In October, the European Union applied stricter rules to the sale and use of teeth whitening products. Based on advice from the European Scientific Committee on Consumer Safety, tooth whitening or bleaching products containing up to 0.1 per cent hydrogen peroxide - the active compound in whitening products - will continue to be freely available to consumers. Products with concentrations higher than 0.1 per cent, up to 6 per cent, will only be sold to dentists.

The General Dental Council, the organisation that regulates dental professionals in Britain, has since successfully prosecuted cases involving tooth-whitening companies or individuals who were not registered with it and who illegally carried out the business of dentistry.

The HKDA has also taken measures to further reinforce regulations governing tooth whitening treatment and the use of dental whitening products to eliminate unsupervised bleaching procedures in Hong Kong.

Says Yung: "The HKDA put forward its views to the government earlier this year, and we hope our input is instrumental to the government's future move of reviewing the regulation of medical beauty treatment and procedures."

All tooth-bleaching materials are based primarily on hydrogen peroxide or carbamide peroxide, which has the potential to irritate mucous membranes in the oral cavity and in the gastric tract if the product is swallowed accidentally.

Conditions such as a pre-existing tissue injury or the concurrent use of alcohol and or tobacco while using tooth whiteners may also exacerbate their toxic effects. Hydrogen peroxide, even at low concentrations, may be especially harmful to oral tissues if they have been previously injured. Dr Kenneth Luk, a registered dentist, recommends that people consult their dentists before using over-the-counter teeth whitening products. Even though these products contain low percentages of hydrogen peroxide, Luk says it's important to protect exposed root surfaces, sensitive teeth and restore poor fillings which whitening gels may diffuse along the margin. Teeth with amalgam fillings may run the risk of discolouration.

Home kits include a sheet of plastic that needs to be warmed in water and moulded by customers to create a teeth-whitening tray. It's imperative to make a tight fit to minimise leakage into the gums and throat. He adds: "Whitening has been around since the 1980s. I haven't come across any reports in Hong Kong on its adverse effects. But the risk of possible burning of gum, cheek and throat because of the high concentration cannot be ruled out."

Due to the potential risks of acute and long-term effects brought about by tooth bleaching treatments, application of tooth whitening products and bleaching light, the HKDA states that "tooth bleaching should not be regarded as a simple cosmetic treatment. It should only be performed by registered dentists, and should be regulated as a practice of dentistry.

"For optimal safety and to ensure proper diagnosis and treatment, examination by a dentist should be a must even for application of home-based tooth whiteners in whatever formats such as gum shields, strips or paint-on products, which may contain relatively low concentrations of hydrogen peroxide."

There are several teeth bleaching options available, such as in-office whitening, home use whitening with custom trays, teeth whitening strips and toothpastes.

In-office whitening (also known as power bleaching, professional whitening or chair-side whitening) is done by a dentist under carefully monitored conditions. It is the safe, controlled and pain-free use of a bleaching gel, which has results that are visible immediately.

It's more suitable for people with severe discolouration due to ageing or foods like, tea, coffee and curry. BriteSmile and Zoom are among the most popular in-office whitening systems, which involve a single hour-long session with the dentist. Although the results are fast, reliable and predictable, it's expensive (HK$6,500 to HK$10,000).

According to Dr Sandeep Jain, dentist at Diestel & Partners, the results of in-office bleaching can last from a few months to two years, depending on a person's food habits and oral hygiene.

Although the procedure uses a higher concentration of hydrogen peroxide (3 per cent to 16 per cent), Jain says it's safer as it's done under a dentist's control. For long-term results, he advises supplementing in-office systems with take-home whitening kits.

For take-home whitening kits, the dentist makes patients customised trays that have to be worn for about an hour each day for a week. It contains a lower concentration of hydrogen peroxide (0.1 to 6 per cent) than in-office whitening.

Patients have full control of how white they want their teeth to be. They can use the customised trays for over two years every three to four months, whenever they feel teeth are getting darker.

It's good for moderately discoloured teeth and can be used in with in-office bleaching. It's not very expensive (HK$2,500 to HK$4,000) and still good, but needs discipline from the patient.

Sometimes, tooth colour can be improved by professional cleaning such as scaling and polishing to remove stains and tartar. But tooth discolouration, particularly intrinsic discolourations, may not be amenable to bleaching, as it could be a sign of a disease or condition that requires dental therapy.

So a pre-treatment examination and routine monitoring of bleaching by dentists, is necessary.

Dental examination may reveal conditions such as bruxism or temporomandibular dysfunction that may be aggravated by tray bleaching. Radiographs may be necessary in the screening and diagnosis of conditions that may manifest as tooth discolouration.

Non-dental personnel are not equipped to undertake such dental procedures, since they are not educated in the use of disease screening or diagnostic tests.