Chlorophenols are intensely offensive byproducts of disinfection that occur only when the concentration of disinfectant is marginal-that is, barely sufficient to kill pathogens, but without leaving much of a continuing residual to fight any subsequent contamination or chemical impurities. The T&O is strong, bitter, and iodine-like and is usually described as “medicinal” There are several; the worst is called 2.4-dichlorophenol and has an odor threshold in the low parts-per-trillion range. The chemical “phenol” is a six-carbon ring (a benzene ring) with an –OH group attached somewhere. The huge molecules of tannins and lignins that give the brown-yellow color to swamp water (and also to tea) are highly “phenolic” in nature, meaning there are a great many benzene rings with one or several OH groups attached, all linked up into an irregular polymer chain with a molecular weight in the millions. These tannin and lignin molecules are the main source of THMs and other disinfection byproducts when the disinfectant (chlorine. Chlorine dioxide or ozone) chops them into little pieces and then chlorinates the fragments. The fragments that are, or contain, a phenol quickly become chlorophenols, which may easily be further oxidized (to destruction), but only if there is sufficient chlorine or ozone. A chlorophenol problem can also arise if water-using equipment contains phenolic materials such as PPO (polyphenylene-oxide)-based plastic or rubber compounds that let phenol and derivatives leach into the treated water, and the residual chlorine level is very low. The foul T&O may occur only randomly or rarely and can even plague equipment already protected by filtration systems. All it takes is a few ppt (parts per trillion) each of chlorine and a phenol, both of which are undetectable at that level by any standard analytical method.