Yeah, this is something that confuses me too. When I did my foundation course I was very distinctly told by an educator that it is the law, and that a local spa had been banned from doing nails until they got the necessary filtration installed, they were one of the unlucky ones who had been given a surprise inspection apparently.
Then, I went to work in the big wide world of nails, and realised that very few people actually having filtration, and that you dont get inspections as the industry isnt regulated!! I have searched for the regulation that says you have to have it, and I havent been able to find it. Regardless, I do use filtration for the sake of my own health, but it would be nice to know for definite what the regulations are. In the Habia code of practice it suggests that having filtration forms part of an ideal working practice but in no part does is state that having filtration is the law and that you cant work without it.