A year ago I was working on a new aussie show which dealt with suicide, and unfortunatly, my auntie attempted suicide during the run and I was the one who found her. I could barely stomach the show after that happened. I approached the PM and explained everything, including that I felt I may not be able to do a good job due to the fact. Unfortunatly, I had to train up an SM over three nights, but it was 100 times better than doing the next 3 months.
Provided your PM is not a hearless monster, if you explain it, they will usually accomidate you. After all, they want the best for the show too.
If it was just one act in a show with more than one act, suggest that maybe you could get the sound or LX op to call it during the period you cannot cope with, and leave the space for some air.
You don't have to be some emotionless, clinical, uber efficent god, even if many of us try to appear so to the cast and crew. Most people will be really good about it. Occasionally (especially when working with children) they will broach the subject callously and without regards for your feelings, but they are the minority.