Blood Alcohol Content (Blood Alcohol Concentration)



Blood alcohol content or blood alcohol concentration (abbreviated BAC) is the concentration of alcohol in a person's blood. BAC is most commonly used as a metric of intoxication for legal or medical purposes. It is usually measured in terms of mass per volume, but can also be measured in terms of mass per mass. Blood alcohol concentration is given in many different units and notations, but they are all relatively synonymous with each other numerically.

The number of drinks consumed is a poor measure of BAC, largely because of variations in weight, sex, and body fat. However, it is generally accepted that the consumption from sober of one standard drink of alcohol (e.g. 14 grams (17.74 ml) ethanol content by U.S. standard) will increase the average person's BAC roughly 0.02% to 0.05% and would return to 0% about 1.5 to 3 hours later (at a dissipation rate of around 0.015% per hour).

Effects At Different Levels

Unless a person has developed a high tolerance for alcohol, a BAC rating of 0.20% represents very serious intoxication (most first-time drinkers would be unconscious by about 0.15%), and 0.35%-0.40% represents potentially fatal alcohol poisoning. 0.40% is the accepted LD50, the dose that is lethal for 50% of adult humans. In extreme cases, individuals have survived and remained conscious despite having BACs as high as 0.914. the most extreme case appears to be a 30-year-old male who survived a blood alcohol concentration of 2.50%, more than three times the lethal limit; "admitted to hospital in a deeply comatose state", his life was saved by "swift invasive therapy including peritoneal dialysis".

Progressive Effects of Alcohol
BAC (%)
Behavior
Impairment
0.01-0.029
  • Average individual appears
    normal
  • Subtle effects that can be
    detected with special tests
0.03-0.059
  • Mild euphoria
  • Sense of well-being
  • Relaxation
  • Talkativeness
  • Joyous
  • Decreased inhibition
  • Alertness
  • Judgment
  • Coordination
  • Concentration
0.06-0.10
  • Blunted Feelings
  • Disinhibition
  • Extroversion
  • Impaired Sexual Pleasure
  • Reflexes
  • Reasoning
  • Depth Perception
  • Distance Acuity
  • Peripheral Vision
  • Glare Recovery
0.11-0.20
  • Over-Expression
  • Emotional Swings
  • Angry or Sad
  • Boisterous
  • Reaction Time
  • Gross Motor Control
  • Staggering
  • Slurred Speech
0.21-0.29
  • Stupor
  • Loss of Understanding
  • Impaired Sensations
  • Severe Motor Impairment
  • Loss of Consciousness
  • Memory Blackout
0.30-0.39
  • Severe Depression
  • Unconsciousness
  • Death Possible
  • Bladder Function
  • Breathing
  • Heart Rate
>0.40
  • Unconsciousness
  • Death
  • Breathing
  • Heart Rate

Test Assumptions

Blood alcohol tests assume the individual being tested is average in various ways. For example, on average the ratio of BAC to breath alcohol content (the partition ratio) is 2100 to 1. In other words, there are 2100 parts of alcohol in the blood for every part in the breath. However, the actual ratio in any given individual can vary from 1300:1 to 3100:1, or even more widely. This ratio varies not only from person to person, but within one person from moment to moment. Thus a person with a true blood alcohol level of .08 but a partition ratio of 1700:1 at the time of testing would have a .10 reading on a Breathalyzer calibrated for the average 2100:1 ratio.

A similar assumption is made in urinalysis. When urine is analyzed for alcohol, the assumption is that there are 1.3 parts of alcohol in the urine for every 1 part in the blood, even though the actual ratio can vary greatly.

Breath alcohol testing further assumes that the test is post-absorptive-that is, that the absorption of alcohol in the subject's body is complete. If the subject is still actively absorbing alcohol, his body has not reached a state of equilibrium where the concentration of alcohol is uniform throughout the body. Most forensic alcohol experts reject test results during this period as the amounts of alcohol in the breath will not accurately reflect a true concentration in the blood.

Metabolism And Excretion

Alcohol is removed from the bloodstream by a combination of metabolism, excretion, and evaporation. The relative proportion disposed of in each way varies from person to person, but typically about 92 to 98% is metabolised, 1 to 3% is excreted in urine, and 1 to 5% evaporates through the breath. A very small proportion (less than 0.5%) is also excreted in the sweat, tears, etc. Excretion into urine typically begins after about 40 minutes, whereas metabolisation commences as soon as the alcohol is absorbed, and even before alcohol levels have risen in the brain. (In fact, in some males, alcohol dehydrogenase levels in the stomach are high enough that some metabolization occurs even before the alcohol is absorbed.)

Alcohol is metabolised mainly by the group of six enzymes collectively called alcohol dehydrogenase. These convert the ethanol into acetaldehyde (an intermediate that is actually more toxic than ethanol). The enzyme acetaldehyde dehydrogenase then converts the acetaldehyde into non-toxic Acetyl-CoA.

