Estimates of fluorosis prevalence in the York review (2000)

In 2000, a research team at the University of York published the findings of its systematic review of 88 previously conducted studies into the relationship between water fluoridation and dental fluorosis (1).

The studies had been carried out in 30 countries, including some with naturally occurring fluoride levels in water of up to 5 parts per million (i.e., five times higher than that used in UK fluoridation schemes).

The review found that, from the available evidence, there is a relationship between the fluoride level in water and the amount of dental fluorosis.  As the fluoride level increases, the amount and severity of dental fluorosis in the population also increases.  This was not a new discovery.  The relationship between fluoride levels in water and the amount of dental fluorosis has been known about since the 1930s.

The York team estimated the prevalence of dental fluorosis of all levels of severity (including the mildest forms) to be 48% in fluoridated areas (both naturally fluoridated and artificially fluoridated) and 15% in non-fluoridated areas.

However, the difference in prevalence between fluoridated and non-fluoridated areas was much smaller when levels of cosmetically significant fluorosis were compared.  In this case, the York team estimated the prevalence to be 10% to 12% in fluoridated areas and 6% in non-fluoridated areas.

The York team’s estimates for ‘fluoridated’ areas combined data from studies carried out in both artificially fluoridated communities and those with naturally occurring fluoride in water at the same or higher concentrations.

After the York findings had been published, the Medical Research Council asked the team to undertake further analysis of prevalence figures.  The new analysis revealed that the risk of cosmetically significant fluorosis was greater in naturally fluoridated areas than in artificially fluoridated areas even when the fluoride concentrations in water were exactly the same.

Many of the studies of naturally fluoridated areas that were reviewed by the York team had taken place in countries with hotter climates than the UK and where water intake is typically higher and the risk of fluorosis correspondingly greater at any given fluoride concentration. The relevance of the York figures was therefore called into question.

This led the MRC to look specifically at data from European sources, including the study illustrated on this page.  Taking account of such information, the MRC suggested that a more realistic prevalence figure for fluorosis of aesthetic concern would be around 3% to 4% in artificially fluoridated areas and around 1% in non-fluoridated areas. (2)

  1. McDonagh M, Whiting P, Bradley M, Cooper J, Sutton A, Chestnut I (2000): A systematic review of public water fluoridation.  Centre for Reviews and Dissemination, University of York.

  2. Medical Research Council (2002): Working group report: Water fluoridation and health.

POSSIBLE CONFOUNDING FACTORS  IDENTIFIED BY THE YORK REVIEWERS

“There are likely to be many possible confounding factors in cross-sectional studies of dental fluorosis.
Temperature and altitude are two that are frequently mentioned, but not controlled for in these studies (i.e., the studies reviewed by the York team).”

“People living in climates with a higher mean temperature drink more water, thus being exposed to more total fluoride. Higher altitude has also been thought to be associated with the development of
fluorosis, although the mechanism for this is unclear. 

“Fluorosis can be difficult to distinguish from other developmental defects of enamel.”