There are no chemicals. The water comes straight up from about 1000 feet underground where it is geothermally heated. But yes you have to get clean before hand. Generally when you first go in there is a locker room type area where you get undressed. When you go out to the bath itself, around the wall there are usually a number of stools and buckets sitting next to shower hoses. You sit down, fill the bucket, get soapy, then rinse off before getting in the onsen. Don't worry. They're clean.