I would assume the record has improved since 1999 (When I was there), one of teh biggest problems Boulby had was face explosions caused by gas pockets in some of the "Dirty Ore" areas, if a teleminer hit one of these it could be thrown back 30m plus. Which is dangerous in itself without the sudden release of methane. This has been managed out by the use of radio controlled continuous miners and the requirement for miners in the Potash wear oxygen self rescuers as well as the MSA self rescuers - contrary to popular belief the MSA self rescuers DO NOT create Oxygen - the convert Carbon Monoxide (Harmful) to Carbon Dioxide (Harmless).
What else is notable from seeing recent pictures is there is a requirement to wear Orange High Vis clothing - this was not the case in 1999 where the issued clothing was dark Green!
The biggest risk factor though was the heat and exhaustion, it is hard to see how this can be sensibly managed at Boulby, particularly in the North, under the North Sea - you can 't put in additional vent shafts. Basically the air travels down the downcast shaft - it can be around 25 degrees c at the pit bottom (Purely what the vent air has picked up from friction and heat transfer from the shaft sides), it then travels around 9Km to the furthest point of the mine, picking up heat all the way from the rock and equipment, before returning 9kM picking up even more heat!