Firstly there's the physical environment of being far from fresh air for very long periods of time. Secondly there's the effect on mental health of being in dark tunnels, often bent over. Thirdly, there's the underlying fear that some disaster will occur. The noise level may be immense from the drills.
All of these play a part in increasing the stress and reducing the life expectancy pf the mine worker. In addition, there is the poor quality, if not polluted atmosphere of where they work. If the work entails bending down or restricting their movement, the physique of the worker will be affected.