In previous posts, I have explained many of the reasons why breathalyzers are inaccurate and unreliable. See, for example, "Breathalyzers — and Why They Don’t Work"; "Warning: Breathalyzer in Use"; "Convicting the ‘Average’ DUI Suspect"; "Why Breathalyzers Don’t Measure Alcohol"; "Driving Under the Influence of… Gasoline?; "How to Fool the Breathalyzer". (These and many other sources of error are explained more fully in my book, Drunk Driving Defense, 6th edition.)
One of the most common sources of error in breath alcohol analysis is simply testing the subject too early — while his or her body is still absorbing the alcohol. Let’s take a common example. At a restaurant Sarah shares a bottle of wine with a friend. She nurses one glass over a one-hour dinner. Nearing the end, another glass is poured from the bottle and she finishes this. The two friends then order an after-dinner drink. Noting the time, Sarah quickly finishes the drink and leaves. She is stopped by the police one block from the restaurant. After questioning and field sobriety tests, she is taken to a police station and tested on a breathalyzer. The machine shows her blood alcohol concentration (BAC) to be .09% — over the legal limit. She is booked for DUI and jailed.
Sarah’s true BAC, however, was much lower. If a blood sample had been taken instead of a breath test, the results would have shown only .05% — well under the legal limit. Absorption of alcohol continues for anywhere from 45 minutes to two hours after drinking or even longer. Peak absorption normally occurs within an hour; this can range from as little as 15 minutes to as much as two-and-a-half hours. The presence of food in the stomach can delay this to as much as four hours, with two hours being common. During this absorptive phase, the distribution of alcohol throughout the body is not uniform; uniformity of distribution — called equilibrium — will not occur until absoprtion is complete. In other words, some parts of the body will have a higher blood alcohol concentration (BAC) than others.
One aspect of this non-uniformity is that the BAC in arterial blood will be higher than in veinous blood (laws generally require blood samples to be veinous). During peak absorption arterial BAC can be as much as 60 percent higher than veinous. This becomes very relevant to breath alcohol analysis because the alveolar sacs in the lungs are bathed by arterial blood, not veinous: The diffusion of alcohol through the sacs and into the lung air will reflect the BAC of the body’s arterial blood. Therefore, the breath sample obtained by the machine will be reflective of pulmonary BAC — which, during absorption, will be considerably higher than veinous BAC (and higher than the BAC in other parts of the body). After extensive research, one of the most noted experts in the field of blood alcohol analysis has concluded:
Breath testing is not a reliable means of estimating a subject’s blood alcohol concentration during absorption….. There is a significant likelihood that a given subject will be in the absorptive state when tested under field conditons. Because of large differences in arterial BAC and veinous BAC during absorption, breath test results consistently overestimate the result that would be obtained from a blood test — by as much as 100% or more. In order to have some idea of the reliability of a given breath test result, it is essential to determine by some objective means whether the subject is in the absorptive or post-absorptive state. In the absence of such information, an appropriate value for the uncertainty associated with the absorptive state should be applied to all breath test results.
Simpson, "Accuracy and Precision of Breath Alcohol Measurements for Subjects in the Absorptive State", 33(6) Clinical Chemistry 753 (1987). The most recognized expert in the field, Professor Kurt Dubowski of the University of Oklahoma, agrees with Simpson: "When a blood test is allowed, an administered breath test is discriminatory, because in law enforcement practice the status of absorption is always uncertain."