## 2 Exposure
### Methods and NM Aggregation  
The current method of assessing worker exposure to airborne particles in the workplace involves measurement of the mass concentration of health-related fractions of particles in the worker's breathing zone. The main exception to this methodology are particle-number-based metrics for exposure for fibres and for micro-organisms [18,19]. However, nanoparticles carry extremely small masses and therefore generally contribute negligibly to the integral mass concentration of the inhalable or respirable dust fraction [20]. Besides, such a measure cannot distinguish exposure to engineered NMs from background levels of similar sized anthropogenic or ambient particles. Other metrics such as particle number concentration or surface area may be better descriptors for the biological effects of nanoparticles. The issue of exposure metrics has extensively been addressed by [PERSON] and [PERSON] [21], and they conclude that effective approaches for measuring exposure to a wide range of manufactured NMs/nano-objects will require methods for measuring aerosol number, surface area and mass concentration. The most widespread method for determining airborne sub-micron particle number concentrations as a function of particle size, _i.e._, particle number size distribution, is based on electrical mobility analysis of the particles [22]. This technique usually comprises three main components: (1) a particle charger to charge particles predictably depending on their size; (2) a mobility analyser which classifies the particles of one polarity according to their electrical mobility; and (3) a particle counter that determines the number concentration of the mobility-classified particles. These three components are usually employed in what has become the workhorse for occupational exposure measurements: the Scanning Mobility Particle Size (SMPS) or the somewhat more sensitive Fast Mobility Particle Size (FMPS). The International Standards Organisation (ISO) standard ISO/TR 27628 further describes the available methods to measure the above mentioned metrics of nano-objects [23]. However, for SMPS, no standard method has been agreed upon to produce reference particle number concentration [22]. Furthermore these instruments are bulky, expensive and complicated to use, and are therefore usually only operated by research groups and not by practitioners in SMEs for example. Fortunately, smaller and even portable devices are now appearing (e.g., from the EU FP7 NanoDevice project [24]). The SMPS also gives no information on the chemical identity of the counted particles. The latter is also true for the available small portable devices like the Condensation Particle Counter (CPC), diffusion size classifiers [25] or surface area monitors. Though they give no information of the size distribution they can be used for assessing sources of NMs or the effectiveness of control measures. The state of the art method to obtain information about size and shape as well as chemical identity and state of agglomeration is the subsequent analysis by electron microscopy of a taken sample. But no standards for sampling or analyse exist yet [20]. One major finding of most current studies is that during the production and handling of nanoparticles the workplace concentration of particles below 100 nm is close to the background concentration in most manufacturing companies. This background aerosol consists of ubiquitous ultrafine particles from sources in or outside e.g., particles emitted from diesel engines by trucks or forklifts, welding fumes or even vacuum cleaners with electric motors close to the process. Aggregates or agglomerates above 100 nm in size are quite often detected at the workplace and correlate with operations, further complicating detection and discrimination of NMs by existing measurement devices [26]. This is in line with theoretical calculations indicating that most of the particles emitted from processes are agglomerated when reaching the exposed person [27].