Hidden behind the security fences inside Dunedin's Mt Grand water treatment plant, a small piece of metal inside a plastic pipe slowly rotates.
The metal device is called an auger, and its job is simple - it chips away tiny pieces of a soft-looking white substance oozing from inside the pipe.
The white flakes drop into another pipe and disappear, whisked away to mix with Dunedin's drinking water.
It is a process that continues throughout each day, and it is supposed to be a good thing.
The white flakes are bits of sodium silicofluoride - or, simply, fluoride - a chemical added to Dunedin's drinking water since 1966, and now piped to 85% of the city's homes.
It is a public health measure credited by health professionals with reducing rates of tooth decay by 15%, and is described by the United States-based Centres for Disease Control and Prevention (CDC) as one of the top 10 public health achievements of the 20th century.
But there is another view - held by some internationally and by a small but committed band of campaigners in Dunedin - that sees fluoridation not as a saviour, but as unwanted "mass medication".
Put simply, opponents fear all who drink fluoridated water risk being poisoned.
It is a bold claim that has sparked heated exchanges, both within the Dunedin City Council's chambers, in email correspondence and through newspapers' letters to the editor sections across the country.
It appears a David and Goliath struggle - those opposed to fluoride are taking on the weight of Dunedin's medical institutions, including the University of Otago School of Dentistry, Public Health South and the Otago District Health Board, as well as the Ministry of Health, who all support water fluoridation.
However, anti-fluoride campaigners can claim some successes, after arguing their case with such vehemence that Dunedin city councillors voted for a "compromise" agreement on the city's fluoridation policy at a meeting on November 3.
After weeks of lobbying, many councillors were left admitting they could not decide between the various claims and counter-claims. Instead, they voted to maintain the existing split between fluoridated and non-fluoridated areas of the city, and committed to community consultation before any changes.
However, questions remain about those behind the campaign against fluoridation - among other things, who are they? And how would they know?
In Dunedin, Olive McRae (26) is one of those leading the charge against fluoride. She is the Dunedin representative of Fluoride Action Network New Zealand (FANNZ), a campaign group and incorporated society that emerged in Wellington in 2003, affiliated to the older international Fluoride Action Network.
Ms McRae formed the Dunedin branch last year and has since been actively campaigning against water fluoridation across Otago. Her voice has been heard by Dunedin's councillors, health experts and residents across the city.
However, Ms McRae conceded, when interviewed by the Otago Daily Times, she had no medical or dental, qualifications.
"I'm a full-time mother," she said. "I have no medical or dental expertise."
Despite that, she has for the past year mounted a persistent counter-argument while enjoying a quiet rural existence in a small organic community at the Volco Organic Farm on Mt Cargill, on the road to Waitati.
Surrounded by vegetable plots and a variety of animals, the 16-strong community aimed to be self-sufficient in all ways, including water supply, she said. The community collected rainwater, topped up by nearby springs as required, with not a single flake of fluoride added.
It was a choice of lifestyle that went hand-in-hand with her campaign against fluoridation: "It fits in with the objectives of living in this place," she said.
It is also a position that fits in with motherhood, and the belief her 3-year-old daughter is at risk, she said.
"When I became a mother, I decided to make the choice not to medicate my daughter as the risks far outweigh any perceived benefit.
"I am lucky enough to be able to make an informed choice about the medication I expose my daughter to. Unfortunately, many parents do not have the information available to them in order to make an informed choice."
She compared her branch, and the national FANNZ movement, to other non-profit community organisations established by concerned citizens.
"We are all volunteers with no vested interests other than the health and personal rights of the people in our communities.
"We feel strongly about the issue of fluoridation as we believe it is harmful to health and that every citizen should have the right to a public water supply free from medication."
Her position on fluoride is made immediately obvious to visitors to the farm. Parked outside is a small, beaten-up van covered in anti-fluoride slogans including Ms McRae's email address: nomassmedication@gmail.com.
Ms McRae conducts her campaign from home, pulling together research by international organisations, such as the National Academy of Science and American Dental Association, peer-reviewed studies and reviews.
"I do not do any medical or scientific research as I am not qualified to do so. I gather information that is readily available to the public, yet needs to be highlighted more," she said.
It is also from this secluded headquarters that a controversial pamphlet emerged in February with an explosive claim. Bone cancer, it suggested, was caused by drinking fluoridated tap water.
