Currently, about two-thirds of US households have fluoridated water, and the CDC has made nationwide fluoridation of drinking water a top priority for the prevention of cavities. The use of fluoride, however, is something of a hot button issue. For example, a recent study showed that consumer-generated videos versus industry-generated videos on YouTube were much more likely to have an anti-fluoride sentiment. This is not new. In 1945, Grand Rapids, Michigan, became the first US city to implement community fluoridation of the water supply, and in the decades that followed, several conspiracy theories developed around the practice, including speculation of a plot to impose a communist regime in the United States.

Of course, fluoride is not just found in tap water. It's also found naturally and fortified in certain foods, can be found in varnish or gels at the dentist's office, and is a common ingredient in mouth rinses and toothpastes. So how does fluoride prevent cavities, and what does the research say about the benefits of its use?

Now, in general, there's bacteria on your teeth, which can form micro-colonies. And when these micro-colonies coalesce, it creates a layer of dental plaque. Bacteria have a hard time getting into the tooth due to the outer layer of enamel, which is composed of a hard substance called hydroxyapatite, a type of calcium phosphate crystal with the chemical formula Ca10(PO4)6(OH)2.

Now, if the bacteria on the tooth's surface start to overgrow, they can metabolize sugary foods and drinks and generate acid, which can cause the hydroxyapatite to break down. Without hydroxyapatite, the enamel surface of the tooth can weaken and allow bacteria to enter, causing permanent damage. This is called a cavity. If it goes on long enough, the bacteria can infect the root of the tooth, and this can be extremely painful.

So this is where fluoride comes in. Fluoride can replace the hydroxyl group in hydroxyapatite to create fluorapatite, with the chemical formula Ca10(PO4)6(F)2. In general, fluorapatite is more dense and less soluble than hydroxyapatite, so it's less likely to demineralize. Fluoride also accumulates in your teeth over time, so while fluoride treatments are useful throughout your life, treatments are generally more important during childhood and adolescence.

Alright, so how well do fluoride treatments work? One meta-analysis that looked at twenty studies exploring the effect of fluoridation of drinking water on cavities in children found that fluoride in the water leads to a thirty-five percent reduction of tooth decay of baby teeth, and a twenty-six percent reduction of tooth decay of permanent teeth. Another meta-analysis looked at twenty-two studies exploring the effect of applying a fluoride varnish to children's teeth every three to six months at the dentist's office, and found a thirty-seven percent reduction in baby teeth and a forty-three percent reduction in tooth decay in permanent teeth. A third meta-analysis looked at twenty-five studies focusing on fluoride gel treatments given at least one time a year at the dentist's office, generally to children older than six years old, which showed a reduction of twenty-eight percent in tooth decay. A fourth meta-analysis looked at thirty-five studies on the effect of fluoridated mouth rinses given daily to every other week, and found a twenty-seven percent reduction in tooth decay.

Now, if we look at meta-analysis of studies that focus on fluoridated toothpaste, one found that it decreases tooth decay, and the effect is stronger with higher fluoride concentrations in the toothpaste, higher frequency of tooth brushing, and supervised brushing. This was further supported by another meta-analysis that looked carefully at the amount of fluoride in the toothpaste. This study showed that brushing with fluoridated toothpaste with at least 0.1 percent concentration of fluoride significantly prevents tooth decay in children and adolescents aged sixteen years or less. There is a twenty-three percent reduction in tooth decay at concentrations of 0.1 percent to 0.125 percent fluoride, and up to a thirty-six percent reduction in concentrations ranging from 0.24 percent to 0.28 percent fluoride.

Now, while fluoride can be toxic at doses well above those used in dental hygiene, the main concern with using fluoride has generally been fluorosis. Fluorosis is not a disease, but rather a cosmetic condition caused by overexposure to fluoride. Fluorosis is very common, with up to forty-one percent of children and adolescents having it in some form. Usually it takes the form of subtle white patches on the teeth that are barely noticeable. In moderate to severe cases, which occur in less than four percent of the population, there can be significant mottling of the teeth with brown staining. While fluorosis is not generally a health concern, it can lead to social stigma.

A meta-analysis of twenty-five studies looked at whether brushing teeth with fluoridated toothpaste is linked to fluorosis, and found that brushing the teeth of an infant under one year may increase the risk of fluorosis, but that relationship was weak. There was a stronger relationship for children between the ages of twelve months and six years of age. If these children brushed with a toothpaste that was higher than 0.1 percent fluoride or more, they had a thirty percent chance of developing fluorosis. This means that the risks of tooth decay, which are decreased over fluoride concentrations of 0.1 percent fluoride or more, must be balanced with the risk of fluorosis, which increases at fluoride concentrations over 0.1 percent fluoride.

Alright, as a quick recap: Tooth decay and dental cavities are one of the leading health concerns around the world. In general, fluoridation of the water supply, and applying fluoride to the teeth via varnishes, gels, mouth rinses, and toothpastes, has helped decrease tooth decay. While fluoride is generally considered safe at the concentrations used in fluoridated products, overexposure can lead to fluorosis. In some cases, risking fluorosis may be preferable to risking tooth decay in children at high risk for tooth decay; therefore, it may be beneficial to discuss all treatment options with your dentist.

In addition, the American Dental Association recommends preventing overexposure by brushing children's teeth with only a grain-of-rice-sized smear of fluoridated toothpaste between when the teeth begin to erupt to three years of age, and up to a pea-sized bead of toothpaste for children over three years old. For children that are old enough to brush their own teeth, it's also a good idea to monitor their brushing to decrease the likelihood that they swallow the toothpaste, which can also lead to overexposure and fluorosis.