Ascorbic acid (vitamin C) induced hydroxyl radical formation was measured in household drinking water samples using the hydroxyl radical sensitive probe coumarin-3-carboxylic acid. Vitamin C, a reducing agent that is commonly used as a food additive, triggered a significant hydroxyl radical generating reaction when added to the tap-water samples tested. The capacity of ascorbic acid to trigger hydroxyl radical formation in the tap-water samples was dependent on the flushing time before the samples were taken indicating that the water in the copper piping had been contaminated by copper ions. In line with this, high concentrations of copper were measured in the hydroxyl radical generating first-draw samples. Moreover, a strong correlation was found between the hydroxyl radical generation capacity seen in the coumarin-3-carboxylic acid based microplate assay and the DNA damage seen in an agarose gel assay using the pBluescript plasmid. In the water samples showing high capacity to hydroxylate coumarin-3-carboxylic acid, a rapid formation of the open circular form of the plasmid could also be seen indicating a copper assisted hydroxyl radical attack on the DNA. In conclusion, our results show that addition of vitamin C to household tap water that is contaminated with copper ions, results in Fenton type reactions that continuously generate harmful and reactive hydroxyl radicals.