What is fluoride?

Fluoride is a mineral found widely in nature, released from rocks into soils and water. Certain plants – including plants consumed by humans, such as tea – absorb and concentrate fluoride.

“It’s not an industrial waste product. It’s a natural substance,” said Prof Oliver Jones, a professor of chemistry at RMIT University.

Fluoride has been added to drinking water and toothpaste for decades because it strengthens teeth and prevents cavities.

When fluoride is incorporated into tooth enamel, it forms a mineral known as fluorapatite, which is better able to withstand acids, said Prof Loc Do, a professor of dental public health at the University of Queensland. “It makes the enamel more resistant to bacteria that are causing decay,” he said.

The World Health Organization’s safe limit for fluoride in drinking water is 1.5 milligrams of fluoride per litre of water – or 1.5 parts per million. The typical concentration of fluoride in regular toothpaste is between 1000 and 1500 parts per million.

How long has fluoride been added to drinking water?

In the early 1900s, dentists noticed that rates of tooth decay were lower in areas where people had splotchy staining on their teeth – a condition caused by excess fluoride and now known as dental fluorosis. They realised that areas with naturally higher fluoride levels in water had lower rates of dental decay.

In 1945, Grand Rapids, Michigan, became the first place in the world to add fluoride to its drinking water. Fluoridation in the US is not mandatory and occurs on a voluntary basis, but by 2012, more than 67% of the overall US population had access to fluoridated tap water.

In 2011, the US Health and Human Services Department lowered its recommended level of fluoride in drinking water to 0.7 milligrams of fluoride per litre – 0.7 parts per million. For five decades before that, the upper limit was 1.2.

Does fluoride in water have any health effects? Is it safe?

A number of international health agencies have endorsed fluoride in drinking water as effective and safe. The US Centers for Disease Control and Prevention named fluoridation one of the 10 great public health interventions of the 20th century.

Contrary to Kennedy’s claims, studies have found no evidence linking fluoride in regular drinking water to conditions such as osteoporosis, bone fractures or cancer.

As with any chemical compound, ingestion of fluoride at high levels can have adverse effects, such as dental fluorosis – a condition indicated by staining of the teeth – and skeletal fluorosis, a condition in which fluoride is incorporated into the bones. These only occur with chronic exposure to fluoride at far higher levels than the safe drinking limits.

“Everything is a poison at high enough levels – even water,” Jones said.

In August, the US’s National Toxicology Program released a review suggesting a link between fluoride exposure above 1.5 milligrams of fluoride per litre of water and lower IQ in children. However, experts have pointed out that there is a lack of high quality research on the topic.

“Some studies … were considered as having a high risk of bias in other systematic reviews,” Do said. Do’s own research has found no association between drinking fluoridated water before age 5 and changes in behavioural development or executive functioning.

What countries add fluoride to water?

Dozens of countries worldwide artificially fluoridate drinking water. As of 2012, about 435 million people globally have access to water with fluoride at recommended levels.

The US, UK, Canada, New Zealand, Australia and Brazil are all among the countries that add fluoride to water in at least some regions.

Do said adding fluoride to water was a good method to reduce socioeconomic inequality in dental health. “It’s a passive way to reduce dental caries.”

What could change if Donald Trump wins the US election?

Kennedy, who Donald Trump has promised to put in charge of health initiatives, said on Saturday that the Republican would push to remove fluoride from drinking water on his first day in office if elected president.

Kennedy has emerged as one of Trump’s top surrogates after dropping out as independent candidate. On Sunday, Trump told NBC News that he and Kennedy not spoken about fluoride, but that the plan “sounds OK to me.”

Why has fluoride been removed from water in the past and what was the effect?

Authorities in several jurisdictions have removed fluoride from drinking water by public mandate, such as in Calgary, Canada in 2011, or by ministerial decree, such as in Israel in 2014.

Research shows that dental cavities in children increased in Calgary after the end of fluoridation, while the rates of dental treatment significantly increased in Israel. Calgary is set to re-introduce fluoridation in 2025.

Germany stopped fluoridating its water in the 1970s and never reintroduced it, but since 1991 has sold salt with added fluoride.