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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, by mass per volume, or a combination. For example, a BAC of 0.20% can mean 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[1]
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
>.40
  • Unconsciousness
  • Death
  • Breathing
  • Heart Rate
Standard Drink Chart [2]
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
Male
Female
Approximate Blood Alcohol Percentage[3]
Drinks Body Weight
40 kg 45 kg 55 kg 64 kg 73 kg 82 kg 91 kg 100 kg 109 kg
90 lb 100 lb 120 lb 140 lb 160 lb 180 lb 200 lb 220 lb 240 lb
1
.05
.04
.05
.03
.04
.03
.03
.02
.03
.02
.03
.02
.02
.02
.02
.02
.02
2
.10
.08
.09
.06
.08
.05
.07
.05
.06
.04
.05
.04
.05
.03
.04
.03
.04
3
.15
.11
.14
.09
.11
.08
.10
.07
.09
.06
.08
.06
.07
.05
.06
.05
.06
4
.20
.15
.18
.12
.15
.11
.13
.09
.11
.08
.10
.08
.09
.07
.08
.06
.08
5
.25
.19
.23
.16
.19
.13
.16
.12
.14
.11
.13
.09
.11
.09
.10
.08
.09
6
.30
.23
.27
.19
.23
.16
.19
.14
.17
.13
.15
.11
.14
.10
.12
.09
.11
7
.35
.26
.32
.22
.27
.19
.23
.16
.20
.15
.18
.13
.16
.12
.14
.11
.13
8
.40
.30
.36
.25
.30
.21
.26
.19
.23
.17
.20
.15
.18
.14
.17
.13
.15
9
.45
.34
.41
.28
.34
.24
.29
.21
.26
.19
.23
.17
.20
.15
.19
.14
.17
10
.51
.38
.45
.31
.38
.27
.32
.23
.28
.21
.25
.19
.23
.17
.21
.16
.19
Subtract approximately .01% every 40 minutes after drinking.

To use a simple Blood Alcohol Content calculator from the Wisconsin Department of Public Safety, refer to the following link: Blood Alcohol Calculator

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). 1 milliliter of blood is approximately equivalent to 1 gram of blood, 1.06 grams to be exact. Because of this, units by volume are similar but not identical to units by mass.

UnitDimensionsEquivalent toUsed in
1 percent BAC by volume 1/100 (%) g/mL = 1 cg/mL 9.43 mg/g, 0.217 mmol/L United States
1 permille BAC by volume 1/1000 (‰) g/mL = 1 mg/mL 0.943 mg/g, 0.0217 mmol/L Netherlands, Lithuania, Poland
1 basis point BAC by volume 1/10,000 () g/mL = 100 μg/mL 94.3 ppm, 2.17 μmol/L Britain
1 permille BAC by mass 1/1000 (‰) g/g = 1 mg/g 1.06 mg/mL, 0.0230 mmol/L Finland, Norway, Sweden
1 part per million 1/1,000,000 (ppm) g/g = 1 μg/g 1.06 μg/mL, 0.0230 μmol/L
1 thousandth Molarity 1 mmol/L 4.6 cg/mL,[4] 4.34 cg/g Medical personnel

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, Republic of Ireland, New Zealand (drivers under 20 it is .03), Luxembourg, Malaysia, Malta, the United States]],[5] Canada], and the United Kingdom.[6] In the United States, operators of common carriers, such as buses, are restricted to a 0.01% limit, and the Federal Aviation Regulations governing U.S. pilots prohibit operation of an aircraft within eight hours of consumption or while having as little as 0.04 percent of alcohol by weight in the blood.[7] For further information on U.S. laws, see Alcohol laws of the United States by state.
  • In Lithuania the legal limit is 40 mg/dL.
  • In India, the limit in blood is 30 mg/dL.[9]
  • In Spain], the limit was reduced in 2003 from 50 to 25 mg/dL for drivers and cyclists. For drivers in their first two years after gaining a driving licence, the limit is 15 mg/dL. [[3]]

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 and Finland, 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
    • Getting caught driving while drunk 3 times in 5 years; or 2 times in 3 years results in arrest.

Test assumptions

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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

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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.

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 L of water weighs 1 kg, and 1 L = 1000 mL). (This math is wrong, 73.5 divided by 2.205 should be 33.3(repeating). Which Would be 33,333ML of water.)

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 dehydrogenase (the enzyme which breaks down alcohol) and the effects of oral contraceptives. [4]

See also

Notes

  1. Alcohol's Effects from Virginia Tech
  2. Standard Drinks Chart from Virginia Tech
  3. BAC Charts from Virginia Tech
  4. Information obtained from Alcotest 7410 GLC Calibrator's Manual. Last Amended October 2000 page 3A-6.
  5. 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.
  6. Also, in most jurisdictions, police and the Royal Canadian Mounted Police are authorised to suspend drivers' licences temporarily if they test at lower levels.
  7. http://caselaw.lp.findlaw.com/casecode/uscodes/18/parts/i/chapters/17a/sections/section_343.html
  8. Between 0.05% and 0.08%, drivers can be fined 135€ and have six points removed from their licence. Above 0.08% the punishment is more severe with possible imprisonment of up to two years, heavy fines and licence suspension. http://www.securiteroutiere.gouv.fr/infos-ref/regles/l-alcool-au-volant.html (in French)
  9. 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.

References

  • The Handy Science Answer Book. Pittsburgh: The Carnegie Library, 1997.
  • Taylor, L. Drunk Driving Defense, 6th edition. New York: Aspen Law and Business, 2006.
  • Perham, N. R., Moore, S. C., Shepherd, J. P. & Cusens, B. (2007). Identifying drunkenness in the night time economy. Addiction, 102(3), 377-380

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