Sunday, April 26, 2009

Blood Alcohol Content

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 of Alcohol 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. However, there have been cases of people surviving and even remaining conscious at BACs above 0.40%.





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.



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, BAC 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.

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.
0.00% - Czech Republic, Hungary, Romania, Saudi Arabia, Slovakia, United Arab Emirates
0.02% - Brazil, Estonia, Poland, Sweden, Norway
0.03% - India (note: In the State of Kerala, since of late, it is illegal to even have a sip and drive)[8], Japan[9], Russia
0.04% - Lithuania
0.05% - Argentina (0.02% for motorbikes, 0.00% for truck/taxi/bus drivers), Australia (0.02% for Australian Capital Territory learner, probationary & convicted DUI drivers, 0.00% for learner drivers, provisional/probationary drivers (regardless of age) and DUI drivers), Austria, Belgium, British Columbia, Manitoba (0.05% is a 24-hour suspension and a fine, 0.08% is a D.U.I. charge)(0.00% for drivers with class G1 or G2 licenses in Ontario, or class 7 or 5P in the Northwest Territories, or drivers with a under the Graduated License System in Manitoba, and Alberta, or class 7 or 7L in British Columbia), Bulgaria, Costa Rica, Croatia, Denmark, Finland, France, Germany (0.00% for learner drivers, all drivers 18-21 and newly licensed drivers of any age for first two years of licence), Greece, Iceland, Israel, Italy, Latvia, Luxembourg, Netherlands (0.02% for drivers in their first five years after gaining a driving license), Portugal, Republic of Macedonia, Serbia, Slovenia (0.00% for drivers in their first two years after gaining a drivers licence), South Africa, Spain (0.015% for drivers in their first two years after gaining a driving licence), Switzerland, Thailand, Turkey
0.08% - Canada, Malaysia, Malta, Mexico, New Zealand (0.03% for drivers under 20), Ireland, Singapore, United Kingdom (0.02% for operators of aeroplanes), United States

For further information on U.S. laws, see Alcohol laws of the United States by state (0.01% for operators of common carriers, such as buses, 0.04% for pilots, Federal Aviation Regulations within eight hours of consumption).

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).

Currently, the only known substance that can increase the rate of metabolism of alcohol is fructose. The effect can vary significantly from person to person, but a 100g dose of fructose has been shown to increase alcohol metabolism by an average of 80%.

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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Driving While Intoxicated

Driving while intoxicated is the act of operating and/or driving a motor vehicle while under the influence of alcohol and/or drugs to the degree that mental and motor skills are impaired. It is illegal in all jurisdictions within the U.S. The specific criminal offense is usually called driving under the influence (of alcohol and/or other drugs, DUI), and in some states driving while intoxicated (DWI), operating while impaired (OWI), or operating a vehicle under the influence (OVI). Such laws may also apply to boating or piloting aircraft.

In the United States the National Highway Traffic Safety Administration (NHTSA) estimates that 17,941 people died in 2006 in "alcohol-related" collisions, representing 40 percent of total traffic deaths in the US. Over 500,000 people were injured in alcohol-related accidents in the US in 2003. NHTSA defines fatal collisions as "alcohol-related" if they believe the driver, a passenger, or an occupant of the vehicle (such as a pedestrian or pedalcyclist) had a blood alcohol content (BAC) of 0.01 or greater. NHTSA defines nonfatal collisions as "alcohol-related" if the accident report indicates evidence of alcohol present. NHTSA specifically notes that "alcohol-related" does not necessarily mean a driver or nonoccupant was tested for alcohol and that the term does not indicate a collision or fatality was caused by the presence of alcohol. On average, about 60 percent of the BAC values are missing or unknown. To analyze what they believe is the complete data, statisticians simulate BAC information. Drivers with a BAC of 0.10 are 6 to 12 times more likely to get into a fatal crash or injury then drivers with no alcohol.