Motivation: Recently, the Mauna Loa Observatory in Hawaii repeatedly measured unprecedented inclines of carbon dioxide (CO2) levels over the last years. In patients with chronic lung diseases, studies suggest that increased levels of CO2 reduce macrophage function during inflammatory processes; however, little is known how macrophages might sense CO2 and adapt upon high levels of CO2 during differentiation and activation. We, therefore, aimed to elucidate the effects of CO2 on gene and protein expression during basic inflammatory processes, such as monocyte differentiation and macrophage activation.  Methods: Monocyte differentiation was induced by phorbol 12-myristate 13-acetate (PMA). Primary macrophages (BMDMs) were polarized using different cytokines to induce pro-inflammatory (M1) macrophages (lipopolysaccharides) or immuno-modulatory (M2) macrophages (interleukin-4). Cells were simultaneously subjected to different levels of CO2. Morphological changes, mRNA and protein expression of markers for cell differentiation and macrophage polarization were determined.  Results: High levels of CO2 attenuated PMA-induced cell differentiation of human monocytes. mRNA and protein levels of several pro-inflammatory markers of cell differentiation were reduced upon high CO2 levels. In BMDMs, CO2 significantly reduced transcript levels of M1-marker and exclusively increased the expression of arginase-1 (M2-marker). Moreover, CO2 mitigated PMA-stimulated PKC activity. Experiments with buffered medium revealed that changes in pH were responsible for most, but not all, of the CO2-mediated effects on monocyte differentiation and macrophage polarization. Conclusion: CO2 markedly reduces both monocyte differentiation and macrophage activation. A better understanding of CO2-adaptive molecular pathways and CO2-sensing mechanisms will have great therapeutic impact on all inflammation-driven diseases characterized by a microenvironment with high CO2 levels, including chronic lung and tumor diseases.