Even lurking just a bit on fitness communities shows that people often talk about single digit body fat levels.
Intuitively, this does not seem to be a 'normal' state of the body. People with such low fat levels do not necessarily seem healthy on the first glance, with their paper thin skin, prominent veins and visible facial bones. Anecdotally, I’ve seen a female athlete complaining in the internet that when she was dropping to her lowest fat levels her grandma was asking if she wasn’t sick.
Question arises if such low fat levels are not a health hazard?
I know that athletes often drop fat to lowest possible levels just days before a competition and, once the competition is finished, return to slightly higher levels. So these lowest possible levels seem unsustainable long-term. Even if the 'normal' low fat levels of athletes are still not low enough to cause health hazards, question still arises if these periodical drops to even lower levels are not hazardous.
Note that I am asking about body fat levels well below what is usually considered a 'healthy normal' in most people and not about losing fat from being overweight.