asked the Secretary of State for the Environment what is the full chemical breakdown of the substance or substances currently being used to fluoridate the public water supplies.

Hexafluorosilicic acid (H₂SiF₆) or disodium hexafluorosilicate (Na₂SiF₆) are currently used to fluoridate public water supplies. The chemical analysis of the former by weight is 80 per cent. water and 20 per cent. Na₂SiF₆, with minute traces of other substances such as phosphate and chloride. The chemical analysis of the latter by weight is at least 98 per cent. Na₂SiF₆ with traces principally of water and iron.