[Problem of correction of hygienic standards with special reference to the hardness of drinking water]

When hardness of water is less than 2.2 mg-eq/l the concentration of arsenic of 0.05 mg/l and aluminum of 0.5 mg/l are regarded as threshold. Subthreshold levels do not exceed 0.01 and 0.1 mg/l, respectively. Silicon concentration of 25 mg/l is absolutely safe when using water with hardness of less than 2.5 mg-eq/l.