1. The ratio varies from person to person, and women typically have a lower ratio, meaning that breath tests are biased against them.
2. About 1.8% of people have a ratio below 2,100, about 14% have a ratio higher than normal.
3. The ratio depends on the temperature of the breath. The ratio is only 2,100 at 37 °C, and the average temperature of the breath is 34°C. If one is running a fever, the machine will read a higher blood alcohol content, and if one is cold, it will read lower. This is further confounded by the variation in average body temperature between individuals.
4. The ratio will change if the person is still absorbing alcohol into his system. The amount of ethanol vapor will be higher during the absorption phase, which lasts from 20 to 90 minutes, depending on the types and quantities of foods and drinks consumed.
5. The human body generates about three grams of ethanol per day in its normal operation. Ethanol is a byproduct of our digestive system, and the amount generated will vary. People on low calorie, carbohydrate restricted diets will generate more, and their breath can read as high as .02 (1/4 legal limit in the US), without ingesting any external alcohol whatsoever.
6. Ethanol vapor in the lungs takes time to evaporate from lung tissue; faster than normal breathing can lower the reading by as much as 32%, and slow breathing or holding one's breath can raise the reading by the same amount.
So a breath test certainly could be accurate, but it seems about as likely as finding a broken watch showing the correct time. Even if they take you to the station and draw blood, the concentration will vary between arteries and veins, depending if the person is still absorbing alcohol previously consumed. Veins will show a higher reading after absorption is complete, and the law generally states that they are to be used for the blood sampling.
To lower a breath test reading:
1. Lower your body temperature -- remove clothing, drink a lot of cold water, etc.
2. Breathe as fast and deeply as possible before the test.
3. Eat a lot of carbs before drinking.
4. Leave the bar a long time after your last drink.
5. Make sure you don't have a fever (Tylenol and Advil are fever reducers.)
