Arsenic has been reported to be a trace element of
nutritional importance to humans but its functions in the
biological system is not clear (Holum, 1983). Any level of
concentration of silver in drinking water has been disallowed
, both by the World Health Organization (WHO) and
National Agency for Food and Drugs Administration and
Control (NAFDAC), Nigeria. Lead, cadmium and mercury
have been reported not to have any known function in
human biochemistry or physiology, and do not occur
naturally in living organisms (Lenntech, 2004). Hence
dietary intakes of these metals, even at very low concentrations can be very harmful because
they bioaccumulate.