The primary component of silica fume is amorphous silicon dioxide SiO2 in a range of 85 % - 98 %. The main impurities include carbon, silicon carbide and oxides of alkaline (earth) metals.

As silica fume is generated from raw materials which inevitably vary in composition depending on the source, the impurities will also individually vary up to a maximum of 5 %, but the total amount of impurities will not exceed 15 %:
CaO, MgO, Na2O, K2O, Al2O3, Fe2O3, SO3, SiC, Si, C, Cl-< 5 %
Zn < 0.1 %
Mn < 0.1 %
Pb < 0.1 %
Respirable crystalline silica (RCS) < 0.1 %

Silicon carbide in silica fume is in its non-fibrous form, as demonstrated by Scanning Electron Microscopic (SEM) investigations on pre-treated (HF digested) silica fume samples.

The concentration of quartz in silica fume is usually below the limit of quantification (LOQ) (see arrow in the figure) and it has been shown to be concentrated in the coarse fraction of silica fume. In the respirable size fraction the concentration is below 0.1%.

The SWeRF method (Size Weighted Respirable Fraction, has been used to evaluate respirable crystalline silica content in bulk Silica Fume. The SWeRF of a particulate material is calculated by combining the amount of particles of a certain size with their probability of reaching the alveoli when inhaled. SWeRF was proposed as an indication of the potential hazard of a product forming respirable dust when handled.