Blood Alcohol Content - What is BAC?

Friday 19 July, 2013.

Blood alcohol content is a terminology used to denote the concentration of alcohol in blood and provides a common, standard platform to measure and compare the levels of intoxication.

Blood alcohol content (BAC), the medical term used for measurement of alcohol concentration in blood is also known as Blood Alcohol

Concentration

It can be denoted as a percentage by mass per volume or by mass or a combination of both. For example, Blood alcohol content (BAC) of 0.10% means 1 gram of alcohol per 1000 grams of blood. Or in other way, it also means 0.1 gram of alcohol per 100 milliliters of blood. In some countries, it is also measured as grams of alcohol per 1 liter of blood (g/1000 ml).

As there is a great variation in individual physiology and tolerance level to effects of alcohol from person to person, the number of drinks consumed is not an appropriate measure of intoxication. The amount of intoxication varies greatly with body weight, fat percentage of body and gender. Hence, neither the number of drinks consumed by a person nor Blood Alcohol Content makes the precise indicators of level of intoxication. But still BAC creates a standard platform to compare the intoxication of individuals because of consumption of alcohol

Alcohol concentration in blood can be determined by tests carrying out in laboratory. But for law enforcement investigations, it is not always possible to carry out blood sample tests to determine BAC. So, BAC is determined from Breath Alcohol Concentration (BrAC) using an instrument known as Breathalyzer. Breathalyzers estimate the percentage of alcohol found on breath as alcohol maintains equilibrium throughout the body once ingested due to its high affinity towards water.

Test Assumptions

These tests are carried out while assuming that the individual is an average in many ways. To get explained this, let us look at the predetermined BAC average ratio. The assigned standard value for this is 2100:1.That means on an average, for every part of breath, 2100 parts of alcohol are found in breath. But in actual cases, this ratio can vary from 1300 parts to 3100 parts for every part of breath. Similarly, while analyzing urine to measure BAC, it is assumed that the average ratio is 1.3:1, although the actual proportion varies considerably from person to person.

Moreover, all these analyzing tests are based on the assumption that, there is a complete absorption of alcohol in the body of individual and it is disbursed evenly and completely throughout the body. Thus all these tests are done by assuming that the state of equilibrium has been achieved by the body.

Effects of Alcohol at Different Levels of BAC

When ingested, the intoxication levels gradually increase with an increase in blood alcohol content.

Euphoria: This is the initial stage of intoxication, characterized by BAC level at 0.03 to 0.12%. In this phase, person is much more relaxed, over confident and appears without any inhibitions.
Lethargy: In this phase, BAC ranges from 0.09 to 0.23%. The person stars loosing body balance, his vision gets blurred and appears sleepy.
Confusion: BAC gets increased at 0.17 to 0.28%. Name of this phase rightly denotes the status of the subject as he appears intense in his emotions and feelings. Dizziness and Nausea are peculiar features of this stage.
Stupor: This stage is marked by the BAC count ranging from 0.25 to 0.39%, where persons impaired start loosing and again gaining consciousness at frequent intervals. Due to this feature, there is high amount of risk of coma.
Coma: BAC ranges from 0.35 to 0.50%. Patient becomes unconscious, body reflexes become poor, there is drop in pulmonary beats with decrease in breathing.
Death: BAC exceeds the mark of 0.50%. This is a very fatal stage marked by Central Nervous System failure, which ultimately leads to death.

Legal Limits

Although, a particular BAC has varied effects on body from person to person, Blood Alcohol Content is widely used to determine the levels of intoxication and it provides a standard, common platform for a rough conclusion of the status of the subjects. Most of the countries have prescribed a certain level of BAC as bar and don't allow the maneuvering of vehicles, boats, air crafts and heavy machinery beyond these levels.

Alcohol Regulation Laws differ in their limits from one state to another. While in United States, 0.10% level is assigned for safe driving for common carriers and 0.04% BAC is assigned for aviators; countries like Australia, Argentina prescribed 0.05% level of BAC. In India the permissible limit is 40 mg/deciliter, while countries like Russia, Norway, Poland, Sweden permits the level of 20 mg/deciliter. Countries like Saudi Arabia, Malaysia, Romania, Czech Republic are zero tolerant countries.

BAC Calculation

Certain mathematical approach is used to estimate the BAC. Although, this mathematical estimation of BAC is not as precise as a breathalyzer, but it is very handy to determine the level that is not testable at given moment. Retrograde Extrapolation is one of the most used mathematical processes to calculate BAC at the time of driving is estimated by projection which involves consideration of rate of the absorption and elimination of alcohol between driving and actual testing.

Thus, BAC is the tool used by Physicians, forensic experts and Law department for varied purposes.