Andrew Maynard argues against defining engineered nanomaterials for regulatory purposes (Nature 475, 31; 2011). But such a definition is urgently needed, especially for particulate nanomaterials. The aim should be to identify a general class of materials for attention — whether they are benign or hazardous.

Nanomaterials have many properties not shared by their larger-scale counterparts, some of which have safety implications. More and more products containing novel nanomaterials are reaching the market.

In light of legitimate public concerns and the resultant political responses, a revision and adaptation of legislation is considered necessary. The European Cosmetic Products Regulation of 2009 and a European Parliament legislative resolution on food information adopted in July 2011 both stipulate that nanomaterial ingredients should be strictly labelled. The European Parliament has called for the adoption of a “comprehensive science-based” definition of 'nanomaterial'.

Maynard's point that such materials are heterogeneous is justified. However, they all have structures on the nanoscale, which modify their other properties. Size is therefore the most appropriate parameter on which to base a broad definition (see also G. Lövestam et al. EUR 24403 EN, European Commission Joint Research Centre; 2010). A definition is required for labelling purposes, and would assist industry and regulators in identifying where specific safety assessments might be necessary. We acknowledge that it would need revision in line with fresh scientific evidence.