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Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in blood. It is measured either as a percentage by mass, or by mass per volume. For example, a BAC of 0.20% means 1 gram of alcohol per in each 500 grams of an individual's blood.
In many countries, the BAC is measured and reported as grams of alcohol per 100 milliliters of blood (g/100 mL). Because the specific gravity of blood is very close to the specific gravity of water (its main component), the numerical values for BAC (%) and (g/100 mL) do not differ to any consequential degree. The simple "percent" metric is more convenient, and is a pure ratio not dependent on any particular measurement system.
The number of drinks consumed is a poor measure of intoxication largely because of variation in physiology and individual alcohol tolerance. A single drink containing one ounce (28.3 grams) of alcohol will increase the average person's BAC roughly 0.03%, 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 (even though that is just the trademark of one manufacturer of the devices).
Effects at different levels
| Number of Drinks | Blood Alcohol Content | Typical Effect |
|---|---|---|
| 1 | 0.02-.03% | Noticeable cognitive changes |
| 5 | .15-.20% | Person obviously intoxicated, delirium |
| 12 | .30-.40% | Loss of consciousness |
| 24 | .50% | Death |
Units of measurement
There are several different units in use around the world for defining blood alcohol concentration. Each is defined as either a mass of alcohol per volume of blood or a mass of alcohol per mass of blood (never a volume per volume). Below are two tables of approximately equivalent units.
| Measurement with Units | Units also known as: | Commonly used in |
|---|---|---|
| 0.01 g/100 mL | g/dL, % g/mL | USA |
| 0.10 mg/mL | g/L, ‰ g/mL (permille g/mL) | Netherlands, Lithuania, Poland |
| 10 mg/100 mL | mg/dL, % g/L, % mg/mL | Britain |
| 0.01 g/100 g | %, percent by mass | |
| 0.10 mg/g | ‰, permille by mass, g/kg | Sweden, Norway |
Because the density of blood is 1.06 g/mL, there is a very close approximation between mass/volume and volume/volume measurements. For this reason, a mg/mL is approximately the same as a mg/g. An exact conversion is 1 mg/g = 1.06 mg/mL.
In the vernacular and even in popular media, the abbreviation BAC is often used as a unit. For example, in the United States, BAC has become synonymous with g/100 mL and percent by mass. When discussing BAC across international boundaries, it is best to use appropriate units.
Legal Limits
For purposes of law enforcement, BAC is used to define intoxication and provides a rough measure of impairment. Although degree of impairment may vary among individuals with the same BAC, it can be measured objectively and is therefore legally useful and difficult to contest in court. Most countries disallow operation of motor vehicles and heavy machinery above prescribed levels of BAC. Operation of boats and aircraft are also regulated.
Limits by country (BAC: Blood Alcohol Content)
The alcohol level at which a person is considered to be legally impaired varies by country. The list below gives limits by country. These are typically BAC (blood alcohol content) limits for the operation of a vehicle.- Countries with a 0.08% limit include Mexico, New Zealand, Britain, the United StatesDrivers under 21 (the American drinking age), however, are held to stricter standards under zero tolerance laws. Adopted in varying forms in all states, these laws hold the driver to much lower BAC levels for criminal and/or license suspension purposes, commonly 0.01% to 0.05%. Many states have statutory regulations regarding driving while "under the influence" of an intoxicant and a different law for driving beyond the legal blood alcohol concentration. and CanadaAlso, in most jurisdictions, police and the RCMP are authorized to suspend drivers' licenses temporarily if they test at lower levels.. For further information on US laws, see Alcohol laws of the United States by state.
- Countries with a 0.05% limit include Argentina, AustraliaThe limit for Learner drivers and Provisionary Drivers is 0.00%. It is enforced by random breath testing., Austria, Belgium, Bulgaria, Croatia, Denmark, Finland, France, Germany, Greece, Iceland, Israel, Italy, Macedonia, the Netherlands, Portugal, Slovenia, Spain and Turkey.
- In Lithuania the legal limit is 40 mg/dL.
- In India, the limit in blood is 30 mg/dL. This is according to section 185 of Motor Vehicles Act 1988. On first offence, the punishment is imprisonment of 6 months and/or fine of 2000 Indian Rupees (INR). If the second offence is committed within three years, the punishment is 2 years and/or fine of 3000 Indian Rupees (INR). The clause of 30 mg/dL was added by an amendment in 1994. It came into effect beginning 14 November 1994.
- Zero BAC: Countries reported to have limit blood alcohol concentration to zero are the Czech Republic, Hungary, Japan, Malaysia, Romania, Saudi Arabia, and Croatia.
Limits by country (BrAC: BReath Alcohol Content)
In certain countries, alcohol limits are determined by the Breath Alcohol Content (BrAC), not to be confused with BAC.- In Greece, the BrAC limit is 39 micrograms of alcohol per 100 millilitres of breath. The limit in blood is 90 mg/dL.
- In The Netherlands, the BrAC limit is 220 micrograms of alcohol per litre of breath (μg/l, colloquially known as "Ugl").
Other limitation schemes
- For South Korea, the penalties for different BAC levels include
- 0.01-0.049% = No Penalty
- 0.05-0.09% = 100 days of license cancellation
- >0.10% = Cancellation of car license.
- >0.36% = Arrest
- If caught driving when drunk 3 times in 5 years; or 2 times in 3 years = arrest.
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. However, the actual ratio in any given individual can vary from 1300:1 to 3100:1, or even more widely. Thus a person with a true blood alcohol level of .08 but a partition ratio of 1700:1 would have a .10 reading on a Breathalyzer.
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.
Another example is retrograde extrapolation, in which someone's BAC at the time of driving is found by extrapolating backwards from a later test. To estimate how much alcohol has been eliminated in the interim between driving and testing, one must know the rates of absorption and elimination. The rate of elimination in the average person is .015 percent per hour, although again this can vary. 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.
In addition to the assumptions which affect chemical tests of BAC, there are similar assumptions in field testing. For example, the "horizontal gaze nystagmus" test estimates the BAC based on the "angle of onset" (the angle at which a suspect's eyes begin jerking). This is often accompanied by other field sobriety tests, such as standing on one leg or reciting the alphabet from a certain letter to another letter. The BAC is obtained by subtracting the angle from 50 degrees; jerking at 35 degrees, for example, would mean the suspect has a BAC of .15 percent. Just as with the chemical tests, this is based on the angle for the average person, which doesn't necessarily apply to everyone. In most states, however, the test is considered unreliable to determine BAC and is limited to use as a pass-fail field sobriety test.
Metabolism & 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).
References
The Handy Science Answer Book c 1997 by the Carnegie Library of Pittsburgh, p351
See also
External links
- Breath Analyzer Accuracy by David J. Hanson.
- Legal BAC limits by state
- Interactive Blood Alcohol Content Calculator
"Blood alcohol content"