Give a brief explanation about the international standards for drinking water.

(N/A) The international standards for drinking water are as follows:
$(i)$ Fluoride: The concentration of fluoride ions should be monitored. Deficiency below $1 \ ppm$ causes tooth decay. Fluoride is added to bring concentration to $1 \ ppm$ or $1 \ mg \ dm^{-3}$. It converts hydroxyapatite,$[3(Ca_{3}(PO_{4})_{2}) \cdot Ca(OH)_{2}]$,into harder fluorapatite,$[3(Ca_{3}(PO_{4})_{2}) \cdot CaF_{2}]$. Concentrations above $2 \ ppm$ cause brown mottling of teeth,and above $10 \ ppm$ cause harmful effects to bones and teeth.
$(ii)$ Lead: Lead pipes contaminate water. The prescribed upper limit is $50 \ ppb$. Lead can damage the kidney,liver,and reproductive system.
$(iii)$ Sulphate: Excessive sulphate $(>500 \ ppm)$ causes a laxative effect.
$(iv)$ Nitrate: The maximum limit is $50 \ ppm$. Excess nitrate causes methemoglobinemia ('blue baby' syndrome).
$(v)$ Other metals: The maximum concentrations for common metals are:

Metal	Maximum concentration ($ppm$ or $mg \ dm^{-3}$)
$Fe$	$0.2$
$Mn$	$0.05$
$Al$	$0.2$
$Cu$	$3.0$
$Zn$	$5.0$
$Cd$	$0.005$