Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in blood. It is usually measured as mass per volume. For example, a BAC of 0.02% means 0.2 ‰ (permille) or 0.02 grams of alcohol per 100 grams of individual’s blood, or 0.2 grams of alcohol per 1000 grams of blood. Blood alcohol concentration is measured in many different units and in many different fashions, but they are all relatively synonymous for each other. In many countries, BAC is reported as grams of alcohol per liter of blood (g/L). Because the specific gravity of blood is close to 1, the numerical value of BAC measured as mass per volume and that of BAC measured as mass per mass do not differ to any consequential degree other than the placement of the decimal point. For example, 1 g/L is equivalent to 0.94 g/kg. 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. It is also reported in grams per litre, which is an equivalent measurement.
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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, sex, 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.