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 Alchol Testing per volume, or a combination. For example, a BAC of Acohol Testing 0.20% (2.0 ‰) can mean 2 grams of alcohol per 1000 grams of an Alochol Testing individual's blood, or it can mean 0.2 Alchool Testing grams of alcohol per 100 millilitres (also called a decilitre) of blood.
In many countries, the BAC is Alcool Testing measured and reported as grams of alcohol per 1000 millilitres (1 Alohol Testing litre) of blood (g/1000 mL). Because the specific gravity of blood is very close to the specific Alcohl Testing gravity of water (its main component), the numerical values for BAC Alcoho Testing (%, percent) and (g/1000 mL, permille) do not differ to any consequential degree other than the Aclohol Testing placement of the decimal point.
The number of drinks consumed Alcohool Testing is a poor measure of intoxication largely because of variation in physiology and individual alcohol tolerance. A Alcphol Testing single drink containing one ounce (28 grams) of alcohol will increase the average person's BAC Alcohil Testing roughly 0.03%, but there is much variation according to body weight, gender, and body Alcoholl Testing fat percentage. Furthermore, neither Alcoohl Testing BAC nor the number of drinks consumed are necessarily accurate indicators of the level of impairment. Tolerance Allcohol Testing 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).
- 1 Effects at different levels
- 2 Units of measurement
- 3 Legal Limits
- 3.1 Limits by country (BAC: Blood Alcohol Content)
- 3.2 Limits by country (BrAC: Breath Alcohol Content)
- 3.3 Other limitation schemes
- 4 Test assumptions
- 5 Metabolism & excretion
- 5.1 Retrograde extrapolation
- 6 Blood alcohol content calculation
- 7 References
- 8 See also
- 9 External links
- 10 Notes
Effects at different levels
- See also: Effects of alcohol on the body
Unless a person has developed a high tolerance, a BAC rating of 0.20 represents very serious intoxication (most first-time drinkers would be passed out 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.
Effects of Alcohol at Different Blood Alcohol Contents
|Number of Drinks
||Blood Alcohol Content
||Noticeable cognitive changes
||Person obviously intoxicated, delirium
||Loss of consciousness
||Severe coma and 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.
Approximately Equivalent BAC Measures
|Measurement with Units
||Units also known as:
||Commonly used in
|0.01 g/100 mL
||g/dL, % g/mL
||g/L, ‰ g/mL (permille g/mL)
||Netherlands, Lithuania, Poland
|10 mg/100 mL
||mg/dL, % g/L, % mg/mL
|0.01 g/100 g
||%, percent by mass
||‰, permille by mass, g/kg
||Finland, Norway, Sweden
Note: The first three mass/volume units are not exactly equivalent to the last two mass/mass units.
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.
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, United Kingdom, Republic of Ireland, the United States  and Canada . Surprisingly, in the United States, operators of common carriers, such as airplanes and buses, are restricted to a 0.10% limit. For further information on US laws, see Alcohol laws of the United States by state.
- Countries with a 0.05% limit include Argentina, Australia , Austria, Belgium, Bulgaria, Croatia, Denmark, Finland, France, Germany, Greece, Iceland, Israel, Italy, Macedonia, the Netherlands, Portugal, Serbia, Slovenia, Spain and Turkey.
- In Lithuania the legal limit is 40 mg/dL.
- In India, the limit in blood is 30 mg/dL. 
- Norway, Sweden, Russia and Poland have 20 mg/dL limits.
- Zero BAC: Countries reported to have limit blood alcohol concentration to zero are the Czech Republic, Slovakia, Hungary, 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 results in arrest.
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 & 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).
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 their 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 29.4 kilograms of water. 29.4 kilograms of water is equivalent to 29,400 mL of water (1 kg = 1 L, 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 acetaldehyde (the enzyme which breaks down alcohol) and the effects of oral contraceptives. 
- The Handy Science Answer Book. Pittsburgh: The Carnegie Library, 1997.
- Taylor, L. Drunk Driving Defense. New York: Aspen Law and Business, 6th edition, 2006.
- Alcoholic Beverage
- Driving under the influence
- Legal BAC limits by state
- Breathalyzer.net FAQ - Alcohol Testing Equipment Information.
- Alcohol Metabolism
- Simalc, alcohol blood level simulator, tool for prevention and education
- ^ Drivers 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.
- ^ Also, in most jurisdictions, police and the RCMP are authorised to suspend drivers' licences temporarily if they test at lower levels.
- ^ http://caselaw.lp.findlaw.com/casecode/uscodes/18/parts/i/chapters/17a/sections/section_343.html
- ^ The limit for Learner drivers and Provisionary Drivers is 0.00%. It is enforced by random breath testing.
- ^ 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.
Computing a BAC Estimate. US Department of Transportation (Oct 1994). Retrieved on 21 July 2006.
Categories: Alcohol | Alcohol law | Drunk driving | Metabolism