CO₂ sensors are essential for determining if a room is too 'stuffy' - high CO₂ makes humans grumpy and tired. That's why it's always nice to take a deep breath outside where CO₂ is about 400 ppm. However, until now, CO₂ sensors were always really chunky (like the SCD-30) or pricey (like the SCD-40). Inexpensive sensors like the SGP30 approximate the CO₂ using VOC gas concentration, but they don't actually measure CO₂.