Skin CO₂ sniffing for wearable metabolic monitoring

CO₂ is a key physiological parameter used to assess hypoventilation and to estimate metabolic rates. However, current CO₂ monitoring relies on bulky breath-analysis systems that are impractical for continuous use in daily life. Here, we present a wearable on-skin gas-sniffing system that quantifies skin-emitted CO₂ and establishes its physiological relevance through direct correlation with exhaled CO₂ and metabolic rate. Participant studies demonstrate strong correlations between CO₂ flow rates from the skin and breath during both rest and exercise, with skin-emitted CO₂ approximately four orders of magnitude lower than exhaled CO₂. Skin-emitted CO₂ also correlates with metabolic rate, suggesting its potential as a surrogate for breath-based indirect calorimetry. With its wearable form factor and physiological relevance, this skin gas sniffing system enables continuous, noninvasive metabolic monitoring and opens opportunities for studying skin gas exchange.