How is it that a breathalyzer can measure  the alcohol content in someone’s blood, hours after they had their last drink,  based on their breath alone? Exhaled breath contains trace amounts  of hundreds, even thousands, of volatile organic compounds: small molecules lightweight enough  to travel easily as gases. One of these is ethanol,  which we consume in alcoholic drinks. It travels through the bloodstream  to tiny air sacs in the lungs, passing into exhaled air  at a concentration 2,000 times lower, on average, than in the blood. When someone breathes  into a breathalyzer, the ethanol in their breath  passes into a reaction chamber. There, it’s converted to another molecule, called acetic acid, in a special type of reactor that produces an electric current during the reaction. The strength of the current  indicates the amount of ethanol in the sample of air,  and by extension in the blood. In addition to the volatile  organic compounds like ethanol we consume in food and drink, the biochemical processes of our cells  produce many others. And when something disrupts  those processes, like a disease, the collection of volatile  organic compounds in the breath may change, too. So could we detect disease  by analyzing a person’s breath, without using more invasive  diagnostic tools like biopsies, blood draws, and radiation? In theory, yes, but testing for disease is a lot more  complicated than testing for alcohol. To identify diseases, researchers need to look at a set  of tens of compounds in the breath. A given disease may cause  some of these compounds to increase or decrease in concentration,  while others may not change— the profile is likely to be different  for every disease, and could even vary for different stages  of the same disease. For example, cancers are among  the most researched candidates for diagnosis through breath analysis. One of the biochemical changes  many tumors cause is a large increase  in an energy-generating process called glycolysis. Known as the Warburg Effect, this increase in glycolysis results  in an increase of metabolites like lactate which in turn can affect a whole cascade  of metabolic processes and ultimately result  in altered breath composition, possibly including an increased  concentration of volatile compounds such as dimethyl sulfide. But the Warburg Effect is just one  potential indicator of cancerous activity, and doesn’t reveal anything  about the particular type of cancer. Many more indicators are needed  to make a diagnosis. To find these subtle differences, researchers compare the breath  of healthy people with the breath of people  who suffer from a particular disease using profiles based on hundreds  of breath samples. This complex analysis  requires a fundamentally different, more versatile type of sensor  from the alcohol breathalyzer. There are a few being developed. Some discriminate  between individual compounds by observing how the compounds move  through a set of electric fields. Others use an array of resistors  made of different materials that each change their resistance  when exposed to a certain mix of volatile organic compounds. There are other challenges too. These substances are present  at incredibly low concentrations— typically just parts per billion, much lower than ethanol concentrations  in the breath. Compounds’ levels may be affected  by factors other than disease, including age, gender, nutrition,  and lifestyle. Finally, there’s the issue of distinguishing which compounds  in the sample were produced in the patient’s body and which were inhaled  from the environment shortly before the test. Because of these challenges,  breath analysis isn’t quite ready yet. But preliminary clinical trials  on lung, colon, and other cancers  have had encouraging results. One day, catching cancer early  might be as easy as breathing in and out.