The pamphlet, prepared with support by FANNZ and including Ms McRae's contact details, was distributed in Waitati, Warrington and Seacliff before the pending extension of the Dunedin City Council's fluoridated northern water scheme into Waikouaiti.
The pamphlet cited the case of a 17-year-old who died from osteosarcoma - bone cancer - and said there was "an 85% chance" the teen's condition was caused by water fluoridation.
"Would you sacrifice your son's life to cancer because some people won't brush their teeth?" it asked, before offering its own answer: "Fluoridation - it's not worth the risk."
The pamphlet appeared to have the desired effect, with a turnout of up to 50 people at a public meeting held a few days later in Waitati.
However, it also had a second, perhaps unintended, result. The New Zealand Dental Association complained to the Advertising Standards Authority, alleging the pamphlet's claims were "highly misleading" and amounted to scaremongering.
In its decision released last month, the Advertising Standards Authority agreed, saying the pamphlet made "unsubstantiated health claims" and presented "misleading" research results that "played on fear" in breach of the ASA's rules and code of ethics.
The national co-ordinator of FANNZ, Wellington-based Mark Atkin, accused the ASA of double standards, saying district health boards were still allowed to publish "factually incorrect information" about fluoride - and questioned the ASA's jurisdiction.
He refused to take part in the ASA's proceedings or submit evidence substantiating the pamphlet's claims, but continued to insist the claims were true.
"The reader of an advertisement cannot be misled if the claims are true, as they are," he said.
"The fact we have not provided verification. . . does not affect the truthfulness of the statements."
But Ms McRae insisted her organisation always endeavoured to present accurate information to Dunedin residents, and had the support of "many individuals and organisations throughout Dunedin and Otago" including local iwi, the organic sector and other community groups.
There were also "at least four dentists" and "a number of doctors" within Dunedin who opposed fluoridation, but felt their professional reputations were at stake if they spoke publicly, she said.
And, nationally, FANNZ's membership included doctors, dentists and other health professionals, PhD research chemists, lawyers, and other professionals, she said.
However, Ms McRae is surprisingly cagey when it comes to discussing her own organisation. She will not give telephone interviews, insisting questions are emailed in advance for written responses, and will divulge neither the size of the Dunedin membership nor the backgrounds of those involved.
"It's a committee decision that we don't give out any details about our members personally, our members' numbers or anything else," she said.
National co-ordinator Mr Atkin, in a series of email exchanges with the ODT, also refused to provide a detailed membership breakdown, saying only membership was "substantial". He declined to comment when asked why exact details were withheld.
"I have given you a full response to your inquiry at a level that is appropriate and relevant," was his final word on the request. It is an approach that doesn't wash with some Dunedin city councillors, elected, as they are, to weigh the evidence on both sides of the fluoridation argument before making a decision.
Some have received upwards of 50 emails in recent weeks from those on both sides of the argument, before - and immediately following - the "compromise" vote on November 3.
However, Cr Michael Guest - a staunch supporter of fluoridation - told the ODT research by anti-fluoride campaigners was "tunnel-visioned and dishonestly selective", and did not believe the anti-fluoride movement represented Dunedin public opinion.
"I say the evidence is not balanced. I recognise people have strong views and that a lot of people are anti-fluoride, but unfortunately, I believe a lot of people have been unjustifiably scared off by false and unreasonable claims by those in the anti-fluoride lobby."
And Cr Richard Walls agreed, saying: "I don't see them as representative at all. There's always just two or three people - there's never been a large group of people," he said.
"Those that have been getting in touch with us are very small in number. There don't seem to be many fronting it in Dunedin. Until they disclose their numbers that's the opinion I have."
It is a concern that appeared to be shared by Public Health South officials and the New Zealand School and Community Dental Services Society.
A recent report to councillors warned PHS officials were worried any city-wide referendum could be co-opted by "extreme" views and "emotive publicity".
"If the community as a whole is not well informed, one or more interest groups may significantly influence the decision," the report said.
And, following the November 3 council meeting, the New Zealand School and Community Dental Services Society issued a press release congratulating Dunedin councillors for - essentially - voting to preserve the city's existing fluoridated water supply.
"It was a sensible decision, but also courageous in many respects, because supporting water fluoridation always attracts criticism from a small but vocal minority," society president Dr Neil Croucher said.
But Ms McRae continues to call for a city-wide referendum on the future of fluoridated water, as a step towards achieving a public water supply "free from medication".
"We don't believe the Dunedin councillors have a democratic mandate to fluoridate," she said. gained just 99 votes, the lowest of eight candidates.
