Nitrous causes brain damage in medical staff. It damages cobalamin (vitamin B12), which is no big deal for a one-off exposure but for people to work with it every day causes problems. See:

http://www.anesthesia-analgesia.org/content/60/5/297.short

http://www.nejm.org/doi/pdf/10.1056/NEJM199210013271405

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1418219/pdf/bmjc...

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2162364/pdf/anes...