Carbon dioxide released by the burning of fossil fuels isn’t just bad for the planet — it’s also bad for your brain. Scientists know that humans’ cognition declines as carbon dioxide levels rise in the air they breathe. Kristopher Karnauskas at the University of Colorado Boulder and his colleagues analysed how indoor CO2 levels might increase under various emissions scenarios. If greenhouse gases continue to rise unabated, atmospheric CO2 levels will increase from roughly 410 parts per million to 930 parts per million by 2100. Indoor CO2 levels track outdoor CO2 levels, which means that in a typical school classroom, where people are also breathing out CO2, the level could reach 1,400 parts per million, the team concluded.