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Ash is comprised of a number of different minerals, but only
some of them may be toxic. Crystalline silica is one of the
volcanic minerals of greatest concern; it has been shown to
be a potential human carcinogen in industrial settings and is
the cause of the debilitating respiratory disease silicosis. In
addition, the surface of an ash particle can be modified by
the presence of reactive metals from the volcano, especially
iron, which can result in the generation of extremely harmful
free radicals in the body.
A particular type of eruption, known as a ‘dome-forming’
eruption, may be uniquely hazardous to respiratory health.
During these eruptions, domes form when viscous or
sticky lava piles up on the crater of the volcano instead
of flowing away. These domes are extremely unstable and
prone to collapse. Ash from dome-forming eruptions is
particularly hazardous because it is very fine-grained and
can contain an abundance of crystalline silica in the form
of cristobalite, which crystallises within the dome prior to
collapse. For example, cristobalite can comprise nearly
one quarter of the ash from the ongoing (1995-present)
eruption of the Soufrière Hills volcano on the Caribbean
island of Montserrat. These results prompted the need for
the silicosis hazard to be considered at other dome-forming
eruptions near populated areas. For my research, we studied
a selection of volcanoes in South Asia and Central America,
where recognising hazards based on volcanic setting can
significantly aid decisions made by hazard managers
during a crisis.
Toxicological research on cristobalite-rich ash from
dome-forming eruptions in Guatemala, Indonesia, Mexico,
Japan, and the USA, found ash to be far less harmful to the
lungs than the cristobalite concentrations might indicate.
These data correspond with previous studies on ash from
Soufrière Hills volcano, which found ash to be only mildly
toxic. The International Agency for Research on Cancer
(IARC) has noted that crystalline silica is not carcinogenic
in all instances, depending on ‘internal characteristics and
external factors’ which regulate toxicity.
For volcanic cristobalite, the diminished toxicity likely
results from the following ‘internal characteristics and
external factors’: first, individual ash particles are a mix
of phases so people will always be exposed to particles
that consist of a mixture of different minerals. Second,
all volcanic cristobalite has substitutions of aluminium
for silicon within the crystal; aluminium is known to make
crystalline silica less toxic. Lastly, the concentration of
cristobalite in volcanic ash may be insufficient to be a
significant burden to the lungs.
These results, combined with epidemiological studies on
Montserrat and following the eruption of Mount St Helens,
find that the risk of silicosis from volcanic ash rich in silica
is lower than anticipated.
Images of volcanic ash taken by scanning electron microscopy (SEM).
Breathing in ash particles can affect human health in different ways depending
on their size. Tiny ash particles can penetrate deep into the lungs causing
damage and disease.
Hazard and exposure
to volcanic ash