VIENNA, Sept 16, 2009 (AFP) - They can make fabric resistant to stains, improve the taste of food and help drug research, but nanoparticles could also pose a danger to human health, experts warned Wednesday 16 September.

Susanne Stark, of the Consumer Information Association, told a seminar in the Austrian city of Salzburg that companies should be forced to indicate on labels whether a product contains the tiny particles.

"There are more questions than answers on the effects of nanoparticles" on human health, the chemist said.

Cosmetic and food products should indicate whether their products contain nanoparticles by 2012, she said.

Nanoparticles, measuring no more than 100 nanometres (0.00001 centimetre), have helped to revolutionise how everyday products are made.

The particles are used to make stain-resistant paint or creamy cosmetic and food products. They can also change colour to indicate whether a product has expired.

But their risk to humans remains largely unknown.

Nanoparticles can enter the body through the mouth and nose, the digestive system or the skin, and spread inside the body through blood vessels, said Hans Peter Hutter, a doctor specialised in environmental hygiene in Vienna.

"These tiny particles could without a doubt go all the way to the placenta," he said. But he warned that little was known about their behaviour inside human tissue.