Fluoride was used as an industrial chemical, a pesticide, and a medical treatment long before it became the cavity-fighting ingredient we associate with toothpaste and tap water. Its earliest applications had nothing to do with teeth. Sodium fluoride served as an insecticide, a rat poison, a wood preservative, and even a medication for overactive thyroid, all before the first city added it to drinking water in 1945.
Fluoride as an Industrial Byproduct
Fluoride compounds first entered large-scale production not because anyone wanted fluoride itself, but because it was an unavoidable waste product of two major industries: aluminum smelting and phosphate fertilizer manufacturing. ALCOA, the dominant American aluminum company in the early twentieth century, produced large quantities of sodium fluoride waste as part of its manufacturing process. Disposing of it was expensive, and the compound’s toxicity made it an environmental headache.
Fertilizer production created an even messier problem. Highly toxic hydrogen fluoride and silicon tetrafluoride gases are released when phosphate rock is processed. Before the 1970s, Florida fertilizer plants simply vented these gases into the atmosphere, giving central Florida some of the worst air pollution in the country. Farmers and ranchers eventually pressured manufacturers into installing pollution scrubbers during the 1960s. Those scrubbers converted the toxic vapors into fluorosilicic acid, a hazardous but more manageable liquid. That same liquid has been shipped from Florida fertilizer factories to water treatment plants across the United States for the past half century.
Pest Control and Poison
Sodium fluoride is toxic to essentially all living organisms, and people recognized this property early. It was widely used as an insecticide, a rodenticide (rat poison), a herbicide, and a fungicide for preserving timber. As an insecticide, its tendency to also kill plants limited it mostly to bait formulations rather than broad spraying. The commercial product ranged from 93 to 99 percent purity and was supposed to be dyed a distinct color to prevent people from confusing it with table salt or flour, a real risk given how similar the white powders looked.
A Treatment for Overactive Thyroid
In the 1930s, German physicians began prescribing fluoride compounds to patients with hyperthyroidism, a condition where the thyroid gland produces too much hormone. A 1937 German study reported that patients with Graves’ disease became symptom-free and able to return to work within six to eight weeks of treatment with a fluoride-based drug called Pardinon. Later research in 1958 confirmed the mechanism: daily doses of 5 to 10 milligrams of fluoride suppressed thyroid hormone production and lowered basal metabolic rate in hyperthyroid patients.
This use is largely forgotten today, replaced by more targeted thyroid medications. But it remains relevant to modern debates about fluoride safety, since critics point to the thyroid-suppressing effect as evidence that low-level fluoride exposure could affect thyroid function in healthy people.
How Fluoride Became a Dental Tool
The connection between fluoride and teeth was an accidental discovery. In the early 1900s, a dentist named Frederick McKay noticed that residents of Colorado Springs had severe brown staining on their teeth, a condition called “mottled enamel.” The staining was ugly, but McKay also observed something unexpected: those same stained teeth were remarkably resistant to cavities. It took decades of investigation before researchers identified naturally occurring fluoride in the local water supply as the cause of both the staining and the cavity protection.
The chemistry behind this is straightforward. Tooth enamel is made of a mineral called hydroxyapatite. When fluoride ions are present, they swap into the mineral’s crystal structure, replacing hydroxyl groups and forming a harder, more acid-resistant version called fluorapatite. This substitution is energetically very favorable, and the resulting mineral resists dissolving in two ways: the fluoride ions themselves stay locked in the crystal rather than washing away, and they bind surface calcium ions more tightly, making the whole structure harder for acid to break down.
The Grand Rapids Experiment
In January 1945, Grand Rapids, Michigan became the first city in the world to add fluoride to its public water supply. The compound used was sodium fluoride, sourced as a waste product of aluminum production. The trial ran for 15 years, comparing cavity rates in Grand Rapids children with those in the unfluoridated neighboring city of Muskegon. The results were striking: the cavity rate among children born after fluoridation began dropped more than 60 percent.
That single statistic drove rapid adoption. Cities across the country began fluoridating their water supplies through the 1950s and 1960s, transforming fluoride’s public image from industrial toxin and rat poison into a public health tool.
Fluoride Toothpaste Arrives
The success of water fluoridation prompted interest in putting fluoride directly into consumer products. In 1950, Procter & Gamble partnered with Dr. Joseph Muhler at Indiana University to develop a fluoride toothpaste. After more than 40 clinical trials confirmed it worked, the company launched Crest with Fluoristan (a stannous fluoride compound) into test markets in 1955, then expanded nationally in January 1956. It became the first commercially successful fluoride toothpaste and set the template for virtually every toothpaste sold today.
How Current Use Compares to Original Applications
The doses involved in modern dental use are orders of magnitude smaller than fluoride’s original applications. The U.S. Public Health Service currently recommends a water fluoride concentration of 0.7 milligrams per liter, a level chosen to maximize cavity prevention while minimizing dental fluorosis (the faint white speckling that echoes the brown staining McKay first noticed in Colorado). Compare that to the 5 to 10 milligrams per day used to suppress thyroid function in medical patients, or the concentrated sodium fluoride formulations sold as rat poison.
Fluoride’s journey from aluminum waste and pest control agent to a fixture of preventive dentistry is one of the stranger transformations in public health history. The same toxicity that made it effective at killing insects and rodents, its ability to interact powerfully with biological systems, turned out to protect teeth when applied at far lower concentrations.