One of the most toxic elements is Fluorine. The fluoride ion is the most electronegative and reactive ion known to man. Translation -- it will rip out electrons from glass so that it can fill its outer shell. The major source of Fluorine in the United States is from phosphate production toxic waste in Florida.
Big business has convinced millions of idiots that this waste product instead of being deposited in a waste disposal repository should be introduced into the water supply as a means of furthering dental health. Dental Fluorosis, mottling of the teeth, is the frequent result. The fluoride ion displaces the Carbonate radical in natural teeth and bones. The result is a mottled Calcium Fluoride precipitate as part of the dental structure. The identical thing also happens to the bones except we can't normally see this. The result is that the bones and teeth become more brittle with diminished strength.
The fluoride ion is the smallest ion known to man. It cannot be removed by filtration, even by Reverse Osmosis, regardless of what any advertisement claims. The best way to remove any soluble fluorine ion is to precipitate it with calcium ion (Calcium hydroxide) in solution and precipitate it out as highly insoluble Calcium Fluoride.
Generally only people under the age of 50 or so will have dental fluorosis as the addition of fluorine into municipal water supplies probably only started in the 1960's or later.
More dangerous is the single displacement reaction of Iodine by Fluorine in the thyroid gland. Fluorine displaces Iodine and the thyroid gland consequently ceases to function. When that happens you can expect many health problems as the thyroid gland provides substances that control many body functions. Remember that there is no such thing as equality in this world and that Fluorine is unequal with Iodine.
The fluoride bond is so strong that there are only about 5 fluorine compounds that exist that are water soluble, i.e. Hydrogen Fluoride, Sodium Fluoride and Fluorosilic acid.
Note that anti-depressants, e.g. Paxil, most typically have a fluorine compound as their active ingredient. Go to a feed store and note that insecticides and herbicides frequently have fluorine as one of their ingredients.
Another little known fact of the global warming manure is that new refrigerants are being mandated to be made with fluorine instead of historic chlorine. The refrigerants will eventually breakdown down in the atmosphere and the earth will have more toxic fluorine in the environment instead of having it in the earth's rock for the last millions of years. Thank you dumb asses for helping to poison the rest of us.
This is not an exhaustive website regarding health and fluorine but you can do further research on the web.