Henrik Laursen, coordinator of the nano team in the Commission's environment department, said the EU executive had received around 200 replies to the consultation. He said: "It is clear that at a certain level many stakeholders are saying different things, and there is no absolute scientific definition."
He said the Commission would not be rushed into making a decision because, once made, it would not be a working model but would immediately have a significant binding effect.
But Chiara Giovanini, a spokeswoman for ANEC, the European Consumer Voice in Standardisation, noted that the "lack of an agreed definition is creating legal uncertainties for regulatory purposes, and hindering the development of adequate safety test and measurement methods".
She called on the Commission to adopt the draft definition of nanomaterials contained in the consultation at the end of last year "without further delay".
Laursen said that although the Commission would consider the public consultation and the advice of key scientific bodies such as the EU's scientific committee on emerging and newly identified health risks (SCENIHR), ultimately the definition would be "a policy decision".
The Commission is believed to be attempting to frame a definition before the end of the summer. However, the finer detail of how to define nanomaterials is the subject of fervent disagreement between stakeholders behind the scenes.
Wim de Jong, vice-chair of the SCENIHR, told EurActiv that his organisation had recommended to the Commission that the number of particles, rather than the weight of the particles, be used as a guide for determining the definition.
"This is important because the potential hazards of using these particles relates to the number of them within a particular product," he said.
But other stakeholders are opposed to using numbers as a guide to defining nanomaterials. For example, the European Chemical Industry Council (Cefic) has recommended to the Commission that weight be used instead.
Cefic's reply to the Commission consultation reads: "Weight is generally used in all chemical legislation and test procedures and should therefore be used instead of particle number concentration."
Carlos Peña, director of emerging technology programmes in the office of the chief scientist at the US Food and Drug Administration, the US consumer regulator, said that the US had not defined nanotechnologies and was instead trying to keep watch over how they are introduced into several different sectors – such as food, drugs and cosmetics – to ensure that regulation within those sectors takes account of their use.
Henrik Laursen, coordinator of nano team at the European Commission's environment department, said: "We still have some decisions to take but what we will come up with eventually will not be a working definition, it will be a definition that will be applicable [...] There is no room for us to introduce a definition by trial and error, we are expected to make sure we act and we need to come up with something."
Laursen said that within the draft definition provided by the Commission there was a review clause "so there is a possibility to return to the issue it to see if we got it right". He added: "The definition will not and cannot solve all the issues, but it will be the beginning of the process."
Qasim Chaudry, the principal research scientist at the UK Food and Environment Research Agency, said his agency was currently considering the first case of nano ingredients that would become available for consumer use in the cosmetics industry. The outcome of this case would, he said, "set a precedent for future assessment".
He added: "The challenges include the fact that there is no definition of nanotechnology, and the fact that we are setting new systematic standards for dealing with nano."
Chiara Giovanini, a spokeswoman for ANEC, said: "We support the proposed science-based approach according to which the size distribution of a material should be presented as size distribution based on the number concentration (i.e. the particle number), and not on the mass concentration of a nanomaterial product. This is because a small mass concentration may contain the largest number fraction."
She added: "Materials on this scale present different properties compared with 'bigger' particles (e.g. greater reactivity and mobility in the body) and are increasingly being used to create new products. Products claiming to contain nanomaterials are already widely available on the European market and it is imperative to ensure that they are safe for consumers to buy. We believe that the starting point of any sound regulatory approach is agreement on a definition of what exactly nanomaterials and nanotechnologies are."