Summarize the international standards for the concentration of various metals and ions in drinking water.

(N/A) The international standards for drinking water are summarized as follows:
$(i)$ Fluoride: The concentration of $F^-$ ions in drinking water is critical. Deficiency (below $1 \ ppm$) causes tooth decay. It is often added to reach $1 \ ppm$ or $1 \ mg \ dm^{-3}$ because 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 levels above $10 \ ppm$ are harmful to bones and teeth.
$(ii)$ Lead: Lead contamination often occurs via lead pipes. The prescribed upper limit is $50 \ ppb$. Lead exposure can damage the kidney,liver,and reproductive system.
$(iii)$ Sulphate: Excessive sulphate $(> 500 \ ppm)$ causes a laxative effect; moderate levels are generally harmless.
$(iv)$ Nitrate: The maximum limit is $50 \ ppm$. Excess nitrate can cause methemoglobinemia (blue baby syndrome).
$(v)$ Other metals: The maximum recommended concentrations for common metals are provided in the table below:

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