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Stuart Johnston from the British Dental Association gives his tipsÌýfor responsible teeth whitening.

The woman on the stall said it was a gentle and safe treatment with no side effects or sensitivity afterwards. When we asked what would happen during the treatment she said, "You sit in the chair, I apply the whitening gel on your teeth and the whitening gel is activated by blue light, a very healthy blue light."

Back in 2007, Sonic Whitening's parent company, All White 3000 was in trouble with the Advertising Standards Authority for misleading customers. It was advertising 'laser whitening' treatment, when it doesn't use lasers. Three years later, Sonic Whitening's leaflets still promise 'laser white teeth'. When we asked whether they use lasers, the woman on the stall told us that they call it a laser, but it is actually a blue LED light.

Many salons argue that legally, they are allowed to whiten teeth. They point to a 2001 House of Lords judgement which classed teeth whitening products as cosmetics, not medicines. On the other hand, the Welsh Assembly Government says that all teeth whitening procedures, including bleach and laser treatments, constitute the practise of dentistry.

The legal situation is confusing, but the Cosmetic Products Safety regulations make it clear that a product's ingredients must be displayed or disclosed to consumers. We asked the woman at the Sonic Whitening stall what was in the whitening gel, but she wouldn't tell us. "I can't tell you exactly what the ingredients are in the gel", she said, "but it is different than in the dentist". When we wrote to the company they refused to tell us and said only a competitor or market researcher would ask such a question.

Stuart Johnston from the British Dental Association was concerned by the company's unwillingness to reveal this information. "How can you give consent if you don't know what chemicals are being used?" he said. "It's going to whiten your teeth, so it has to have some fairly robust chemistry to it. There's no protection for the rest of the mouth, nothing to stop it getting on your gums and nothing to stop it being swallowed. This is a chemical that hasn't been defined, so that really would concern me."

Sonic Whitening's treatment takes less than an hour and appears very different to the tooth whitening procedure followed by dentists like Dr Pamela White, who has been whitening teeth at her Cardiff surgery for 12 years. After a consultation, she makes an exact mould of her patient's mouth. For two weeks they use a low dose of whitening gel at home, before having their final treatment in surgery. At a cost of more than £500, Pamela is not surprised that people are tempted by a quick fix on the high street.

The gel used by most dentists contains hydrogen peroxide, a chemical proven to whiten teeth, but the process can take several applications. Sonic Whitening won't reveal what it uses, but dentists say they're concerned as some salons use whitening products which haven't been proven to be effective.

One of the chemicals, chlorine dioxide, can permanently damage the enamel of the teeth, as Dr Pamela White explains. "It's the same as putting household bleach in your mouth. One girl who I can think about, had her teeth whitened in a hairdressing salon, it had totally stripped enamel from four front teeth. They were very stained, very mottled, very ugly and she was very distressed about it."

On its website Sonic Whitening claims to employ professionals who are registered with the General Dental Council. However, the company refused to tell us who they were, or whether any of them worked on the Cardiff stall. We were able to confirm that the company's director Thai Tran is not a registered dentist. The woman who spoke to the X-team on the stall said that she was a 'qualified dental cosmetologist". Both the General Dental Council and the British Dental Association said they don't recognise this qualification.

When we asked Sonic Whitening for its response to the concerns we'd raised, the company told X-Ray that it's carried out thousands of successful treatments with no adverse effects. It said it regards teeth whitening as a form of 'pampering', not dentistry, and says its products aren't dangerous and are 'neutral' to teeth, although it said it would welcome clearer legislation on this issue.

And as for the secret whitening ingredient, we repeatedly asked the company to tell us what was in its gel. Eventually it told us that it contains sodium perborate, which together with other active ingredients 'will leave your teeth beautifully white without any side effects'.

Sodium perborate is a chemical which is currently permitted but that could soon change, as there's evidence that it could be toxic and have a harmful effect on reproductive health and the development of unborn children. The European Commission is working on a proposal to ban sodium perborate for consumer use - and that includes its use in teeth whitening products.

Have you had your teeth professionally whitened? If so, we'd like to know what your experiences have been. Email us at x-ray@bbc.co.uk.

Stuart Johnston from the British Dental Association gives his tipsÌýfor responsible teeth whitening.