The Devil Lies in the Details: Small Structural and Chemical Changes in Iron Oxide Pigments Largely Alter the Biological Outcomes in Macrophages

Because of their technical qualities, such as resistance to fading and to high temperatures, mineral pigments are still widely used. Among mineral pigments, iron oxide pigments represent a widely used class, because of their diversity of shades (from yellow to red to brown to black) and low toxicity compared to heavy metals-based pigments. However, low toxicity does not mean the absence of adverse effects. We thus investigated the biological effects of two different subtypes of Pigment Red 101, i.e., hematite, produced by two different processes, namely a wet precipitation process and a calcination process. Macrophages were chosen as a target cell type because they represent the main scavenger cell type that is in charge of handling particulate materials in the body. During this comparison, we realized that the calcined pigment was contaminated from the start by bacterial endotoxins, which induced intense inflammatory responses and biased the comparison. After depyrogenation, the calcined pigment proved to dissolve to a higher extent in macrophages, but to show less intense adverse effects (e.g., alteration of the mitochondrial transmembrane potential, oxidative stress and inflammatory responses) than the precipitated pigment in a recovery exposure mode, allowing us to investigate delayed effects of the pigments. Thus, despite their identical pigment number, pigments differing in their structure and in their synthesis induce different responses from living cells, even if administrated in equivalent amounts. This should be taken into account for some applications, such as tattooing. Moreover, endotoxin contamination should also be checked to increase workers' and users' safety.