However, the city's anti-fluoride campaigners do have their supporters - both within the community and among city councillors. Debates within council chambers have, at times, become heated, and some votes have been close.
When Cr Guest described anti-fluoride campaigners as "akin to quacks and snake oil merchants" during a committee meeting last month, an angry Cr Fliss Butcher retorted by raising the spectre of thalidomide while arguing against "mass medication".
Another, Cr Paul Hudson, told the ODT he had been researching fluoride and its effects since the 1980s and was opposed to the chemical's use.
"I'm not satisfied we are doing the best thing as far as our citizens are concerned," he said. "We have moved on a lot in the last 40 years."
He supported calls for a city-wide referendum on fluoride's continued use, and planned to raise the issue again at the council's next infrastructure services committee meeting on November 24.
"If we have referendums associated with rubbish and drainage, surely we should be consulting people about what we are compulsorily putting into their drinking water," he said.
Nor is Ms McRae alone on the campaign trail. Among others willing to put their names to the anti-fluoride campaign is Dr Bruce Spittle (63), a retired Dunedin psychiatrist turned fluoride researcher. He is the only Dunedin member of the International Society for Fluoride Research, an organisation he describes as neutral and committed only to publishing peer-reviewed research.
Dr Spittle was managing editor of the society's quarterly journal, which was distributed to a mailing list of "about six" people in New Zealand and institutions including the New Zealand's parliamentary library and the University of Otago, he said.
When he did speak out - something he did frequently through emails to councillors and newspaper letters - Dr Spittle said he did so as an individual, not a representative of the international society for which he worked.
He also did so as someone who "can't claim any dental qualifications", but was well schooled in the fluoride debate through his own research, and by attending international fluoride conferences, he said.
Dr Spittle said there were "maybe half a dozen" committed and active anti-fluoride campaigners in Dunedin, but insisted the movement was gaining momentum and public support.
"The people who don't like it [fluoride] are really quite numerous. It's the nature of things that only a small group of people start off trying to change things. The reason they make the noise is they are sure this is something that needs changed.
"The evidence is just not being addressed. It's being labelled as junk science, but it really isn't like that. I think there's an attempt to discredit the people bringing it forward."
Some councillors certainly agreed the argument, at times, became too personal - more diatribe than debate - although exact descriptions varied: Cr Hudson thought correspondence was "actually . . . quite good"; Cr Dave Cull described it as "passionate" and "rather vociferous"; Cr Neil Collins said it was "at times . . . most unsavoury"; and Cr Walls labelled it "quite offensive" and "quite patronising".
Ms McRae distanced her branch from the criticism of councillors, and said she, too, had been the victim of personal attacks.
Hate mail had been sent to her personal address and the tyres of her van had been slashed.
"FANNZ does not in any way promote abuse or threatening behaviour. We despise it," she said.
It is a debate that shows no sign of letting up - despite council's November 3 vote, fluoride is again likely to crop up when Cr Hudson goes into battle for a city-wide referendum later this month.
And there appears to be no easy solution.
As Cr Cull explained: "There's no possible compromise that will satisfy both sides. Either the stuff is in our water or it's not.
"That's the tricky part."
WATER FLUORIDATION
What is fluoride?
• A naturally-occurring chemical, also sourced from the aluminium industry, added to public water supplies at between 0.7 and 1 part per million to prevent tooth decay. Credited with a 30% to 40% reduction in tooth decay in New Zealand.
Use of fluoride:
• Early 1900s: US-based dentists note patients consuming water with high naturally-occurring levels of fluoride show signs of enamel mottling (dental fluorosis), but also resistance to tooth decay.
• 1962: Following positive field studies in the 1940s-'50s, showing a 50% to 70% decline in tooth decay, fluoride concentration in drinking water of 0.7-1.2ppm is recommended.
• Fluoridation adopted as public health measure in many US cities, endorsed by American Dental Association, the American Medical Association and World Health Organisation. (Source: Centres for Disease Control and Prevention.)
• Water fluoridation begins in New Zealand with 1954 Hastings trial.
• Controversy leads to New Zealand commission of inquiry in 1956-57, considering health benefits and possible dangers, which supports fluoridation and recommends widespread use.
• Fluoridation introduced in Dunedin in 1966, as part of widespread expansion in 1960s. Eighty-four centres now receive fluoridated water in New Zealand.
• Worldwide, 60 countries and hundreds of millions of people receive fluoridated water.
(Source: Ministry of Health.)
