Asbestos is a naturally occurring fibrous material that was widely used in construction due to its heat and chemical resistance, durability and availability.

Undamaged or covered (boxed in or sealed with paint or similar) asbestos containing materials (ACMs) present no immediate health hazard. However, they must still be managed due to the health risk associated with the potential release of any fibres.

Damaged or deteriorated ACMs can release fibres, especially when disturbed, which are known to cause several lung diseases. 

For more information look here.