Iron oxide nanomaterials (FeO NMs) have important biomedical and environmental applications, e.g. drug delivery, chemotherapy, magnetic resonance imaging contrast agents, etc. However, the environmental risks of such FeO are not fully assessed, particularly for soil living invertebrates, which are among the ones in the first line of exposure. Research has showed that longer-term exposure time is required to assess hazards of NMs, not predicted when based on shorter time and are therefore recommended. Thus, in the present study the effects of FeO NMs and FeCl were assessed in the terrestrial environment, using the soil model species (Oligochaeta), throughout its entire lifespan (202 days). Two animals' density were used: 1 (D1) and 40 (D40) animals per replicate, in LUFA 2.2 soil. The endpoints were survival and reproduction monitored over-time, up to 202 days. Results showed that density clearly affected the toxicity response, with higher toxicity and lower lifespan in D1 compared to D40. Overall, FeCl was more toxic than FeO NM in terms of reproduction, however, adult animals can be at higher (long-term) risk when exposed to FeO NM. Differences might be linked to slower Fe kinetics for the FeO NMs, i.e., slower Fe dissolution and release of ions.