Example: A drunk driver gets into an accident. He is transported to the hospital where a nurse takes his blood. Later, his blood alcohol results are determined in the lab. What is the conversion for determining what his breath alcohol concentration would have been if he submitted samples of his breath into a breathalyzer machine?
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The numbers measured would be the same because the different devices convert the number during the measurement. The ratios of what is actually measured in the breath and urine are describe at the reference given.
Alcohol is removed from the bloodstream by a combination of metabolism, excretion, and evaporation. The relative proportion disposed of in each way varies from person to person, but typically about 90 to 98% is metabolised, 1 to 3% is excreted in urine, and 1 to 5% evaporates through the breath. A very small proportion (less than 0.5%) is also excreted in the sweat, tears, etc. Excretion into urine typically begins after about 40 minutes, whereas metabolisation commences as soon as the alcohol is absorbed, and even before alcohol levels have risen in the brain. (In fact, in some males, alcohol dehydrogenase levels in the stomach are high enough that some metabolization occurs even before the alcohol is absorbed.)