You brushed. Then the day happened.

What you notice first

It is rarely pain. It is the edge of a front tooth that looks a little more see-through than it used to. A smile that reads warmer in photos than it did. Cold water that suddenly registers. Or nothing at all, until a dentist points at something on the screen and says it has been there a while.

People reach for whitening at this point, and it disappoints them. The colour is not a stain. Enamel is translucent, and the layer underneath it is naturally yellow. When enamel gets thinner, more of that layer shows through. You are not looking at dirty teeth. You are looking at thinner ones.

Where the mineral goes

Enamel is almost entirely mineral, and it has no living cells to repair itself. What it has instead is a daily exchange. Every coffee, meal, snack or acidic drink drops the pH at the tooth surface and pulls a little mineral out. Saliva then spends the better part of an hour buffering the acid and carrying calcium and phosphate back in.

One exposure a day and the surface keeps up. Coffee at the desk, lunch, a sparkling water, a snack at four and a glass of wine at nine, and the repair phase keeps getting interrupted before it finishes. The balance tips by a fraction, and it tips in the same direction every day.

What that looks like, in order

1. The surface roughens. Stains catch faster, a film settles along the lower teeth by evening, and breath goes off earlier in the day.
2. Cold starts to register. The colour shifts, because the enamel is thinner, not dirtier.
3. Chalky white patches appear near the gum line. This is the last point at which mineral can still move back in.
4. The surface gives way. A filling follows, and the tooth is never original again. Fillings get replaced, each one takes a little more, until there is not enough left to hold one.

Brushing morning and night is the right thing to do. It is also two moments in a day that has fourteen hours of exposures in between, and none of those exposures happens next to a sink.

The mineral with the unfamiliar name

Hydroxyapatite is what enamel is made of. Enamel is almost entirely mineral, and almost all of that mineral is hydroxyapatite: tightly packed crystals of calcium and phosphate, lined up in rods so dense that enamel is the hardest tissue in the body. It sounds like a laboratory word; it is the same crystal that has been in your teeth since they came through. The dentine underneath and your bones are built from it too.

That is why it ended up in oral care. Most ingredients work on the tooth from the outside. Hydroxyapatite is the tooth's own material. In laboratory and clinical studies, fine hydroxyapatite particles attach to the enamel surface and settle into the microscopic irregularities that acid leaves behind, which is why surfaces treated with it look and feel smoother. Japan has used it in toothpaste since the 1990s, and over the past decade it has become one of the most studied mineral ingredients in oral care in Europe.

It is also gentle by nature. It is non-toxic if swallowed, it does not stain, it has no aftertaste, and it suits people who are cautious about what they put in their mouth several times a day.

What helps in the hours between

None of this asks you to give up coffee or wine. It asks for a little attention at the moments the acid arrives.

1. Rinse with water after acidic food or drink. A mouthful of plain water moves the pH back towards neutral faster than saliva alone.
2. Wait before you brush. Right after coffee, juice or wine the surface is softened. Brushing straight away scrubs it; give it 30 to 60 minutes.
3. Group the sips. A coffee drunk in ten minutes is one exposure. The same coffee sipped across two hours is a dozen.
4. Give the surface its mineral at that moment. Not at night, hours later, but right after the exposure, while the exchange is happening.

The first three are habits. The fourth has always been the hard one, because toothpaste needs a sink. Mouthwash needs a sink. The hours that do the damage are spent in cars, offices, meetings and restaurants.

One ordinary day

Five moments, and each one is where the fourth habit belongs.

Made exactly for those moments

There is now a product made for precisely this. CariSpray Adult is a hydroxyapatite oral spray that delivers the tooth's own mineral in seconds, wherever the day happens. Two or three sprays across the teeth, a few seconds, and you carry on. No water, no sink, no spitting. It also carries xylitol, a substance the main cavity-causing bacteria cannot use, and it works alongside whatever toothpaste you already use. One bottle fits in a pocket and lasts about a month.

The tooth in Specimen 01 was brushed for decades. What it never had was anything for the hours in between.