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The Science Behind Blood Alcohol Content (BAC) Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in blood. It is measured either as a percentage by mass, by mass per volume, or a combination. For example, a BAC of 0.02% can mean 0.2 grams of alcohol per 1000 grams of an individual's blood, or it can mean 0.2 grams of alcohol per 100 millilitres of blood.
In many countries, the BAC is measured and reported as grams of alcohol per 1000 millilitres (1 litre) of blood (g/1000 mL). Because the specific gravity of blood is very close to the specific gravity of water (its main component), the numerical values for BAC (%, percent) and (g/1000 mL, permille) do not differ to any consequential degree other than the placement of the decimal point.
In the UK, BAC is reported as milligrams of alcohol per 100 millilitres of blood. For example, a BAC of 0.08% is legally given as a limit of 80 mg per 100 ml [1]. It is also reported in grams per Litre, which is an equivalent measurement [2].
The number of drinks consumed is a very poor measure of intoxication largely because of variation in physiology and individual alcohol tolerance. However, it is generally accepted that the consumption from sober of two standard drinks (containing a total of 20 grams) of alcohol will increase the average person's BAC roughly 0.05% (a single standard drink consumed each hour after the first two will keep the BAC at approximately 0.05%), but there is much variation according to body weight, gender, and body fat percentage. Furthermore, neither BAC nor the number of drinks consumed are necessarily accurate indicators of the level of impairment. Tolerance to alcohol varies from one person to another, and can be affected by such factors as genetics, adaptation to chronic alcohol use, and synergistic effects of drugs.
Alcohol content in blood can be directly measured by a hospital laboratory. More commonly in law enforcement investigations, BAC is estimated from breath alcohol concentration (BrAC) measured with a machine commonly referred to as a Breathalyzer which is a genericized trademark. EFFECTS AT DIFFERENT LEVELS
Unless a person has developed a high tolerance, a BAC rating of 0.20 represents very serious intoxication (most first-time drinkers would be unconscious by about 0.15), and 0.35 represents potentially fatal alcohol poisoning. 0.40 is the accepted LD50, or lethal dose for 50% of adult humans. For a long-time, heavy drinker, those numbers can at least double. In extreme cases, individuals have survived BAC ratings as high as 0.914.
PROGRESSIVE EFFECTS OF ALCOHOL
| BAC | Behavior | Impairment | | .01-.06 | * Relaxation
* Sense of Well-being
* Loss of Inhibition
* Lowered Alertness
* Joyous | * Thought
* Judgment
* Coordination
* Concentration | .06–.10 | * Blunted Feelings
* Disinhibition
* Extroversion
* Impaired Sexual Pleasure | * Reflexes Impaired
* Reasoning
* Depth Perception
* Distance Acuity
* Peripheral Vision
* Glare Recovery | .11–.20 | * Over-Expression
* Emotional Swings
* Angry or Sad
* Boisterous | * Reaction Time
* Gross Motor Control
* Staggering
* Slurred Speech | .21–.29 | * Stupor
* Lose Understanding
* Impaired Sensations | * Severe Motor Impairment
* Loss of Consciousness
* Memory Blackout | .30–.39 | * Severe Depression
* Unconsciousness
* Death Possible | * Bladder Function
* Breathing
* Heart Rate
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>.40
| * Unconsciousness
* Death | * Breathing
* Heart Rate
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Standard Drink Chart
| Alcohol | Amount in ml | Amount in fl. oz. | Colloquial amount | | 80 proof liquor | 37 millilitres | 1.25 oz | one small shot | | beer | 355 millilitres | 12 oz | one can
| | table wine | 148 millilitres | 5 oz | one small glass
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BAC CALCULATOR
* Subtract approximately .01% every 40 minutes after drinking.
TEST ASSUMPTION
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. With such a wide range of error caused by human individuality, a talented DUI defense attorney can educate a Bay Area jury on the flawed science behind this test.
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 90 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.)
Metabolism is 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).
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. 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.
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.
| California Department of Health Food and Drug Laboratory Branch
850 Marina Bay Parkway, G365
Richmond, CA 94804-6403
510-412-6220
510-412-6280
California Department of Justice California Department of Justice
Criminal Record Audits and Security Section
P.O. Box 903417
Sacramento, CA 94203 |
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