal silica, Ludox [Warheit, 1991;Kelly and Lee, 1990] and health effects of amorphous silica chronic inhalation were assessed solely from studies on fused silica, silica gel, and precipitated silica [Groth et al, 1981;Schepers, 1981]. Amorphous silica and crystalline silica are contrasted in studies in which the only amorphous silica used is (a) fumed silica [Reutzel et al, 1991], (b) colloidal silica [Warheit et al, 1991b], (c) fused silica [Rosenbruch et al, 1990], (d) diatomaceous earth [Pratt, 1983], (e) Ludox, Zeofree 80 [Warheit et al, 1995].…”

Food applications and the toxicological and nutritional implications of amorphous silicon dioxide

“…These findings were not present with rats and guinea pigs treated in a similar fashion. 30 Brody et al 7 studied the deposition of translocation of silica inhaled in rats. Rat lungs were analyzed by SEM at varying times after exposure to aerosolized a-quartz.…”

Use of Indicator Cell Lines for Determining Inflammatory Gene Changes and Screening the Inflammatory Potential of Particulate and Non-Particulate Stimuli

“…Amorphous silica, or fumed silica (Aerosil200), is produced through the combustion of hydrosilanes, is found in paints and cosmetics, and is, because of its commercial production value, more likely to be involved in high exposure rates in the human population Groth et al, 1979;Mossman & Churg, 1998). Unlike crystalline silica, which has been implicated in the development of chronic inflammatory lung disease, amorphous silica is water-soluble and may be easily cleared.…”