How teeth whitening actually works.

The phrase teeth whitening hides a very specific reaction. The mouth doesn't get bleached the way fabric does. A short list of small molecules — hydrogen peroxide most of the time — diffuses through enamel and dentin and breaks the bonds that hold pigment in place. The pigment stops absorbing visible light. The smile reads brighter.

That's the whole story. Everything else — strips vs. trays vs. LED light vs. branded toothpaste — is mostly a story about concentration, contact time, and access. Worth understanding before you spend a rupee.

A tooth, briefly ¶

Enamel is the outer 1.5–2 mm of a tooth. It's the hardest tissue in the human body, made of densely packed hydroxyapatite crystals. Under it sits dentin, a softer, more porous, naturally yellowish layer that contains tubules running toward the nerve. Most of a tooth's color comes from the dentin — enamel is closer to translucent, and over a lifetime it slowly thins from chewing and acid wear, letting more of that dentin colour show through.

That's why a healthy adult tooth is rarely paper-white. The architecture is a layered cosmetic — and the layer doing most of the colour talking is underneath.

Two kinds of stain ¶

Discoloration comes in two flavours, and they respond to whitening very differently.

Extrinsic stains sit in the pellicle — a thin protein film that coats enamel within minutes of brushing. Tannins from coffee, tea, red wine; chromogens from curries, berries, beets; and chromogens from tobacco all bind to this film. Mechanical abrasion (a brush, a soft polish) can remove a portion. Mild surfactants and enzymes help further. This is the territory of most "whitening" toothpastes.

Intrinsic stains live inside the tooth — in the dentin and enamel matrix. They build slowly over years (food, drink, ageing) or arrive faster (certain antibiotics in childhood, fluorosis, trauma to the nerve). No amount of brushing reaches them. Peroxide can.

Brushing addresses the film on the tooth. Peroxide addresses the colour in the tooth.

What peroxide does ¶

Hydrogen peroxide (H₂O₂) is a small, polar molecule. Small enough to diffuse through the porous enamel into the dentin in minutes. Once inside, it dissociates into reactive oxygen species — short-lived radicals that oxidize the long, conjugated double-bond chains responsible for most pigment colour. Those chains get broken into smaller, lighter, less light-absorbing fragments.

That's the bleach. The crystal architecture of enamel and dentin isn't touched. The fragments diffuse out, get swallowed in tiny quantities (orders of magnitude below any toxic threshold at consumer concentrations), and the tooth reads paler.

Hydrogen vs carbamide peroxide ¶

You'll see two ingredients on the back of most whitening boxes. Both end up doing the same thing, just on different timetables.

• Hydrogen peroxide — fast and direct. A 6% gel hits peak activity in the first 15–30 minutes. Used in most strips, in-office treatments, and short-wear pens.
• Carbamide peroxide — a slow-release form. A 10% carbamide peroxide solution is roughly equivalent to about 3.5% hydrogen peroxide and breaks down over several hours. Used in tray gels meant for overnight or extended wear.

Carbamide is gentler per minute but does its work over a longer window. Hydrogen is faster but more sensitivity-prone if concentration runs high. There's no "better." There's the format you'll actually use.

What whitening can't do ¶

Three honest limits:

• It can't whiten restorations. Fillings, veneers, crowns, and bonded composite don't bleach. Whiten the natural teeth first, then have any restorations matched to the new shade — not the other way around.
• It can't fix grey from inside the nerve. Tetracycline staining and dead-tooth grey often require in-office internal bleaching or restorative work.
• It can't lift you past your dentin. Your enamel is translucent. After a certain point you're staring at dentin colour. Going further requires veneers, not gel.

A reasonable expectation ¶

Two weeks of consistent, properly worn at-home strips on healthy teeth typically lifts shade by 3 to 6 levels on the standard VITA shade guide. A coffee drinker who quits espresso during the run and waits 30 minutes after meals to brush will see the upper end. Someone who whitens once and goes back to a half-litre of black tea a day will keep about half the gain.

Set expectations early, brush gently with a soft-bristle, give the pellicle 30 minutes after a meal to re-stabilize before whitening, and the chemistry will do the rest.

Whitening is a small reaction, repeated. The work is in showing up for two weeks.

Next: Whitening strips vs. toothpaste — when each format makes sense, and what either can realistically deliver.