FGF21 and obesity due to melanocortin 4 receptor deficiency: Finally, a significant negative association was observed between PM<sub>2.5</sub> concentrations and levels of the novel obesity-related biomarker fibroblast growth factor 21 (FGF-21) (OR: 0.148; 95% CI: 0.025-0.89; <i>P</i> <sub>FDR</sub> = 0.037).