This European Standard provides a guideline on the implications of choice of particle metric to express the exposure to airborne NOAAs e.g. released from nanomaterials and present the principles of operation, advantages and disadvantages of various techniques that measure the different aerosol metrics. The standard specifies which metrics should be measured during a basic assessment and a comprehensive assessment, respectively, as described in WI 00137053. Potential problems and limitations are described and need to be addressed when occupational exposure limit values might be adopted in the future and compliance measurements will be carried out. Specific information is mainly given for the following metrics/measurement techniques: - number/Condensation Particle Counters by optical detection; - number size distribution by electrical mobility; - surface area (electrical charge on available surface); - mass/chemical analyses (e.g. Inductively Coupled Plasma atomic Mass Spectrometry (ICP-MS), X-Ray Fluorescence (XRF)) on size-selective samples (e.g. by impaction or diffusion). This document is intended for those responsible for selecting measurement methods for occupational exposure to airborne nano-objects.