Blood alcohol content

For people who check facts

Blood Alcohol Content (or Blood Alcohol Concentration), often abbreviated BAC, is the concentration of alcohol in blood, measured, by volume, as a percentage. For example, a BAC rating of 0.20 means 1 part per 500 in an individual's blood is alcohol.

Number of drinks consumed is a poor measure of intoxication, because of variation according to body weight. One drink (alcoholic unit) will increase the average person's BAC to roughly 0.04%, but there is much variation according to body weight and body fat percentage. Measured with a machine commonly referred to as a Breathalyzer (even though that is just the trademark of one manufacturer of the devices), BAC is used as an ideally objective (and therefore, legally uncontestable) measure of the level of impairment of an individual, especially as it pertains to the operation of a motor vehicle.

Neither BAC nor the number of drinks consumed are necessarily accurate indicators of the level of impairment. Genetics make people's alcohol tolerance vary, and long-term heavy drinking can greatly increase the base tolerance.

Most countries disallow operation of motor vehicles and heavy machinery at prescribed levels of BAC, which vary both by country and by situation. In Sweden, driving with any non-zero BAC rating is illegal. By contrast, the policies of the United States are much more liberal: In some states, a BAC rating as high as .099% is considered not intoxicated, though the federal government is pushing for a reduction of the limit to 0.08%. In some states, drivers under 21 (the American drinking age) are considered legally impaired at lower levels (perhaps 0.02%) as part of a zero tolerance policy.

In Australia, the limit is 0.05% in most states and territories, and either 0.02% or zero for inexperienced drivers (learner drivers or those on probationary licenses). It is enforced by random breath testing.

Despite the liberal intoxication limits of many countries, one should not assume that driving with a BAC rating of, say, .079% is as safe as driving while sober. At a BAC rating of 0.05%, the probability of a driver having an accident is more than four times its base level. Despite this, some drivers believe their driving actually improves with small amounts of alcohol. This assertion is completely false: Their perception of their driving improves because they are more relaxed, but their actual motor competence is markedly lower than when sober.

A BAC rating of 0.20% represents serious intoxication, and 0.35% represents potentially fatal alcohol poisoning. 0.40% is the accepted LD50, or lethal dose for 50% of adult humans. In extreme cases, individuals have survived BAC ratings as high as 0.74%, but only with medical attention.