SILICA IN THE WORKPLACE
2
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Silica: Its Occurrence, Uses and Health Effects
What is Silica?
Silica is the name of a group of minerals that contain silicon and oxygen in a chemical combination and
have the general formula SiO
2
. Silica may be free, in which case only SiO
2
is present, or combined, in
which the SiO
2
is combined chemically to some other atom or molecule. This difference is important to
recognize, since the silica problem exists only with free silica. Labels on packages and product analysis
sheets and Safety Data Sheets (SDS) must be read carefully for this reason. Free silica may occur as
amorphous free silica, of which there are many forms, and crystalline silica, of which there are five
principal forms. Certain materials contain both amorphous and crystalline silica. Silica-related diseases
are associated only with crystalline silica. The most common examples of crystalline silica are beach or
bank sands. A third form of free silica is fused silica, which is produced by heating either the amorphous
or crystalline forms. Heating amorphous silica causes conversion to the crystalline form under
appropriate conditions.
The interrelationships between the crystalline and non-crystalline forms of silica are extremely
important. Products containing amorphous silica are often heated to high temperatures during
use in the workplace. A material initially containing no crystalline silica when installed may contain
considerable quantities when removed.
Figure 1 illustrates the interrelationship between the various forms of silica.
