safety - 312 questions (appears canonical since [dangerous] is a synonym)
health-and-safety - 57 questions
I can't see any reason why these should remain as separate tags. Something unsafe is, by definition, unhealthy.
Thanks for the suggestion. I saw a few questions that used both, and nothing that stands out needing only a single tag. I've approved your proposed synonym. This can still be undone if others have a reason to disagree.