A more difficult question is what Anna should do. My observations in ethical quandaries have been that more often that not, the more information available to affected individuals, the better. I believe she should immediately tell all the students what she has found out and advise them to at the very least not eat in the laboratory.
Graduate students are intelligent people.
They don't need professors to take care of them. If they find out that the experiments are most likely contaminating the indoor air with lead, they will take precautions, irrespective of what the professor will say. That is a tricky question, because the answer depends on the alternatives. As the case study reads, constructing expensive hoods would require spending money that would be made available by firing several students. Dan would not be one of the students fired because the laboratory would be constructed for his experiments.