Human skin emits light (albeit the glow is extremely weak) and a wide variety of small molecules that may be sometimes "sniffed" by dogs or even other humans. These chemicals tell a story about our health and wellness, things we eat and drink, touch and breathe. Mosquitoes use such emissions to assess our "attractiveness" from indicators such as Indoles (unpleasantly smelling but healthy "inner soil" biomarker) or carbon dioxide (amount of which correlates with the size of the person producing it) in the air.
Sampling air for health indicators is much less invasive than drawing blood and could be an invaluable diagnostic tool. Many applications of gas sensors have been proposed over decades ranging from sweat patches to sensorized garments. Where are we now? What is the 2016 state of the art?