There are many things that could be added to water to lower dental cavities, if people think it's appropriate to use additives in drinking water.
Aloe Vera, a plant, may be as effective as chlorhexadine, a dental chemical used in mouthwashes, to reduce cavities.
But for now fluoride is generally added to water, and there are reasons to question why it is added to drinking water.
Tap water is generally used for drinking by poorer people. In the United States the per capita consumption of bottled water is over 40 gallons per year. Poor people obviously buy much less than 40 gallons and well off people buy more, but the average is 40 gallons or so.
Bottled water consumption has been rising dramatically, and in 2017 bottled water pulled ahead of soda and other drinks to become the most consumed beverage.
Water distiller sales likewise have increased in recent years, and like bottled water distillers are generally used by people in better financial circumstances.
Several years ago some Harvard scientists did a meta study looking at the effects of fluoride on intelligence. They found that increased fluoride consumption by children was associated with lower iq.
Fluoride supplementation is common in the United States, and their study was not widely reported except in alternative media. Two scientists criticized the study based largely on how significant the results were, in other words the two critics felt the significance was exaggerated.
The original authors then responded to that criticism.
Worth noting that the studies used were mainly from China, because for whatever reason there simply are no similar studies on fluoride done in the United States.
Fluoride is an abundant element in the earth's crust. Just by eating almost any natural food you will get plenty of fluoride.
Fluoride at higher levels is associated with problems. There is no benefit to consuming too much fluoride, except fewer cavities, but there are some severe consequences to consuming too much fluoride, even aside from potentially reduced iq.
The Dairy industry lobby in the United States is quite powerful.
As a result of pushing dairy on the public, calcium consumption has risen, even if it is not in a form suitable for adults.
One of the important variables in fluoride toxicity is calcium level. In other words if you are deficient in calcium you are more susceptible to certain more severe problems from higher fluoride consumption.
If you Google "fluorosis" in the United States, a symptom of too much fluoride, most of the results involve the teeth. The first Wikipedia link says
"People with fluorosis are relatively resistant to dental caries (tooth decay caused by bacteria), although there may be cosmetic concern. In moderate to severe fluorosis, teeth are weakened and suffer permanent physical damage."
In other words the suggestion is that if too much fluoride were a problem for a specific individual, there would be plenty of warning e.g. discolored teeth etc.
But are there other manifestations of fluorosis? Actually there are, but again you have to look at studies done in other countries.
"Epidemiological studies during 1963-1997 were conducted in 45,725 children exposed to high intake of endemic fluoride in the drinking water since their birth. Children with adequate (dietary calcium > 800 mg/d) and inadequate (dietary calcium < 300 mg/d) calcium nutrition and with comparable intakes of fluoride (mean 9.5 +/- 1.9 mg/d) were compared. The toxic-effects of fluoride were severe and more complex and the incidence of metabolic bone disease (rickets, osteoporosis. PTH bone disease) and bony leg deformities (genu valgum, genu varum, bowing, rotational and wind-swept) was greater (> 90%) in children with calcium deficiency as compared to < 25% in children with adequate calcium who largely had osteosclerotic form of skeletal fluorosis with minimal secondary hyperparathyroidism."
That abstract ends with
"Deep bore drinking water supply with fluoride < 0.5 ppm and improvement of calcium nutrition provide 100% protection against the toxic effects of fluoride and are recommended as the cost effective and practical public health measures for the prevention and control of endemic fluorosis."
In other words the 'solution' to skeletal / dental fluorosis is to make sure water has less than 0.5 ppm of fluoride, and make sure people have enough calcium.
Water in the United States has fluoride added to bring it up to 0.7 to 1.2 ppm. The suggested standard was recently lowered to 0.7 ppm.
So there is a solution to skeletal, including dental fluorosis.
The solution is to consume more calcium.
Unfortunately for this 'solution' though, the neurological and skeletal system have very different relationships with calcium. While calcium effectively 'dilutes' fluorine in the skeleton, it does not do the same in the nervous system.
So 'treating' skeletal fluorosis with calcium supplementation, something the dairy industry has encouraged, however indirectly, effectively masks non physical, neurological symptoms of fluorosis.
Are there indications of a possible problem with neurological symptoms of fluorosis caused by fluorinating water? Again, you have to look to other countries for the research.
In Mexico a reliable study showed that higher prenatal fluoride exposure led to higher ADHD symptoms, specifically the attention deficit part.
But wait, it gets worse.
Here is the full abstract.
"Experience in practice has suggested that children with attention-deficit hyperactivity disorder (ADHD) tend to have higher numbers of diseased, missing and filled teeth (DMFT score) than children without the condition. To date, however, this impression has not been systematically investigated. A case-control study of children in Otago, New Zealand (case DMFT > or = 5; control DMFT <5; case status determined from School Dental Service dental records) was conducted by postal survey and reference to the School Dental Service records. Cases and controls were matched on age, sex, ethnicity, and school socio-economic status. The purpose of the study was to assess whether having ADHD was associated with higher odds of having high caries experience. Questionnaires were returned for a total of 128 case-control pairs. Conditional logistical regression analysis showed that, after controlling for fluoride history, medical problems, diet, and self-reported oral hygiene, children with ADHD had nearly 12 times the odds of having a high DMFT score than children who did not have ADHD (OR = 11.98; 95% CI 1.13, 91.81). No other factors were significant predictors. Dental practitioners and parents should consider ADHD to be a condition that may affect children's dental caries experience."
New Zealand has fluorinated their water supply for many decades under the guise that their drinking water was 'low' in fluoride. This has led to a sort of competition between real science and government controlled pseudo science.
There are many, many variables involved in any chemical's activity in the body.
Common sense would suggest a researcher would look at different effects from different fluorine compounds, synergies between fluorine and other minerals, etc. But 'authorities' don't need common sense. Who needs a bank account if they have a gun? No matter what 'science' shows, people with authority can conquer it.
So, again, from the above abstract.
"...diseased, missing and filled teeth (DMFT score) ....after controlling for fluoride history, medical problems, diet, and self-reported oral hygiene, children with ADHD had nearly 12 times the odds of having a high DMFT score than children who did not have ADHD
At this point, science says fluoride increases the 'attention deficit' part of ADHD, but not necessarily the hyperactivity. See the Mexican study above as an example.
But here we have New Zealanders with both a very strong indicator of low fluoride i.e., rough teeth, and a moderate indicator of higher fluorine exposure than their peers i.e., ADHD.
The study is from 2004 and appears to be the result of scientific idiocy.
What they were trying to demonstrate, and why, could probably be uncovered with a little research.
tldr / Fluoride taken with calcium hides skeletal fluorosis, and skeletal fluorosis is a visible warning sign for neurological fluoride toxicity, so the net effect of giving dairy to people supplemented with fluoride is to hide neurological symptoms.