Tooth bleaching using peroxide-based materials is an effective procedure in dentistry. Studies provide evidence on the safety of bleaching by dental professionals. However, concerns and controversy remain regarding the safety of bleaching systems that do not involve dental professionals, especially those available at mall kiosks, salons, spas, and cruise ships. Bleaching causes tooth sensitivity and/or gingival irritation in a significant portion of patients. Bleaching requires proper examination, diagnosis, and treatment plan, which can be performed appropriately only by dental professionals; therefore, involvement of dental professionals in bleaching is necessary to maximize the efficacy and minimize potential risks.
In-office tooth bleaching has been a dental procedure for more than a century; however, at-home tooth bleaching was not available until 1989, when it was introduced by Haywood and Heymann. With its demonstrated efficacy, lower cost than in-office bleaching, and the convenience of self-application, at-home bleaching quickly gained popularity and has now become an integrated procedure in aesthetic dentistry. Nowadays, in addition to the bleaching products available from dental professionals, over-the-counter (OTC) and infomercial at-home bleaching products are available directly to consumers, and they can be applied with a custom or preformed tray, with a brush, or as a strip. In recent years, tooth bleaching similar to in-office procedures but performed under nondental settings, such as mall kiosks, spas, and cruise ships, has become available.
Current tooth bleaching materials, whether used in office or at home, almost exclusively use peroxide compounds as the active ingredient, with carbamide peroxide and hydrogen peroxide (H 2 O 2 ) being the most common. Chemically, carbamide peroxide is composed of approximately 3.5 parts of H 2 O 2 and 6.5 parts of urea, so that a bleaching material of 10% carbamide peroxide contains approximately 3.5% H 2 O 2 . Attempts were made to introduce at-home whiteners that claimed to contain no peroxide; however, such products did not gain acceptance because of the lack of evidence on their efficacy and controversy over their nonperoxide claim. Typically, H 2 O 2 concentrations of in-office bleaching products range from 25% to 38%, whereas at-home formulations contain 3.0% to 7.5% H 2 O 2. However, in the recent years, there has been a trend of elevating the H 2 O 2 concentration in at-home bleaching materials, and products containing up to 15% H 2 O 2 have now become available directly to consumers for home use.
The efficacy of peroxide-containing tooth bleaching products has been debated. In general, data accumulated over the last 2 decades also suggest that tooth bleaching is a relatively safe procedure. However, controversy concerning its safety has continued since the introduction of the home-use materials, and there have been reports of adverse effects of bleaching on oral tissues and restorative materials. This article provides an overview of safety controversies in bleaching in relation to the biologic properties of H 2 O 2 and discusses the proper use of bleaching to maximize benefits while minimizing potential risks.
Biological properties of H 2 O 2 and safety concerns with bleaching
H 2 O 2 is a well-investigated chemical. It was discovered in 1818 and detected in human respiration in 1880. The well-known Fenton reaction was proposed in 1894. Peroxidase and catalase, which are 2 important enzymes in H 2 O 2 metabolism, were discovered in 1898 and 1901, respectively. Shortly after the discovery of another enzyme, superoxide dismutase (SOD), in 1969, H 2 O 2 was recognized as an important by-product in oxygen metabolism, and the research efforts on the biologic properties of H 2 O 2 have been significantly increased since then.
H 2 O 2 is a normal intermediate metabolite in the human body, with a daily production of approximately 6.48 g in the liver. One of the key characteristics of H 2 O 2 is its capability of producing free radicals, including hydroxyl radicals that have been implicated in various stages of carcinogenesis. Oxidative reactions of free radicals with proteins, lipids, and nucleic acids are thought to be involved in several potential pathologic consequences; the damage by oxidative free radicals may be associated with aging, stroke, and other degenerative diseases. To prevent potential damage to cells during oxidative reactions and repair any damage sustained, there are various defensive mechanisms available at cellular and tissue levels. Enzymes such as catalase, SOD, peroxidase, and selenium-dependent glutathione peroxidase, which exist widely in body fluids, tissues, and organs, effectively metabolize H 2 O 2 . Human saliva also contains these enzymes. In fact, salivary peroxidase has been suggested to be the body’s most important and effective defense against the potential adverse effects of H 2 O 2 . A study on infants, juveniles, adults, and adults with impaired salivary flow found rapid decomposition of H 2 O 2 in dentifrices. After brushing for 1 minute with 1 g dentifrice, less than 2% of the prebrushing dose of H 2 O 2 (30 mg) was detectable in the oral cavity of the subjects.
Much of the safety concerns with home-use bleaching originate from H 2 O 2 used in the materials, especially the known toxicology of free radicals. The oxidative reactions and subsequent damage in cells caused by free radicals are thought to be the major mechanisms responsible for the observed toxicity of H 2 O 2 . There have been concerns of potential systemic adverse effects if the bleaching material is ingested as well as local adverse effects on enamel, pulp, and gingiva because of the direct contact of the material with the tissues. The safety controversies over peroxide-based tooth bleaching have prompted not only scientific deliberations but also legal challenges to its use in dentistry.