Which of the following ions present in low concentration in drinking water is essential for normal growth of teeth but harmful to teeth at high concentration ?

Fluoride is a mineral considered essential for dental health as it promotes the remineralization of tooth enamel, making it more resistant to acid attacks and preventing dental caries. At low concentrations (typically around 0.7 to 1.0 mg/L), it is beneficial for the normal growth and maintenance of teeth. However, high concentrations in drinking water lead to adverse effects. Excessive intake during the developmental years causes dental fluorosis, characterized by staining, mottling, and pitting of the enamel [1]. Chronic exposure to even higher levels can result in skeletal fluorosis, leading to stiff joints, bone deformities, and 'Knock-Knee syndrome' [2]. While calcium is a structural component of enamel, it does not exhibit this specific dose-dependent toxicity profile in drinking water where low levels are essential but high levels are specifically 'harmful to teeth' in the manner of fluorosis.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 29: Environment Issues and Health Effects > ag.l.s.Fluorine > p. 414
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > 3. Hazards of ground water pollution: > p. 76