Fluorine, chlorine, bromine, iodine, and astatine are halogens. They have in common only 7 electrons in their outer electron shell, making them very reactive. Fluorine has the lowest electronegativity of all elements which is why it is so dangerously reactive. Astatine is very rare, but the other four have found uses as killers. Among the applications, we use fluorine as rat poison, chlorine and bromine to kill bacteria in water and iodine in hospital rooms to swab skin before surgery.
There is no known body requirement for fluorine or bromine yet we get large doses of fluorine in fluoridated water and toothpaste, and of bromine as a pesticide on fruits and vegetables and as a flame retardant. You are breathing bromine as it evaporates from your electronic components, and probably sleep on it in the flame retardant in your mattress.
But we do need chlorine and iodine, lest we die from deficiencies of these two elements. Chlorine is part of the hydrochloric acid our stomachs make to digest our food, and is part of white blood cells to be released to kill invading pathogens. Iodine is necessary for our bodies to make thyroid hormones which regulate the burn rate in our cells. Every cell in our bodies has receptor sites to attract iodine.
These four elements compete for receptor sites in our cells. Without industrial civilization we would be exposed to very little fluorine and bromine, and if we consumed chlorine and iodine in the proper ration our bodies would function quite well. But introduce fluorine and bromine and iodine gets driven out by the stronger electronegativity of fluorine and bromine which attach at the cell receptor sites that are there for iodine. Additionally, hydrochloric acid it diminished as we produce hydrofluoric acid instead; perhaps this is the common problem of acid reflux.
When we don't have enough iodine, a whole world of problems arise. Our thyroid glands can enlarge in an attempt to produce more hormone (goiter). Babies don't develop properly when deficient in iodine resulting in low intelligence and nervous system problems. When the other cells of our body are running too slow from lack of iodine, then they don't produce sufficient hormones so we open up a host of problems. Of particular interest is apoptosis, the programmed death of cells. When cells are slowed down this process goes array, and cancerous growth of cells results. Cancer cells are our cells that don't stop growing and don't die to be replaced.
The minimum requirement for iodine is set by government at 150 mcg, a very small amount. Yet we could easily get 8 to 10 mg a day of sodium fluoride and who knows how much bromine. Since fluorine and bromine have a higher electronegativity than iodine, they out compete iodine for receptor sites and the water soluble iodine gets flushed out, leaving us deficient. Is it any wonder that thyroid disease is common. Breast cancer, ovarian cancer, endometrial cancer, thyroid cancer, and prostate cancer plus the precursors to these diseases that involve enlargement, fibrous growth, and cysts are all related to iodine deficiency. When you brain cells are running at a low metabolic rate you are less intelligent which is why sodium fluoride reduces intelligence about 10 points.
We indeed are suffering much from industrial waste.
"Bend over and grab your ankles" should be etched in stone at the entrance to every government building and every government office.