It can be toxic stuff. The original workers there may have got a lot of exposure so I wonder if the guides who work there now have to limit how long they spend up there. I found a study showing it may not be too bad if they take precautions. The I found this by someone who visited this same lighthouse. We were only up there for a few minutes.

I vaguely remember something about science teachers having some higher risks from exposure to mercury in experiments at school. The rules on using it may have changed since I was in those classes. I had a toy maze game with mercury in it and that may not be allowed now.