There're specific guidelines for using a nasal irrigation device such as a neti pot. When it comes to using water, it should definitely be sterilized, and using tap water leaves a person open to rare but severe brain infections. Tap water might contain specific brain-affecting amoeba, which can make a person severely sick. But this infection is rare. Doctors advise not to use tap water in any case, and if you can't find sterilized water, it's best to boil your tap water for 5 minutes to kill any bacteria.