For tVOCs and inaccurate, inferred CO2 readings, absolutely. Sensors like the BME680 or SGP30 are ~4mm square. For real CO2 sensors, not yet. The smallest accu

For tVOCs and inaccurate, inferred CO2 readings, absolutely. Sensors like the BME680 or SGP30 are ~4mm square. For real CO2 sensors, not yet. The smallest accurate sensor I've worked with is the SCD30 and it's much too large to fit into a watch body as-is. Sensirion has come up with the SCD40 though, which is 12x12x7mm and should make a wearable CO2 sensor feasible.