Because asbestos is often mixed with another material, it's hard to know if you're working with it or not. But, if you work in a building built before the year 2000, it's likely that some parts of the building will contain asbestos.
If you inhale asbestos fibres (which are long and thin) they can become lodged in the tissue of your chest and your body’s natural defences may not be able to easily break them down. This can lead to lung diseases (mainly cancers), particularly if you are repeatedly exposed to fibres over a number of years.
Source: http://www.hse.gov.uk/asbestos/