Ongole, Sept. 2: Like many other compounds, fluoride is good in small quantities and becomes harmful in large quantities. Doctors use fluoride in small quantities for the prevention of dental cavities in children. It is common practice to add very low concentrations of fluoride to tooth pastes and drinking water (less than 1ppm). But in the water consumed by people in Prakasam villages, the concentration of fluoride is about 60 to 70 ppm which is much above the permissible levels of 4 to 6 ppm. Besides this, levels of silicon, magnesium and Creatinine are also high in this water. Skeletal fluorosis, which has a crippling effect, occurs because of the deposit of fluoride in the bones. Apart from this, high concentrations of fluoride in water also cause teeth mottling, kidney diseases as well as malformation of bones in kids.