Water Quality Standards

Mercury is present in the inorganic form in surface and ground waters at concentrations usually of less than 0.5 µg/litre. Levels in air are in the range of 2–10 ng/m3. Mean dietary intake of mercury in various countries ranges from 2 to 20 µg per day per person.

The kidney is the main target organ for inorganic mercury, whereas methylmercury affects mainly the central nervous system.

In 1972, JECFA established a provisional tolerable weekly intake (PTWI) of 5 µg/kg of body weight of total mercury, of which no more than 3.3 µg/kg of body weight should be present as methylmercury. In 1988, JECFA reassessed methylmercury, as new data had become available, and confirmed the previously recommended PTWI of 3.3 µg/kg of body weight for the general population, but noted that pregnant women and nursing mothers were likely to be at greater risk from the adverse effects of methylmercury. The available data were considered insufficient to allow a specific methylmercury intake to be recommended for this population group.

To be on the conservative side, the PTWI for methylmercury was used to derive a guideline value for inorganic mercury in drinking-water. As the main exposure is from food, a 10% allocation of the PTWI to drinking-water was made. The guideline value for total mercury is 0.001 mg/litre (rounded figure).