What to do if opponent has poor hygiene? Are there regulations on what to do when your opponent has poor hygiene and it negatively influences your performance?

I do not want to be disrespectful or shame anyone, but bad breath or body odor really can negatively impact your mind, especially if you have to spend hours opposite of the person. It is not fair to have to play under such conditions (in my opinion).
According to Article 12.2.3 of the FIDE Laws of Chess it is one of the roles of the arbiter to -


  12.2.3 ensure that a good playing environment is maintained


Your first course of action should be to bring the matter to the attention of the arbiter. It is not a pleasant thing for him or her to deal with but it is part of the (usually unpaid) job of the arbiter to resolve such problems.