A contaminant is anything found in your water that is not a water molecule, which is why we prefer to think of them as impurities. Contaminants/impurities can be good or harmful to your health, depending on the substance and quantity. For example, E. coli is a very small organism that is harmless at low levels but can make people sick in large enough quantities. On the other hand, chlorine, which is used in the water treatment process as a disinfectant against bacteria like E. coli, is also an impurity, but it is added at very safe levels to ensure high-quality drinking water.