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.