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J R Soc Med. 2004 January; 97(1): 49.
PMCID: PMC1079281


Fluorosis is one of some 90 causes of incisor teeth mottling.1 Fluoridated toothpaste imposes such a hazard if, in under eight-year-old children, too much is used. Iain McLean and his colleagues, although in another context, (October 2003 JRSM2), rightly say ‘the taking of true measurements is advisable’. And no more valid, I suggest, than when parents apply toothpaste to a child’s toothbrush. Dentists3 ask them to judge (guess) this as ‘an amount... no greater than a small pea, with the emphasis on small’: too much and the dentition’s integrity is hazarded. With use of such ‘guestimates’ the end product, measured by the amount of fluoride, may vary widely.

I measured the sizes of nationally marketed frozen peas in packets bearing a message that ‘only the sweetest and smallest are selected’: the diameter of the smallest was 0.5 cm and that of the largest 1 cm. Since the volume of a sphere is given by 4/3πr3 the larger pea’s volume was eight times greater than that of the smaller. I found it difficult to relate size to volume, as perceived by diameter; had the diameter of the larger pea been three times that of the smaller, the volume would have been twenty-seven times greater. Surely we cannot expect a mother, when supervising a youngster’s tooth-brushing, to judge unerringly the size of a pea. If children’s toothpaste were marketed with a dispenser to take out the guesswork it would help overcome the danger of mottling.

Lord Kelvin gave us a maxim: ‘When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind...’.4 Heavens preserve us from small peas.


1. Small BW, Murray JJ. Enamel opacities: prevalence, classifications and aetiological considerations. J Dent 1978;6: 33–42 [PubMed]
2. McLean I, Anderson CM, White C. The accuracy of guestimates. J R Soc Med 2003;96: 497–8 [PMC free article] [PubMed]
3. British Society of Paediatric Dentistry. A policy document on fluoride supplements and fluoride toothpaste for children. Int J Paediatr Dent 1996;6: 39–142 [PubMed]
4. Kelvin WT. Popular Lectures and Addresses [1891–1894]. London: Macmillan

Articles from Journal of the Royal Society of Medicine are provided here courtesy of Royal Society of Medicine Press