Why do I need fluoride in my water?

Don’t worry, you won’t get too much fluoride from brushing your teeth and drinking the water in Aotearoa.

Before your baby’s first tooth even appears the nurse might hand you a teeny toothbrush and a tube of toothpaste with fluoride in it.

This moment marks the start of a child’s lifelong teeth-brushing journey – one made all the more important for those who live in a region where the water isn’t fluoridated.

But why, when we’re following health advice to brush twice daily with fluoride toothpaste, do we need fluoride in our water? The issue again made headlines when Dr Ashley Bloomfield left the Ministry of Health in July. One of his last acts as directorgeneral of health was an order for 14 councils to start fluoridating drinking water – a move that means 60% of the population will have fluoridated water. The order follows a 2021 law change that removed the responsibility from local authorities.

Fluoride is a naturally occurring mineral that’s already in our water and food. It’s good for teeth because it strengthens the enamel to protect teeth from decay, and when minor damage does occur, it helps teeth heal. The combination of brushing with fluoride toothpaste and having it in the water supply means there is a constant low level of fluoride in saliva helping to protect teeth from damage caused by things like sugar.

But, the amount of fluoride naturally occurring in New Zealand water isn’t enough to have a protective effect. That’s where water fluoridation comes in.

There’s naturally about 0.2mg of fluoride per litre of water, so councils increase it to between 0.7mg/l and 1mg/l – the level recommended by the World Health Organisation (WHO). Simply put, we need it in both because not everyone brushes their teeth properly.

A 2009 Oral Health Survey found only 43% of Kiwi kids were brushing twice daily with fluoride toothpaste, and only 65.3% of adults.

While toothbrushing is up to the individual, water reaches everyone – and people living with fluoridated water have significantly less decay than those in non-fluoridated areas. New Zealand dental experts say having it in the water allows for a population approach to protecting teeth. And everybody benefits.

In the United States the Centres for Disease Control and Prevention found fluoridated water reduced tooth decay by about 25%, and labelled fluoridated water one of the 10 great public health achievements of the 20th century.

But is it possible to get too much? Some groups claim – loudly – that fluoride can cause fluorosis (discolouring of teeth), negatively impact people’s IQ and cause bone fractures and cancer.

This is not going to happen as a result of drinking water in New Zealand – to reach a toxic level an adult man would have to drink more than 1500 cups a day. There’s no significant link between fluoridated water and cancer, and most of the studies into the impact on IQ come from China – where levels of fluoride are up to 20 times higher than New Zealand, and exposure to other substances, including arsenic, have been ignored. AWHO report states that in areas where there have been cases of skeletal fluorosis and bone fractures, people were consuming 14mg of fluoride a day. To consume 10mg in New Zealand you would need to drink 40 cups.

As for dental fluorosis, a 2014 review of the scientific evidence by the Royal Society of New Zealand and the Prime Minister’s chief science adviser found this is possible from drinking water fluoridation in New Zealand, but even then it was not at a high enough level to be noticeable.

❚ Reporting disclosure statement: This story was written with expert advice from New Zealand Dental Association spokesman Dr Rob Beaglehole. It was reviewed by The Whole Truth: Te Māramatanga expert panel member Associate Professor Lisa Te Morenga.