Blood alcohol content, BAC, is the concentration ratio of alcohol in your blood stream. It is measured in three different ways – a percentage by mass, mass per volume or a combination of both. In most countries, blood alcohol content is measured in grams of alcohol per 1000 milliliters of blood.
It is very difficult to tell what your blood alcohol content is based on how many drinks you have – this is due to the extreme variations in physiology and each person’s alcohol tolerance. However, generally two standard drinks with 20 grams of alcohol will increase a sober person’s blood alcohol content to 0.05%, and consuming one drink an hour after that will maintain the BAC at that level. There is still a wide variety of variations to this calculation based on body weight, body fat percentage and gender. A person’s tolerance to alcohol will also impact the BAC levels.
Blood alcohol content is directly measured by a laboratory in a hospital, but most commonly if you are stopped for suspected impaired driving, you will be subjected to a breathalyzer, which measures the breath alcohol concentration.
For the average person, a blood alcohol concentration of .20 represents serious intoxication however most first time drinkers would be passed out by 0.15. Fatal alcohol poisoning levels begin at 0.35 while 0.40 is the accepted lethal dose for half of the adult population. At the same time, a heavy drinker could potentially double those numbers.
Alcohol has progressive effects on behavior and impairment of mind functions. At a BAC of 0.01 to 0.06, behavior is relaxed, joyous and you experience a loss of inhibition, lowered alertness and a sense of well-being. The functions that are impaired are thought, judgment, coordination and concentration. From 0.06 to 0.10, you have blunted feelings, much less inhibition, extroversion and impaired sexual pleasure. Your reflexes are impaired, as is your depth perception, reasoning, distance acuity, peripheral vision and glare recovery. You are considered impaired at 0.08 BAC. At 0.11 to 0.20, you are experiencing over-expression, emotional swings, you’re loud and boisterous and feeling angry or sad. Your reaction time, gross motor controls and speech are impaired and you are likely staggering. At 0.21 to 0.29, you lose your understanding, you are in a stupor and your sensations are impaired. You have loss of consciousness, severe motor skills impairment and even memory blackouts. At 0.30 to 0.39, you experience unconsciousness and severe depression and death is possible. You lose control of your bladder functions, your heart rate and breathing are at risk as well. Over 0.40, you are unconscious and likely losing control of your breathing and heart rate, and death is more than possible.
In the United States, the legal blood alcohol content limit is 0.08%, and that is the same in Mexico and Canada as well. Common carriers such as buses are restricted to a 0.01% BAC limit and for aircrafts, pilots are not allowed to fly within eight hours of consumption of alcohol or while having 0.04% BAC volume.