Blood alcohol

Blood alcohol (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. In many countries, the BAC is measured and reported as milligrams of alcohol per 100 milliliters of blood (mg/100ml).

Number of drinks consumed is a poor measure of intoxication, because of variation according to body weight. One drink (unit of alcohol) will increase the average person's BAC to roughly 0.04, 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. Individual alcohol tolerance varies, and can be affected by genetic or nutritional factors, drugs, other degrees of impairment, and long-term heavy drinking.

Alcohol content in blood can be directly measured by a hospital laboratory. More commonly, for law enforcement purposes, BAC is estimated from breath ethanol content measured with a machine commonly referred to as a Breathalyzer (even though that is just the trademark of one manufacturer of the devices). 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, the BAC has the advantage of being simpler to measure objectively, and therefore harder to contest, than impairment of driving.

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 a BAC rate of over 0.02 is illegal. By contrast, the policies of the United States have historically been more liberal; however as of 2004, 47 States and the District of Columbia had adopted a BAC of .08. As of 2005, all states in the US have adopted a BAC of 0.08. In some states, drivers under 21 (the American drinking age) are considered legally impaired at lower levels (perhaps 0.02, or even a mere trace) as part of a zero tolerance policy.

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, but only with medical attention.