Under the Microscope: The public water supply has been fluoridated at a low level by the authorities since 1964. Many studies have shown that this is a very effective preventive measure against tooth decay and little or no evidence of side-effects has been reported beyond a low incidence of tooth discolouration.

Ingestion of fluoride at significantly higher levels would cause problems and concern has been voiced (Dan Fagin, Scientific American, January 2008) that, when intake of fluoride from toothpaste and from various foods and drinks is added to intake from fluoridated public water, levels of ingestion of fluoride could creep into the lower ranges where more serious side-effects are precipitated, particularly in the very young. However, the matter is being closely monitored and the experts still enthusiastically endorse fluoridation.

Very early in the 20th century it was noticed that children born in Colorado Springs, US, had stained teeth, but not adults who moved there, and it was noted that the mottled teeth were highly resistant to decay. It was suspected that the staining was caused by something in the drinking water and young children whose permanent teeth had not yet erupted or developed enamel were at the highest risk of developing the stain. This phenomenon was also found in other areas and it was noted that in all cases fluoride levels were naturally high in the local water – typically 2.5mg per litre or higher. The staining of the teeth was named fluorosis.

It was later determined that levels of fluoride in water at or below 1mg/litre (one part per million – 1ppm) posed little or no risk of fluorosis and pressure was exerted to add fluoride to public drinking water at levels that would prevent cavities without triggering fluorosis. In 1945, Grand Rapids, Michigan, became the first city in the world to have fluorine added to its water. Today, 59 per cent of the American public receive fluoridated water. Fluoridation of public water supplies later spread to other countries, including Ireland.

Teeth are covered with a layer of hard white enamel. The primary mineral in enamel is hydroxylapatite, whose crystals are composed of calcium, phosphate, hydrogen and oxygen. Food particles stick between our teeth. Oral bacteria consume these particles, excreting lactic acid in the process. This lactic acid can dissolve hydroxylapatite and form cavities. Fluoride, either ingested in water or topically applied to teeth in fluoridated toothpaste, can combine with the hydroxylapatite crystals, making them more resistant to lactic acid. Fluoride also enhances deposition of calcium and phosphates from saliva onto hydroxylapatite, thereby renewing damaged enamel and fighting tooth decay.

Early studies of fluoridation showed a reduction of 50 per cent or more in tooth decay. Fluoridated toothpaste was introduced in the early 1970s and this has contributed to the general improvement in dental health in OECD countries in recent decades. Fluoridated water supplies still effect a 50 per cent improvement in dental health compared to non-fluoridated areas, but the absolute value of this 50 per cent improvement has decreased, mainly because of fluoridated toothpaste. However, public health experts consider that sufficient beneficial effects of water fluoridation remain to justify its continuation.

There is no public water fluoridation in Northern Ireland. Tooth decay rates had been similar on both sides of the Border prior to the introduction of water fluoridation. A recent study found that decay rates in Northern Ireland are about 50 per cent higher than in the Republic.

Extensive research, reviewed by

J Mullen in The British Dental Journal, Vol 199, No 7 (2005) shows no clear evidence that the level of fluoridation of water used in Ireland (1ppm) produces any adverse effects other than a low incidence of cosmetic dental fluorosis. The treatment of fluorosis is simple. Most of the tooth discolouration is confined to the outer tenth of a millimetre of tooth enamel and it can be easily abraded away by the dentist.

The Iowa Fluoride Study, directed by Stephen Levy, is the longest running study of the health effects of fluoridation. Part of the study is the measurement of trace fluoride levels in thousands of foods, drinks and toothpastes. Levy and others think that some children are probably ingesting more fluoride than they should. They still support water fluoridation as a proven method of controlling tooth decay, especially where poor oral hygiene is practised, but they believe that the case for water fluoridation in communities with good oral hygiene weaker than before.

Furthermore in 2006 a committee of the US National Research Council issued a report that concluded the current American limit of 4mg/l (4ppm) for fluoride in drinking water should be lowered because consistent exposure of children to fluoride at that level can discolour emerging permanent teeth (fluorosis) and appears to increase risk to adults of fracture and possibly skeletal fluorosis, a painful stiffening of joints. Of course most water is fluoridated at a lower level than 4mg/l, but there is uncertainty about the level of additional fluoride we ingest from food and toothpaste.

The best expert advice we have at the moment is that fluoridation of public water remains a hugely valuable preventive measure, but in some areas we can now lower the level at which we fluoridate water when we take improved oral hygiene and fluoride intake from other sources into account. In this regard the level of fluoridation of Irish water has now been reduced from a level of 1ppm to 0.7ppm.

William Reville is Associate Professor of Biochemistry and Public Awareness of Science Officer at UCC -http:// understandingscience.ucc.ie

© 2008 The Irish Times