A passive vehicular heatstroke prevention system monitors carbon dioxide (CO2) and infrared (IR) energy levels to determine whether a child is present inside a closed vehicle, and, if so, monitors the temperature in the vehicle and, if the temperature in the vehicle exceeds at least one preset critical value, automatically lowers the temperature in the vehicle and contacts the driver/caregiver and/or emergency personnel. The system detects the presence of a child in the closed vehicle by detecting a critical level of carbon dioxide in the air within the vehicle, while monitoring the interior vehicle temperature and takes corrective action to prevent the temperature from exceeding a preset value, such as by activating the vehicle's air conditioning unit and lowering the vehicle's windows, as well as contacting the driver/caregiver and/or emergency personnel.