## 5 General Recommendations for Various Stakeholders
### Environmental and Health Researchers  
Environmental and health researchers and research bodies, whether in the public or private sector, are facing quite a challenge in regard to NMs, not least because some NMs may be persistent and cumulative. Currently no environmental monitoring technology is in place to allow for the monitoring of the expected increased environmental concentrations of persistent NMs. While we recommend this, it is certainly one point of future research which is currently not seen to be an easy task. We suggest that for the time being concentrations in the environment have to be predicted rather than measured.  
Environmental studies have so far been limited to release-related studies such as [PERSON] _et al._[62] on the TiO\({}_{2}\) wash-off from facades. The main way of assessing possible environmental concentration currently is using emission-based approaches and models such as those pursued by [PERSON] _et al._[63] Here they use information on production rates, release fractions, assumed or based on measurement (e.g., sewage plant studies), and environmental transport to model environmental concentrations. These concentrations may be compared with environmental no-effect levels for plants, animals and humans to assess a possible risk. Still the application of the model is limited to _a priori_ information which would be good to overcome in the future.  
We suggest that the traditional approach for determining risk characterization ratios (EUSES) does not work, and EUSES predictions of environmental concentrations may not be applicable to nanoparticles, since nanoparticles are dispersed rather than dissolved. Suspension (and rate of dissolution) is not modelled for conventional chemicals. The tendency of nanoparticles to aggregate and agglomerate is also obviously not considered for conventional chemicals. It is unclear how to model partitioning of NMs attached to solid surfaces because the following are unknown or not included in current models: the extent/rate of dissolution; the extent or rate of agglomeration or settling; and the extent of association with sediment.  
**Recommendation 31**: Distribution models must be updated to encompass NMs, and the focus should be on quantifying distributions e.g., for \"suspended dissolved\". In addition there is a need to focus on obtaining the fundamental understanding of distributions. This knowledge can then be used to develop models. Urgent attention should be given to finding a replacement for the octanol-water partition coefficient (Kow). A parameter other than Kow is required for predicting solid phase-water partitioning, but more research in NM partitioning in environmental phases is needed before such replacements can be made. It is likely that the following parameters will assist in deriving the most relevant descriptors for NM distribution in the environment: size distribution, shape, coating/surface chemistry, synthesis method, state of agglomeration, shape, \"pure\" NMs _vs._ formulation/embedded NMs. Furthermore, affinity for adsorption of natural organic matter (NOM) and zeta potential with NOM as well as dissolution rate may be of high value.  
**Recommendation 32**: Routine methods should be developed for the quantification of NMs in tissues, both total mass concentration and particle number concentration per g of tissue. There are detection limits in environmental samples and there is a technical gap of some three orders of magnitude. Current instrumentation only works down to mg/l, but environmental concentrations are ng-ug/L. The timescale is short and medium term. This is a technical barrier to developing environmental monitoring programs at national levels. There are currently none for NMs in river water or soil.