Samantha Fox, of Ardgowan School, asks :-

Why are people's teeth white? Why aren't they pink or blue?

Tom Kardos, a dentist at University of Otago's Dental School, responded.

The colour of any object is determined by that part of the spectrum of visible light that is reflected from the object. Teeth may show wide variations in colour, some of which are due to variations in chemical composition of the tooth, and others are due to staining on their surface.

The bulk of the tooth is made up of a tissue called dentine. That part of the tooth that we see in the mouth is covered by a layer of enamel. Enamel is an unusual tissue as it is made up of inorganic crystals of a calcium phosphate salt, calcium hydroxyapatite. The inorganic phase makes up 86 per cent, by volume, of enamel, hence it is considered a crystalline tissue with numerous small defects.

When light strikes enamel much of it is reflected and the tooth appears white. This property tells us that the crystalline structure of enamel is imperfect. A perfect crystalline structure would be translucent. It is worthy of note that the whiter a person's teeth are the less perfect the crystalline structure of enamel is.

Sometimes more dense "white spots" can be seen in the enamel. These represent regions in which there is a reduction in the normal inorganic content, such as the retention of protein during development or loss of the inorganic phase in the presence of acid produced by bacteria, i.e. enamel caries. Under the right conditions early enamel caries is reversible, so these latter white spots can disappear whereas those due to protein retention persist.

As we get older the inorganic content of enamel usually increases forming a more perfect crystalline tissue, thus increasingly enamel becomes more translucent. When enamel is translucent, light passes through the enamel and is reflected from the deeper dentine. Dentine contains more protein (25 per cent by volume) than enamel giving the tooth a yellow colour. Loss of whiteness of the tooth with age is due to the maturation of enamel.

Dentine may also become translucent as its inorganic content increases with age. Under these conditions light passes through both the enamel and dentine before being reflected from the dental pulp, the tissue in the center of the tooth. As the pulp contains many blood vessels the tooth now appears pink. A tooth may also appear pink if, for some reason, the thickness of either dentine or enamel is reduced, changing the normal light reflecting properties of the tooth.

Staining of deposits on the tooth surface, in particular dental plaque, may significantly change the colour of teeth. These stains can be removed as part of dental care.