Nitrate (NO3) can cause methemoglobinemia (infant cyanosis, or "blue baby disease") in infants who have been given water or fed formulas prepared with water having a high nitrate concentration. A domestic water supply should not contain nitrate concentrations in excess of 10 mg/l (expressed as nitrogen). High levels found in shallow wells may be an indication of seepage from septic systems or livestock manure deposits. In some polluted wells, nitrite (NO2) will also be present in concentrations greater the 1 mg/l and is even more hazardous to infants. When the presence of high nitrite concentration is suspected, the water should not be used for infant feeding. Ion exchange and reverse osmosis can be used to remove excess nitrate and nitrite.