The silica dust hazard
What is it and why should I be concerned?
12 – Winter 2017 — The Canadian Design and Construction Report




Every year an esti-
mated 2.3 million
workers out of a
global workforce of
approximately 3.5 billion die as a re-
sult of work related accidents and in-
cidents. An estimated 160 million
suffer as a consequence of work re-
lated diseases with some two million
dying from these diseases annually. It
is estimated five per cent of all cancer
cases may be work related.

In terms of the deaths from work
activities, about eight per cent can be
directly attributed to respiratory dis-
eases namely silicosis, pneumoconio-
sis, asbestos related diseases (ARD),
chronic obstructive pulmonary disor-
ders (COPD), occupational asthma,
metal-lung disease and organic dust
diseases. Respirable Crystalline Silica (RCS)
is an abundantly found naturally occur-
ring substance found in stone, rocks
(marble, sandstone, flint, slate), sand
and clay, as well as in products rang-
ing from brick, concrete, tiles, some
composite materials and some metal-
lic ores to shingles, mortar and plas-
ter. Less harmful forms of silica dust
are amorphous form such as those
found in diatomaceous earth, syn-
thetic amorphous silica or silica gel.

These are not believed to lead to sili-
Type of Material
Crystalline Silica (%)
Tripoli Sandstone
Plastic Composites*
Road Rock
Concrete / Mortar
Shale China Stone
Granite Tile
Slate Clay
Brick Ironstone
Basalt / Dolerite
Limestone Marble
95+% 70 - 90%
19 - 90%
Up to 80%
25 – 70%
40 – 60%
Up to 50%
20 – 45%
30 – 45%
20 – 40%
Up to 40%
Up to 30%
Up to 15%
Up to 15%
Up to 2%
Up to 2%
cosis, lung cancer or other known
common disease associated with
RCS. However, it is possible to chemi-
cally change amorphous silica above
1300 o C into RCS such as when calcin-
ing (roasting or exposing to strong
heat) or welding using acetylene at
greater than 3100 o C. Diatomaceous
earth, as well as burning agricultural
waste, can at these temperatures
change into a harmful form. This is im-
portant in scenarios involving demol-
ishing silos or receptacles which may
have been used to store rice husk.

When and at what quantity is RCS
harmful? RCS is harmful to health when it is
inhaled deep into the lungs, therefore
activities such as drilling, cutting and
crushing are where we have to be
more careful. The dust that is particu-
larly harmful is smaller than a fine
grain of sand. To put this into perspec-
tive, the size of a full stop is about
200-300 m (micrometres) in diameter,
whereas the RCS dust is about 5 m.

These particles are so small that they
cannot be seen with the naked eye.

Those involved in demolition of
older building and facilities can be ex-
posed to bricks which have higher sil-
ica content due to high levels of sand
and limestone, including sand-lime
bricks, concrete blocks, cellular con-
crete, masonry rubble, natural stone
and blast furnace slag bricks.

If a person is exposed to a high
level of RCS, the effects of harm and
onset of silicosis can start from as lit-
tle as a few weeks of exposure, as
was the case in the Hawks Nest Tun-
nel Disaster in the 1930s. In this case,
thousands of workers were exposed
to particularly high grades of silica
dust during the construction of a tun-
nel, in restricted areas with no per-
sonal protection. This led to many
workers developing an acute form of
silicosis within a matter of weeks and
resulted in their untimely deaths. This
figure is an under-estimate and the ac-
tual number could have been many
thousand workers from one incident.

The table below gives a range of
RCS content in certain materials found
on demolition sites, either in pure
form or within other materials. In the
case of plastic composites silica may
be used as filler.

Regulatory authorities have set
standards based on the maximum ex-
posure levels before there is likely to
be significant harm.

In the UK, this is defined as a
Workplace Exposure Limit (WEL) of
0.1mg/m 3 based on an eight-hour
working shift. This rate is lower in
countries such as Canada where the
The Canadian Design and Construction Report — Winter 2017 – 13