Many physiologically active materials are removed from the bloodstream (whether by metabolism or excretion) at a rate proportional to the current concentration, so that they exhibit exponential decay with a characteristic halflife (see pharmacokinetics). This is not true for alcohol, however. Typical doses of alcohol actually saturate the enzymes' capacity, so that alcohol is removed from the bloodstream at an approximately constant rate. This rate varies considerably between individuals; experienced male drinkers with a high body mass may process up to 30 grams (38 mL) per hour, but a more typical figure is 10 grams (12.7 mL) per hour. Persons below the age of 25, women, persons of certain ethnicities, and persons with liver disease may process alcohol more slowly. Many East Asians (e.g. about half of Japanese) have impaired acetaldehyde dehydrogenase; this causes acetaldehyde levels to peak higher, producing more severe hangovers and other effects such as flushing and tachycardia. Conversely, members of certain ethnicities that traditionally did not brew alcoholic beverages have lower levels of alcohol dehydrogenases and thus "sober up" very slowly, but reach lower aldehyde concentrations and have milder hangovers. Rate of detoxification of alcohol can also be slowed by certain drugs which interfere with the action of alcohol dehydrogenases, notably aspirin, furfural (which may be found in fusel oil), fumes of certain solvents, many heavy metals, and some pyrazole compounds. Also suspected of having this effect are cimetidine (Tagamet), ranitidine (Zantac), and acetaminophen (Tylenol) (paracetamol).

There are currently no known drugs or other ingestible agents which will accelerate alcohol metabolism. Alcohol ingestion can be slowed by ingesting alcohol on a full stomach. Spreading the total absorption of alcohol over a greater period of time decreases the maximum alcohol level, decreasing the hangover effect. Thus, drinking on a full stomach or drinking while ingesting drugs which slow the release of acetaldehyde, will reduce the maximum blood levels of this substance, and decrease the hangover. Alcohol in non-carbonated beverages is absorbed more slowly than alcohol in carbonated drinks.

Retrograde Extrapolation

Retrograde extrapolation is the mathematical process by which someone's blood alcohol concentration at the time of driving is estimated by projecting backwards from a later chemical test. This involves estimating the absorption and elimination of alcohol in the interim between driving and testing. The rate of elimination in the average person is commonly estimated at .015 to .020 percent per hour, although again this can vary from person to person and in a given person from one moment to another. Metabolism can be affected by numerous factors, including such things as body temperature, the type of alcoholic beverage consumed, and the amount and type of food consumed.

In an increasing number of states, laws have been enacted to facilitate this speculative task: the BAC at the time of driving is legally presumed to be the same as when later tested. There are usually time limits put on this presumption, commonly two or three hours, and the defendant is permitted to offer evidence to rebut this presumption.

Forward extrapolation can also be attempted. If the amount of alcohol consumed is known, along with such variables as the weight and sex of the subject and period and rate of consumption, the blood alcohol level can be estimated by extrapolating forward. Although subject to the same infirmities as retrograde extrapolation-guessing based upon averages and unknown variables-this can be relevant in estimating BAC when driving and/or corroborating or contradicting the results of a later chemical test.

Blood Alcohol Content Calculation

BAC can be roughly estimated using a mathematical approach. While a mathematical BAC estimation is not as accurate as a breathalyzer, it can be useful for calculating a BAC level that is not currently testable, or a level that may be present in the future. While there are several ways to calculate a BAC, one of the most effective ways is to simply measure the total amount of alcohol consumed divided by the total amount of water in the body-effectively giving the percent alcohol per volume water in the blood.

The total water weight of an individual can be calculated by multiplying his or her body weight by their percent water. For example, a 150 pound woman would have a total amount of water of 73.5 pounds (150 x .49). For easiest calculations, this weight should be in kilograms, which can be easily converted by dividing the total pounds by 2.205. 73.5 pounds of water is equivalent to 33.3 kilograms of water. 33.3 kilograms of water is equivalent to 33,300 mL of water (1 L of water has a mass of 1 kg, and 1 L = 1000 mL).

Gender plays an important role in the total amount of water that a person has. In general, men have a higher percent of water per pound (58%) than women (49%). This fact alone strongly contributes to the generalization that men require more alcohol than women to achieve the same BAC level. Additionally, men are, on average, heavier than women. The more water a person has, the more alcohol is required to achieve the same alcohol:blood ratio, or BAC level. Further, studies have shown that women's alcohol metabolism varies from that of men due to such biochemical factors as different levels of alcohol dehydrogenase (the enzyme which breaks down alcohol) and the effects of oral contraceptives.

It is not strictly accurate to say that the water content of a person alone is responsible for the dissolution of alcohol within the body, because alcohol does dissolve in fatty tissue as well. When it does, a certain amount of alcohol is temporarily taken out of the blood and briefly stored in the fat. For this reason, most calculations of alcohol to body mass simply use the weight of the individual, and not specifically his water content.




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