My nightmare

I have one reoccurring nightmare. In it I am in a giant factory. Three of the walls are concrete, and the other is open, and I can see outside. I am walking along one of the walls near a conveyor belt that doesn't seem to go anywhere. There are lots of people working on stuff being transported on the belts. Suddenly the one nearest to me starts choking. it looks like he can't breathe, and all his skin turns purple. His eyes are bulging out of his head, and he seems to be crying for help. Then, he is fine and he returns to work, but he has changed, he is now a completely different person. I start running down the lines of people. The same thing is happening to all of them, very quickly. I start screaming, as I approach the open wall, and then I wake up.