Nanotechnology is being used to make transparent sunscreen
Cosmetics containing tiny "nano" particles are being used widely despite unresolved issues surrounding their safety, a consumer watchdog warns.
Many skin care products, including sunscreens and wrinkle creams, contain this technology to make them easier to apply and invisible on the skin.
But experts are concerned about their possible long-term effects on the body, Which? reports.
Which? wants more safety checks and tighter regulation of their use.
It says, at the moment, consumers cannot tell which products use nanomaterials as many fail to mention it.
Nanotechnology is the science of manipulating atoms and molecules on the nanoscale - 80,000 times smaller than the width of a human hair.
The cosmetics industry is using it to create new materials with novel properties.
On the flip-side, that might mean unexpected risks.
Which? wrote to 67 cosmetics companies, including all of the main brands as well as smaller ones, asking them about their use of nanotechnology, what benefits they thought it brought and how they ensured product safety.
Seventeen firms responded, and of these, eight were willing to provide information about how they used nanotechnology.
Most of the eight, which included The Body Shop, Boots, Nivea, Avon, L'Oréal, Unilever, Korres and The Green People, used nanotechnology for the UV filters in their sunscreens.
Which? also found evidence of other cosmetics companies offering nanocosmetics online.
These products included nano emulsions - preparations containing oil and water droplets reduced to nano size - used to preserve active ingredients, such as vitamins and anti-oxidants, and for their lightness and transparency.
Another example was a type of nanomaterial called "fullerenes" used in anti-aging cream products.
Scientists have raised particular concerns about potential toxicity of fullerenes if they were able to penetrate the skin.
There is also a concern that the nanomaterials in sunscreens might be able to breach sunburned skin.
The Which? report says all nanocosmetic products should have an independent safety assessment.
The precautionary principle should be applied to products where there are potential risks but where it is not currently possible to assess their safety so that consumers are not put at risk, it says.
Sue Davies of Which? said: "We're not saying the use of nanotechnology in cosmetics is a bad thing, far from it. Many of its applications could lead to exciting and revolutionary developments in a wide range of products, but until all the necessary safety tests are carried out, the simple fact is we just don't know enough.
"The government must introduce a compulsory reporting scheme for manufactured nanomaterials so we are all aware - and only those that are independently assessed as safe should be allowed to be used in cosmetics."
In September 2006, the government launched a voluntary reporting scheme for all engineered nanomaterials to find out what was, or could be, on the market, to guide the development of regulations. This has had a limited response - 12 responses in two years - and is now under review.
A spokeswoman for the Cosmetic, Toiletry and Perfumery Association said: "The industry is working with government to provide more information on the safety of these products.
"The safety assessment of cosmetic products is a legal requirement and that assessment is robust and takes into consideration the particle size of ingredients."
Professor Dame Ann Dowling, chairman of the Royal Society working group on nanotechnologies, said: "The Royal Society has been calling, for the last four years, for companies to make public the safety testing methods they have been using on their nanoproducts. We are disappointed at continuing lack of transparency in this area.
"More research does need to be done on the effects of manufactured nanoparticles on human health and the environment. This is important so that regulation can be built on a proper understanding of any risks."
A European Commission spokeswoman said: "We are working towards improving our ability to assess the safety of all consumer products using nanomaterials including cosmetics.
"The Scientific Committee on Emerging and Newly Identifed Health Risks (SCENIHR) is currently preparing an update of its 2006 opinion on the risk assessment of products of nanotechnologies. This update will be available in January 2009."
Boots said it did not consider its current use of materials was of concern to health.
The Body Shop said its products helped to protect human skin.