Naomi Wu evaluates low-cost CO2 meters by charging them, letting them stabilize (~30 minutes), and comparing readings to the Aranet4 NDIR reference. Several inexpensive units tracked within ~50 ppm; her top picks are the Vitalight/Mini CO2 Detector (~$60), Sensirion USB Gadget (~$60) for travel, and Qingping Air Monitor Lite (~$100) for home, while Aranet4 ($200–$250) remains the gold standard.