Are you on untreated well water? If it’s city water, there shouldn’t be much of anything that would be a problem, considering copper has its own disinfectant qualities, as well. Do you use it EVERY brew?
Oh, btw, the steam generated inside the chiller when it’s in the wort for sanitizing will pretty much take care of the parts that are not submerged. I normally drop mine at 10 minutes, and within 3 minutes it’s spewing steam that will remove skin (don’t ask how I know). There is no part of that chiller that can be touched until I put some cold water through it. I’d say that about 20% of my coil is not submerged on the best of days. Water inside the cooler will be raised to boiling temperature and will change to steam. Anything over 180F should be germ free.