Fluoride in Drinking Water: What It Does, How It Works, and Who Decides

Why this note exists

Fluoride has been added to drinking water in parts of New Zealand for decades. It is one of the longest-running population-wide public health interventions — and one of the most contested.

This note aims to explain:

• what fluoride does in the body when ingested
• whether ingestion actually matters, or if the benefit is mainly topical
• what substance is added to New Zealand’s water, and where it comes from
• how fluoridation decisions are made in New Zealand

This is an explanatory note, not a verdict.

What fluoride is used for

The stated purpose of water fluoridation is to reduce tooth decay (dental caries), particularly in children.

Fluoride helps by:

• strengthening tooth enamel
• promoting remineralisation
• reducing damage from acids produced by oral bacteria

These effects are well established in dental science.

Topical vs systemic fluoride: does ingestion matter?

What modern evidence agrees on

The primary benefit of fluoride is topical.

That is:

• fluoride works mainly by contact with the surface of teeth
• this occurs via toothpaste, mouth rinses, and fluoridated water washing over teeth

This view is widely accepted in contemporary dentistry.

What ingestion was originally thought to do

Early fluoridation policy assumed that swallowing fluoride during childhood helped incorporate it into developing teeth.

More recent evidence suggests:

• systemic ingestion plays a secondary role
• once teeth have erupted, benefit is overwhelmingly topical

This reframes the central question:

If the benefit is mostly topical, is ingestion necessary?

The policy answer has been “yes” — but that answer rests on assumptions about convenience, equity, and population coverage rather than biological necessity alone.

What happens when fluoride is ingested?

Absorption

• Approximately 75–90% of ingested fluoride is absorbed
• Absorption occurs mainly in the stomach and small intestine
• In the acidic stomach, fluoride more readily forms hydrogen fluoride (HF), which crosses cell membranes efficiently

Distribution

Once in the bloodstream, fluoride distributes rapidly:

• ~99% of retained fluoride accumulates in bones and teeth
• the remainder circulates transiently in soft tissues

Excretion

• About 50% of absorbed fluoride is excreted via the kidneys
• Retention is higher in:
  • children (due to bone growth)
  • people with reduced kidney function
  • individuals with acidic urine

Long-term storage

• Fluoride incorporates into bone mineral as fluorapatite
• At high exposures, this can increase bone density while reducing flexibility
• Chronic excessive exposure causes skeletal fluorosis
• At recommended drinking-water levels, skeletal fluorosis is not observed in New Zealand

Effects on the gut

At drinking-water concentrations:

• most healthy adults tolerate fluoride without obvious gastrointestinal symptoms

At higher intakes:

• nausea
• abdominal discomfort
• gastric irritation

Fluoride has antibacterial properties, and animal or laboratory studies show it can affect gut bacteria at sufficient doses.
However:

• human evidence at fluoridated-water levels is limited
• there is no strong consensus that community fluoridation meaningfully disrupts the human gut microbiome

This remains an evidence gap, not a settled finding.

Effects on other systems

Brain

• Fluoride can cross the blood–brain barrier
• High-dose animal studies show neurotoxic effects
• Epidemiological studies at very high natural fluoride levels show associations with lower IQ
• Evidence at fluoridated-water levels is mixed and debated

There is no clear causal mechanism established at low doses, but the topic remains under study.

Thyroid and endocrine system

• Fluoride can interfere with iodine uptake at pharmacological doses
• Historically, it was used to treat hyperthyroidism
• At drinking-water levels, most studies show no clear thyroid effect in iodine-sufficient populations
• Potential concern is higher in iodine-deficient individuals

What substance is added to New Zealand’s water?

In New Zealand, fluoridation typically uses:

• hexafluorosilicic acid, or
• sodium fluorosilicate

These are synthetic fluorine compounds, not naturally occurring calcium fluoride.

Where they come from

• They are by-products of phosphate fertiliser production
• Captured from industrial processes
• Purified to meet drinking-water standards
• Imported into New Zealand

This does not imply they are unsafe — they are diluted to very low concentrations and regulated — but it does mean fluoridation relies on adding an industrial compound, not a naturally occurring mineral.

Who decides on fluoridation in New Zealand?

Historically

• Fluoridation decisions were made by local councils
• Many communities held referendums
• Some areas fluoridated, others did not

This reflected local values and priorities, but resulted in uneven national coverage.

What changed in 2021

In 2021, legislation transferred fluoridation decision-making from local councils to the Director-General of Health.

At the time, that role was held by Ashley Bloomfield.

Under this framework:

• local referendums no longer determine fluoridation
• councils can be directed to fluoridate
• decisions are made centrally, based on public health advice

The stated rationale was:

• reducing oral-health inequities
• standardising protection
• avoiding repeated local disputes

This was a governance change, not the result of new scientific discovery.

Why fluoridation remains contested

The debate is not primarily about whether fluoride reduces tooth decay.

It centres on:

• ingestion vs topical delivery
• universal exposure vs targeted benefit
• individual consent (water is unavoidable)
• cumulative exposure (water + toothpaste + food)
• central authority vs local decision-making

Reasonable people weigh these factors differently.

What the policy assumes

Water fluoridation assumes:

• low-dose lifelong ingestion is safe
• population benefit outweighs loss of individual choice
• centralised decisions are preferable to local variation
• topical and systemic exposure together provide sufficient benefit

Public health authorities consider these assumptions acceptable based on current evidence, while acknowledging ongoing areas of uncertainty.

Closing

Fluoride reduces tooth decay.
Most of that benefit is topical.

Water fluoridation delivers fluoride systemically, to everyone, as a matter of policy. Understanding how fluoride works, what is added, and who decides helps move the discussion beyond slogans — in any direction.