The legal requirement to submit to chemical breath tests as well as the accuracy of such tests (or lack of it) have been detailed on this blog in the past. All of this talk about measuring your breath might prompt a perfectly relevant question: Why measure your breath to determine if something is in your blood?
Alcohol follows a strange pathway when it enters your body. This pathway is described in detail by The Alcohol Pharmacology Education Partnership. Because the particular form of alcohol used in drinks such as beer, wine and liquor (ethanol) is water-soluble, it can pass through the membranes of your stomach and small intestine through a process known as passive diffusion. This is how it enters into the bloodstream via capillaries and is transported throughout your body.
Its first stop is the heart, where the right ventricle them pumps it into the lungs. The carbon dioxide in the blood then reacts with oxygen brought into your lungs when you breathe. This vaporizes some of the ethanol into a gas, which then collects in the lung sacs. The remaining ethanol remains in the blood and travels a circuitous route through the body back to the lungs.
The gaseous ethanol collected in the lung sac then escapes your body as you breathe. This causes the balance of gaseous ethanol to liquid ethanol in the blood to become uneven. More ethanol from the blood is then vaporized to achieve equilibrium. This process continues with each breath as the alcohol is metabolized and works its way out of your system.
As you can see, this process is incredibly dynamic, or ever-changing. Knowing this, you can see why the results of preliminary alcohol screenings (such as breathalyzer tests) have such a high margin of error.