In vitro studies have shown that cobalt substances predominantly induce pre-inflammatory biomarkers, resulting in a grouping of substances either predicted to cause inflammation following inhalation, or those with a different reactivity profile (poorly-reactive). There is a lack of data on whole-organ lung responses following inhalation of these substances, especially relating to the poorly-reactive group. It is of interest to generate tissue-specific histopathological correlation to better ascertain the predictive nature of the lower tier tests (i.e. tier 1 - bioelution, tiers 2a and b - in vitro markers and ToxTracker testing), in order to understand the type of effects caused by the poorly-reactive group and to gauge long-term effects. Eight cobalt substances were tested in vivo in a customized 4-h toxicity test; with animals sacrificed up to 14-days post-exposure. Histopathological severity scores were assigned based on inflammatory and pre-carcinogenic markers. A clear pattern emerged, with the reactive substances causing a persistent increase in one or more of the selected markers, and absence of these markers with poorly-reactive substances. Longer-term studies should be conducted to investigate the repeated dose effects of the poorly-reactive substances.