Asbestos describes any naturally occurring compound of silicon that forms long, thin crystals.  Once popular in building materials for its resistance to fire, soundproofing abilities, low cost and strength, it has been completely banned in Europe and its use in North America is strictly limited due to its health effects.

When inhaled, asbestos fibers become lodged in the lungs and can cause cells to mutate.  It has been linked to cancer, including one particular type known as mesothelioma which is almost exclusively found in individuals who have been exposed to asbestos.  Also common is asbestosis, a form of pneumoconiosis.  

Asbestos is dangerous and should only be handled by qualified personnel.  It is often found in old construction, particularly in insulation for air ducts and plumbing.

Asbestos at Wikipedia

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