How a tooth actually decays

This is the least glamorous thing on the dentistry side of the blog and the one that quietly powers the most answers. Ask a panel about the sugar levy, about fluoridation, about why children have teeth removed under general anaesthetic, and the useful half of every reply is a fact about this mechanism: what dissolves, what repairs it, and what tips the balance either way.

Candidates who have never learned it give themselves away in a particular manner. They talk about sugar "rotting" teeth, which makes decay sound like an event rather than a balance, and it leaves them with nothing to say when an interviewer asks why two children eating the same amount of sugar have different mouths.

A tooth sits in saliva that is supersaturated with calcium and phosphate. Left alone, mineral moves gently into the tooth. Now add sugar. Bacteria in plaque — Streptococcus mutans and lactobacilli among them — ferment it within minutes and produce acid, and the pH at the enamel surface falls. Below roughly 5.5, the critical pH for hydroxyapatite, the crystal begins to dissolve and calcium and phosphate leave the tooth. That is demineralisation.

Then the mouth recovers. Saliva buffers the acid, washes the sugar away and returns the pH above the critical threshold, and the dissolved mineral moves back into the enamel surface. That is remineralisation. A healthy mouth does this several times a day and the net position stays flat or positive. Decay is what happens when the sum comes out negative for long enough: the subsurface loses mineral, a white spot lesion appears, and eventually the weakened surface collapses into a cavity. Only at that point is the damage irreversible — which is why an early lesion is a warning rather than a filling.

Everything a dentist does about caries is an attempt to move that sum. Reduce the acid attacks by cutting the frequency of free sugars — which is why advice is about when and how often rather than how much, and why the bedtime bottle and the all-afternoon squash are the classic offenders. Disrupt the biofilm by brushing, which removes the bacteria doing the fermenting. Strengthen the repair with fluoride. And protect the vulnerable surfaces with fissure sealants on the deep grooves of molars where a toothbrush bristle cannot reach.

Fluoride deserves its own sentence because candidates routinely get it wrong. It works topically — by being present in the mouth at low concentration, repeatedly. It slows the rate of mineral loss during an attack, accelerates remineralisation afterwards, and the mineral that re-forms incorporates fluoride as fluorapatite, which has a lower critical pH of around 4.5 and therefore resists the next attack better. That is why fluoridated water works by washing over erupted teeth many times a day, why toothpaste should be spat and not rinsed, and why swallowing fluoride is not the point.

This is the question that separates candidates, because the mechanism answers it and a list of risk factors does not. Two children with the same sugar intake can have very different teeth, and every reason is a variable in the same equation.

Saliva is the biggest. Flow rate and buffering capacity decide how fast the pH recovers, and both fall with certain medicines, with dehydration and with some medical conditions — which is why a patient on antidepressants or with a dry mouth from radiotherapy is a high caries risk regardless of diet. Plaque matters: how much there is and how long it has been undisturbed. Fluoride exposure shifts the critical pH itself. Tooth anatomy decides where sugar sits. And timing — sugar at bedtime, when saliva flow falls to almost nothing overnight, removes the recovery half of the cycle entirely.

Say that, and you have explained the social gradient in decay without ever landing on the word neglect: the variables that decide the balance are mostly not choices a child makes.

Nobody will ask you to recite the chemistry. It arrives underneath other questions: "How would you explain tooth decay to a child?", "Should we tax sugary drinks?", "Why does fluoride work?", and every scenario about a parent who says they only give juice at mealtimes.

For the explanation question, use the balance and skip the vocabulary. For the policy question, use frequency to explain why drinks are the target. For fluoride, say topical before you say anything else.

The best single source is the government’s evidence-based toolkit for prevention, which sets out what to advise and why in one place. Any first-year dental textbook chapter on the caries process will give you the chemistry in more depth than an interview needs, and the Oral Health Foundation’s pages are the plain-English version to practise explaining.

Two pieces here run on this mechanism. Children’s tooth decay and hospital extractions is the mechanism at population scale, and water fluoridation is the repair half of the balance turned into policy. For the levy that targets frequency, read the sugar levy piece.