The international standards for drinking water are given below :

$(i)$ Fluoride : Fluoride ion concentration should be tested in drinking water. Its deficiency in drinking water is harmful to man and causes diseases such as tooth decay etc. Soluble fluoride is often added to drinking water to bring its concentration upto $1 \mathrm{ppm}$ or $1 \mathrm{mg} \mathrm{dm}^{3}$. The $\mathrm{F}^{-}$ions make the enamel on teeth much harder by converting hydroxyapatite, $\left[3\left(\mathrm{Ca}_{3} \mathrm{PO}_{4}\right)_{2} \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]$, the enamel on the surface of the teeth, into much harder fluorapatite $\left[3\left(\mathrm{Ca}_{3} \mathrm{PO}_{4}\right)_{2} \cdot \mathrm{CaF}_{2}\right] .$ If $\mathrm{F}^{-}$ion concentration above $2 \mathrm{ppm}$ causes brown mottling of teeth. Whereas if $\mathrm{F}^{-}$ion concentration is more than $10 \mathrm{ppm}$ it causes harmful effect to bones and teeth. It is reported from some parts of Rajasthan.

$(ii)$ Lead : Drinking water gets contaminated with lead when lead pipes are used for transportation of water. The prescribed upper limit concentration of lead is about $50 \mathrm{ppb}$ lead can damage kidney, liver, reproductive system etc.

$(iii)$ Sulphate : Excessive sulphate $(>500 \mathrm{ppm})$ in drinking water causes laxative effect, otherwise at moderate levels it is harmless.

$(iv)$ Nitrate : The maximum limit of nitrate in drinking water is $50 \mathrm{ppm}$. Excess nitrate in drinking water can cause disease like methemoglobinemia ('blue baby' syndrome).

$(v)$ Other metals : The maximum concentration of some common metals recommended in drinking water. Which are given in table.