Professional whitening does not scrub colour off the surface or strip away a layer of enamel. The peroxide gel diffuses through the enamel and reacts with coloured molecules inside the tooth, particularly in the dentine underneath. In clinical studies the change shows up as teeth becoming lighter and less yellow, and it is measured with a standard colour system rather than judged by eye alone.
Peroxide travels through the tooth
Enamel looks solid, but peroxide can diffuse through it. A literature review by Joiner (Journal of Dentistry, 2006) concluded that whitening works by peroxide diffusing through enamel and oxidising coloured compounds, particularly within the dentine. Kwon and Wertz (Journal of Esthetic and Restorative Dentistry, 2015) reviewed the research on how whitening works and described a process with three parts: the whitening agent diffuses into the tooth, it interacts with stain molecules, and it causes microscopic changes on the surface and changes within the tooth that affect how it reflects light.
The review also noted that the interaction does not seem to be limited to stain molecules, and that peroxide has some effect on sound enamel and dentine as well. The authors concluded that supervision by a dental professional is important for a successful and safe whitening result.
What "whiter" means when it is measured
Researchers do not judge whitening by eye alone. Tooth colour is usually recorded in the CIELAB system, which describes any colour with three numbers: L* for lightness, a* for red to green, and b* for yellow to blue. Joiner and Luo (Journal of Dentistry, 2017) reviewed how colour science is applied to teeth and concluded that instruments such as spectrophotometers, colorimeters and digital imaging systems can now measure tooth colour and whiteness in a reproducible way.
A double-blind, placebo-controlled trial by Swift and colleagues (Journal of Dentistry, 2009) shows what the numbers can look like in practice. Forty adults used either 6% hydrogen peroxide strips or placebo strips without peroxide for 30 minutes twice a day, and tooth colour was measured from standardised digital photos. After two weeks the peroxide group's teeth had become lighter (L* up by an average of 1.9) and less yellow (b* down by 2.1), while the placebo group changed very little (L* up by 0.4 and b* down by 0.3). With continued use to six weeks, the peroxide group kept improving gradually, by roughly 0.2 for L* and 0.3 for b* each week.
| Colour measure | What it describes | What whitening usually does |
|---|---|---|
| L* | Lightness, from black to white | Goes up: teeth look lighter |
| b* | Yellow to blue | Goes down: teeth look less yellow |
| a* | Red to green | Can also shift, but is less often the focus |
Measuring lightness and yellowness separately gives a more precise picture of the change than a single shade tab. Our article on how many shades whiter whitening can go explains why shade guide numbers are only a rough way to describe that change.
The gel is used up while you wear it
Peroxide breaks down over the time a tray is worn. In a small crossover trial with 20% to 22% carbamide peroxide gels worn in trays, Mailart and colleagues (Journal of Esthetic and Restorative Dentistry, 2023) found the peroxide concentration fell progressively over 120 minutes, and fell more in lower trays than upper ones. In laboratory testing on enamel samples, Cavalli and colleagues (Brazilian Dental Journal, 2019) measured how quickly different gels broke down and suggested that 10% carbamide peroxide could be applied for 4 hours, and 15% or 20% carbamide peroxide or 6% hydrogen peroxide for 2 hours. These were laboratory findings, so your own wear time is set by your dentist.
This is part of why different gels come with different wear times. Our guides to carbamide and hydrogen peroxide and to overnight or daytime tray wear go into the practical side.
Why more gel, stronger gel or longer wear is not better
Because whitening depends on peroxide reaching inside the tooth, it is tempting to think that more of it will give a whiter result. A 2026 structured narrative review by Reis and colleagues (Brazilian Oral Research) examined common beliefs about whitening and concluded:
- higher peroxide concentrations may speed up early whitening but increase side effects and do not improve the final result
- leaving gel on longer than recommended does not produce a perceptible extra colour change and may increase sensitivity
- using more gel or covering a larger area does not improve the final result and may add side effects
- products bought over the counter do not replace professionally supervised whitening for predictability or size of colour change
Following the wear time and amount of gel you are given is therefore part of reaching your result with less risk of sensitivity.
What peroxide does to enamel
The question many people ask is whether this process weakens teeth. Joiner (Journal of Dentistry, 2007) reviewed research on peroxide and enamel and dentine, and concluded that the majority of studies found no significant harmful effects on surface structure and chemistry or on hardness. Studies that did find effects often used laboratory methods that did not reflect conditions in the mouth, or products with a particularly low pH, where acid erosion is the likely explanation. Our article on whether whitening damages enamel covers this question in more detail.
How this shapes whitening at Lumi Dental
All three whitening options at Lumi Dental, Lumi Home, Lumi Refresh and Lumi Radiance, include Boutique take-home whitening with custom trays, so the gel is held against the teeth in a controlled amount for a set time. Teeth are examined before whitening starts, because decay, gum problems and existing fillings or crowns can change what is suitable. No shade is promised, since the starting colour and the way each person's teeth respond differ. The teeth whitening page and our comparison of Lumi Home, Refresh and Radiance explain the options.
Questions about how whitening works
Does whitening work on the surface or inside the tooth?
Mainly inside. Peroxide diffuses through enamel and reacts with coloured compounds, particularly in the dentine, and Kwon and Wertz also describe microscopic surface changes as part of the effect.
Why do some teeth respond more than others?
Joiner's 2006 review lists the type of stain, the starting tooth colour and the person's age among the factors that can affect the result. Our article on why whitening results differ between people looks at this.
Is a whitening toothpaste doing the same thing?
Usually not. Many whitening toothpastes work mainly by removing surface stain. See our article on whitening toothpaste.
Sources
- Joiner A. The bleaching of teeth: a review of the literature. J Dent. 2006;34(7):412-9. PubMed 16569473
- Kwon SR, Wertz PW. Review of the mechanism of tooth whitening. J Esthet Restor Dent. 2015;27(5):240-57. PubMed 25969131
- Joiner A, Luo W. Tooth colour and whiteness: a review. J Dent. 2017;67S:S3-S10. PubMed 28928097
- Swift EJ Jr, Heymann HO, Wilder AD Jr, et al. Effects of duration of whitening strip treatment on tooth color: a randomized, placebo-controlled clinical trial. J Dent. 2009;37 Suppl 1:e51-6. PubMed 19523738
- Mailart MC, Dos Santos KC, Torres CRG, Borges AB. Chemical behavior of 20%-22% carbamide peroxide gels in at-home bleaching: randomized crossover in situ trial. J Esthet Restor Dent. 2023;35(2):416-422. PubMed 36200919
- Cavalli V, Silva BGD, Berger SB, et al. Decomposition rate, pH, and enamel color alteration of at-home and in-office bleaching agents. Braz Dent J. 2019;30(4):385-396. PubMed 31340230
- Reis A, Maran BM, Cordeiro DCF, et al. Clinical myths in dental bleaching: an evidence-based structured narrative review. Braz Oral Res. 2026;40(suppl 1):e062. PubMed 42659424
- Joiner A. Review of the effects of peroxide on enamel and dentine properties. J Dent. 2007;35(12):889-96. PubMed 17964705
This article is general information only and is not a substitute for advice about your own situation from a dentist.
