Waving a packet of carbon nanotubes accusingly at the assembled American politicians during a hearing last month in Congress, Andrew Maynard was determined to make a point. The nanotechnology expert ... had bought the tiny tubes on the internet. They had arrived in the post along with a safety sheet describing them as graphite and thus requiring no special precautions beyond those needed for a nuisance dust. Dr Maynard's theatrics were designed to draw attention to a growing concern about the safety of nanotechnology. Carbon nanotubes may be perfectly safe, but then again, they may have asbestos-like properties. Nobody knows. Indeed, industry, regulators and governments know little about the general safety of all manner of materials that are made into fantastically small sizes. In the past few years the number of consumer products claiming to use nanotechnology has dramatically grown-to almost 600 by one count. Patents are rapidly being filed. For a product to count as nanotechnology, it ... is enough merely for some of the material to have been tinkered with at a small scale. Often that can involve grinding down a substance into particles that may be only a few nanometres big-a nanometre is a billionth of a metre-about 100,000th of the thickness of a sheet of paper. Despite hundreds of years of experience in chemistry, it is not easy to predict how a substance will behave when it is made extremely small. That means, you cannot be sure how it will affect health. Nanoparticulate versions of a material can act in novel ways. When they are very, very small, materials, such as copper, that are soft can become hard. Materials, such as gold, that would not react to other substances become reactive. And when they have been shrunk, materials, such as carbon, that are perfectly safe might become unsafe. Plenty of research suggests that nanoparticles of harmless substances can become exceptionally dangerous.