Asbestos is a mineral that was mined for and used heavily in the construction/manufacturing industry following World War II, largely used in insulation, and building materials. This was due to its properties such as being a good thermal insulator, a good electrical insulator, minimal chemical reactions as well as being relatively fireproof. All these attributed made asbestos one of the best building materials. The only downside to Asbestos which was not widely known at the time was that once these Asbestos containing material are broken, the fibres they release are deadly once breathed in. 

When Asbestos containing materials or ACMs are broken or start to break-down they release exceedingly small fibres into the air, some looks like string others look like needles.