 For indoor environments such as offices or gyms where the principal source of CO2 is human respiration, rescaling some easier-to-measure quantities such as volatile organic compound (VOC) and hydrogen gas (H2) concentrations provides a good-enough estimator of the current CO2 concentration for ventilation and occupancy purposes.Sensors for these substances can be made using cheap (~$20) MEMS metal oxide semiconductor (MOS) technology.The reading they generate is called estimated CO2 (eCO2) or CO2 equivalent (CO2eq).