Breathalyzer v. BAC - What's the Difference? (And why does it matter to you?)
You may hear "breathalyzer" and "blood alcohol content" (BAC) used interchangeably, but in fact the test commonly known as a breathalyzer doesn't actually read your blood alcohol content at all. What a breath test does is attempt to estimate your BAC, and there are numerous reasons that breathalyzer results may be flawed.
There are several methods of testing breath alcohol content, each employing norms, adjustments and estimates to approximate the subject's blood alcohol content—the number that really counts. The breath test is most commonly administered using a portable device for roadside screening, although some law enforcement agencies maintain larger (and somewhat more accurate) machines as well. Some states exclude portable device readings as evidence but permit evidence from the more reliable fixed models, although portable technology is improving and results from some roadside units are now admissible.
Blood alcohol content is critical, and not only because it can be used as evidence of intoxication or impairment. The law in every state currently makes it a crime to operate a motor vehicle with a BAC in of .08 percent or greater. That means that a driver who tests at .08 or above on an admissible test can be convicted of a crime and face a license suspension and possible jail time even if he shows no signs of impairment.
True blood alcohol content readings can only be obtained through testing the subject's blood. However, in many states law enforcement relies solely on breath or urine tests as evidence. That reliance opens the door to numerous possible problems and inaccuracies. For instance:
- With many devices, the translation from breath alcohol content to blood alcohol content relies on assuming a specific ratio (2100:1) between BAC and breath alcohol content. However, the actual ratio for a particular individual may vary between 1700:1 and 2400:1. That means, roughly, that a breathalyzer based reading of .08 could actually mean blood alcohol content of between .065 and .09. That's a particularly significant difference in that operating with a BAC of .08 or greater is in and of itself a crime, whereas with a BAC of .065, a showing of impairment would be required. In short, in most states a driver who passes field sobriety tests but has a BAC of .08 is still guilty—the same driver with a slightly lower BAC is probably not.
- Some breath test machines identify not only ethyl alcohol, but also any other methyl group compound. That means that many other substances that may be detected in human breath may be misread as alcohol content. Some of the clearest problem areas:
- Some machines are confused by the presence of acetone, which may be present in unusually large quantities in diabetics and others who may be maintaining special diets.
- Certain paints, adhesives, and plastic have been demonstrated to give off fumes that generate false positive breath test results, though true BAC tested simultaneously remained at zero. In one dramatic example published in the Journal of Analytical Toxicology, a painter exposed to fumes under controlled conditions and with no alcohol in his system showed a .075 breathalyzer reading, although his true BAC reading based on a blood test was .00.
- Physical activity can impact apparent BAC levels. One study found that running up and down stairs immediately before a breath test could decrease BAC readings by as much as 25%.
- Like breath tests, urine tests rely on an assumption about conversions that may or may not hold true for any individual person. The translation from alcohol content in the urine to BAC is based on a 1.3:1 ratio for urine alcohol content to blood alcohol content, but like the breath-blood ratios, the actual numbers may vary significantly: the 1.3-1 ratio is simply the norm.