The Economist has this interesting article on an emerging field called ‘olfactory diagnostics’. Basically diagnosing ills by analyzing the air exhaled by patients. It has already been shown that more than 3,000 compounds are regularly exhaled, excreted or exuded from the human body.
By analyzing these compounds and their relative quantities a surprising amount of information can be gained. For example specially trained dogs have been diagnosing bladder cancer for years. However, advances are now being made in order to automate and increase the diagnoses possible.
It is plausible that in future incredible amounts of information could be gleaned through analysis of the air we exhale. Imagine the privacy concerns if, for instance, your employer could learn about your eating habits and possible health problems by analyzing the air you exhale at work.