Asbestos is hazardous to human health when the fibres within the material are allowed to escape into the air. This happens when the material is broken, sawn, drilled, sanded and especially when damaged in the workplace.
White asbestos fibres at x64 magnification
Note: One thousandth of a millimetre is a micron. Asbestos fibres can be as small as 5 microns long and 3 microns in diameter. ie. at this level, fibres are invisible to the naked eye - though still dangerous to living tissue - especially lungs.
Even small amounts of asbestos dust may be hazardous to health because the fibres are of such a size and shape that they can be easily inhaled, penetrating the airways and tissues of the lungs quite easily.