Blood Alcohol Content (BAC)

By James Parker
/
February 2, 2022

What Is BAC?

Blood Alcohol Content (BAC) is a way to measure the amount of alcohol that is in an individual’s blood at a given time. This measurement is the key factor for determining scientifically how intoxicated an individual is. 

When BAC is measured, it is measured in grams per 100 milliliters of blood. That means that a BAC measurement represents what portion of an individual’s blood is made up of alcohol. As an example, an individual with a BAC of 0.08 has blood that is 8% alcohol by volume. The higher the portion of an individual’s blood that is made up of alcohol, the more intoxicated they are and the more dangerous it can be for their health.

There are several stages of intoxication associated with certain BAC levels according to doctors and academic sources such as the University of Notre Dame. These varying levels are generally:

  • 0.020-0.039: Feelings of slight euphoria, and increased extroversion.
  • 0.040-0.059: Feelings of warmth and euphoria along with minor impairment of judgment, memory, and caution.
  • 0.06-0.099: The legal limit. Slight impairment of balance, speech, vision, reaction time, and hearing. Impaired reasoning and memory.
  • 0.100-0.129: Significant impairment of movement, judgment, speech, balance, peripheral vision, reaction time, and hearing.
  • 0.130-0.159: Serious lack of physical control. Blurred vision and loss of balance. 
  • 0.160-0.199: Feelings of unwellness and nausea.
  • 0.200-0.249: Inability to walk, total mental confusion, nausea, vomiting, possible blackout.
  • 0.250-0.399: Alcohol poisoning and loss of consciousness.
  • 0.40: Alcohol-induced coma, with the possibility of death due to respiratory failure.

In general BAC can vary by biological sex, weight, height, and temporary factors like how long it has been since the individual has eaten and how much, if any, water the individual drinks alongside their alcoholic beverages. BAC generally increases after drinking alcoholic beverages and decreases over the course of several hours once the individual has stopped drinking alcoholic beverages and their body has time to metabolise the ethanol into water and carbon monoxide.

Key Takeaways

  • Blood Alcohol Content (BAC) is a measure of the alcohol levels present in the blood.
  • BAC is measured to determine whether an individual is intoxicated using several methods including breathalyzers.
  • Breathalyzers can actually fluctuate depending on calibration, sensitivity, and general variance on top of only measuring breath alcohol levels rather than a true serological measurement of BAC.
  • If you were arrested and charged with a DUI based on a BAC that was very close to or just under the legal limit, an experienced DUI attorney may be able to improve the outcome of your case by leveraging experience and expert testimony.

BAC and Driving Under the Influence (DUI) Charges

When it comes to measuring BAC in the context of a DUI charge there are only two thresholds that matter: The “per se” threshold and the “zero tolerance” threshold. The per se threshold is the BAC measurement required for an officer or prosecutor to prove that, as a matter of law, a driver is intoxicated. If a driver has a BAC at or above the per se threshold then it cannot be argued that they were not drunk at the time of the test. Every state has a per se BAC limit of 0.08 except for Utah which has a more strict limit of 0.05. 

The zero tolerance threshold applies exclusively to underage drivers accused of driving while drunk. Since underaged drivers cannot legally drink, they have a much stricter BAC limit to be found to be in violation of the law as a matter of law. These levels can range from 0.02 to 0.0 depending on the state. One state, New Jersey is the only state to have a zero tolerance limit of 0.01.

With these limits as hard lines for drivers, the accuracy of a BAC test can be the difference between wrongful conviction and innocence. Unfortunately, BAC tests can be seriously unreliable and influenced by a variety of factors. The first thing to note is that not all BAC tests are equal. In general, the way that BAC tests rank in terms of accuracy is, in descending order, blood tests, breath tests, and urine tests.

Blood tests are the golden standard for measuring BAC because they draw blood from the accused. However, there are also significant drawbacks to this method. An officer cannot administer a blood test in the field. Instead, the accused must be brought to a clinic where a licensed phlebotomist will take a sample  with the accused’s consent. Some jurisdictions allow samples to be collected with a warrant but in general, because refusing to consent has been linked to harsher sentences and may result in immediate suspension of driving privileges, most people will consent willingly. 

While blood samples are highly accurate, they are neither expedient nor infallible. Testing of the blood can take multiple days and any error in preservation, sterilization, transportation, or even labeling can result in inaccurate results being forwarded to the police.

The most common method of BAC testing is with a breathalyzer. While breathalyzers are convenient and expedient they are also far from accurate. To begin, a breathalyzer does not measure blood alcohol levels, they measure breath alcohol levels. Since the lungs only exhale about 8% of alcohol content in the blood, breathalyzers extrapolate the accused’s BAC. This extrapolation is not only inaccurate from the start, the semiconductor used to measure BAC in cheaper breathalyzers can also react to acetone in the mouth if the accused is a diabetic, cigarette smoke if the accused is a smoker, and chemicals with acetates such as hairspray.

In addition to material error, breathalyzer can suffer from miscalculation if it is incorrectly calibrated, if the software calculating BAC glitches, if there are environmental varnishes or paint fumes in the air, s well as the presence of any foreign substance which may throw off the readings. Breathalyzers are so prone to error or inaccuracy, that in many jurisdictions, a roadside breath reading is not considered admissible evidence in court. A roadside reading can, however, be used as justification to make an arrest. 

Finally, BAC can technically be analyzed through a urine test. This testing method is extremely rare and usually only used when there are no other alternatives available. Urine samples are often inaccurate since the body can take varying amounts of time to break alcohol down in the kidneys and the presence of alcohol can remain in the urine for up to 24 hours, making it difficult to prove that at the time the accused was driving they were intoxicated.

Bottom Line

If you have been charged with a DUI on the strength of a single breathalyzer test or after being brought in for a blood test, you will need an experienced DUI attorney to help achieve the best outcome in your case. A DUI attorney can examine the process by which the prosecutor ascertained your BAC and find the potential flaws in those methods. A DUI attorney can challenge the accuracy of the BAC test, including whether the results were within a margin of error that could indicate a false positive. 

With an experienced DUI attorney, your BAC test may be thrown out or placed on shaky footing that could be the grounds to achieve a better outcome in the case overall.

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