The European Commission (EC) asked the European Food Safety Authority (EFSA) for scientific assistance regarding the possible impact on human health of exposure to ammonium released from water filter cartridges. EFSA was asked whether a level of ammonium between 0.5 mg/L and 5 mg/L in water poses a risk to human health when this water is consumed over a long period of time and in particular by certain vulnerable groups of the population. Ammonia is a naturally occurring compound and an important source of nitrogen for mammals. Large amounts (3-4 g per day, 43-57 mg/kg body weight (b.w.) per day for a 70 kg adult) of ammonium are produced in the gut and excess ammonium is metabolised in the liver and excreted in urine. For adults, the estimated exposure to ammonium in water within the specified concentration range (0.5-5 mg/L) would range from 0.014 mg/kg b.w. per day to 0.14 mg/kg b.w. per day and the estimated exposure would be slightly higher for infants and children. Considering the large amounts of endogenously produced ammonium compared to the exposure from water, it is concluded that additional exposure to ammonium from water at the specified range is negligible and does not pose a risk to human health, even for vulnerable groups.