evidence: Direct human health risks of increased atmospheric carbon dioxide [...] Growing evidence suggests that environmentally relevant elevations in CO2 (<5,000 ppm) may pose direct risks for human health. Increasing atmospheric CO2 concentrations could make adverse exposures more frequent and prolonged through increases in indoor air concentrations and increased time spent indoors. We review preliminary evidence concerning the potential health risks of chronic exposure to environmentally relevant elevations in ambient CO2, including inflammation, reductions in higher-level cognitive abilities, bone demineralization, kidney calcification, oxidative stress and endothelial dysfunction. This early evidence indicates potential health risks at CO2 exposures as low as 1,000 ppm—a threshold that is already exceeded in many indoor environments with increased room occupancy and reduced building ventilation rates, and equivalent to some estimates for urban outdoor air concentrations before 2100. Continuous exposure to increased atmospheric CO2 could be an overlooked stressor of the modern and/or future environment. Further research is needed to quantify the major sources of CO2 exposure, to identify mitigation strategies to avoid adverse health effects and protect vulnerable populations, and to fully understand the potential health effects of chronic or intermittent exposure to indoor air with higher CO2 concentrations. [...] Schaefer, K. E. Effects of increased ambient CO2 levels on human and animal health. Experientia38, 1163–1168 (1982).