Water chlorination levels in US and EU likely increase cancer risk, study finds
(www.theguardian.com)
The residue left over after boiling water is probably mostly calcium (if white) or iron (if orange/brown). Both are from dissolved minerals in the water source. If you need to clean anything stained by either, vinegar should dissolve the minerals with minimal effort. I'd avoid vinegar on anything stone (like countertops), as some types of stone will dissolve in the presence of acid.