Iron oxide nanoparticles (IONP) are currently being studied as green magnet resonance imaging (MRI) contrast agents. They are also used in huge quantities for environmental remediation and water treatment purposes, although very little is known on the consequences of such applications for organisms and ecosystems. In order to address these questions, we synthesised polyvinylpyrrolidone-coated IONP, characterised the particle dispersion in various media and investigated the consequences of an IONP exposure using an array of biochemical and biological assays. Several theoretical approaches complemented the measurements. In aqueous dispersion IONP had an average hydrodynamic diameter of 25 nm and were stable over six days in most test media, which could also be predicted by stability modelling. The particles were tested in concentrations of up to 100 mg Fe per L. The activity of the enzymes glutathione reductase and acetylcholine esterase was not affected, nor were proliferation, morphology or vitality of mammalian OLN-93 cells although exposure of the cells to 100 mg Fe per L increased the cellular iron content substantially. Only at this concentration, acute toxicity tests with the freshwater flea Daphnia magna revealed slightly, yet insignificantly increased mortality. Two fundamentally different bacterial assays, anaerobic activated sludge bacteria inhibition and a modified sediment contact test with Arthrobacter globiformis, both rendered results contrary to the other assays: at the lowest test concentration (1 mg Fe per L), IONP caused a pronounced inhibition whereas higher concentrations were not effective or even stimulating. Preliminary and prospective risk assessment was exemplified by comparing the application of IONP with gadolinium-based nanoparticles as MRI contrast agents. Predicted environmental concentrations were modelled in two different scenarios, showing that IONP could reduce the environmental exposure of toxic Gd-based particles by more than 50%. Application of the Swiss "Precautionary Matrix for Synthetic Nanomaterials" rendered a low precautionary need for using our IONP as MRI agents and a higher one when using them for remediation or water treatment. Since IONP and (considerably more reactive) zerovalent iron nanoparticles are being used in huge quantities for environmental remediation purposes, it has to be ascertained that these particles pose no risk to either human health or to the environment.