In order to evaluate the effect of thermal treatments on the surface reactivity and carcinogenic potential of
diatomaceous earth (DE) products, the physicochemical features of some specimens--derived by heating the same original material--were compared with their cytotoxic and transforming potency. The samples were an untreated DE (amorphous) progressively heated in the laboratory at 900 degrees C (DE 900) and 1200 degrees C (DE 1200) and a commercial product manufactured from the same DE (Chd) from which the finer fraction (< 10-microm diameter) was separated (Chd-F).
Quartz (Min-U-Sil 5) and a vitreous
silica (amorphous) smoothed up with
hydrofluoric acid and were used as positive and negative controls, respectively. All samples were analyzed for their degree of crystallization, for their ability to release
free radicals and
reactive oxygen species, and for their cytotoxic and transforming potencies in Syrian hamster embryo (SHE) cells. X-ray diffractometry showed that DE 900, like DE, was still amorphous, whereas DE 1200 as well as the commercial product (Chd) were partially crystallized into
cristobalite. The ability of the dust to release
hydroxyl (*
OH) radicals in the presence of
hydrogen peroxide, as revealed by the spin-trapping technique, was as follows: Chd-F, DE 1200 > Chd > DE 900 > DE, suggesting that on heating, the surface acquires a higher potential for
free radical release. Most of the
silica samples generated
COO* radicals from the
formate ion, following homolytic
rupture of the
carbon-hydrogen bond, in the presence of
ascorbic acid. A concentration-dependent decrease in cell proliferation and colony-forming efficiency was observed in SHE cultures treated with Chd-F, Chd, and DE. Heating abolished DE cytotoxicity but conferred a transforming ability to thermal treated particles. DE was the only sample that did not induce morphological transformation of cells. According to their transformation capacity, the samples were classified as follows: Chd-F > Chd, DE 1200 > DE 900 >> DE. Taken together, the reported results suggest that (1) the transforming potential of a biogenic amorphous
silica is related to the thermal treatment that transforms the original structure in
cristobalite and generates surface active sites; (2) the reactivity of samples in releasing *
OH radicals correlates to their transforming ability; (3) the finer fraction of the commercial product is significantly more toxic and transforming than the coarse dust; and (4) opposite to
silica dusts of
mineral origin, which loose both cytotoxicity and transforming ability upon heating, heated
diatomite acquires a cell-transforming potency. DE products should be thus considered a set apart of
silica-based potentially toxic materials.