Blood alcohol level: how to calculate it? | Health Magazine | The USA Print

The blood alcohol level reflects the level of alcohol (more precisely ethanol) circulating in the body after the absorption of one or more drinks. “The alcohol we ingest passes through the digestive tract, then into the blood. Since the blood is a real highway, alcohol is then distributed throughout the body, especially in the brain. The liver will metabolize it, that is to say transform it so that it is eliminated. A small part of this alcohol passes through the lungs into the exhaled air. There is about 2000 times less alcohol in the exhaled air than in the blood, but it is enough to be able to measure it”, explains Professor Jean-Claude Alvarez, director of the Paris-Saclay Pharmacology-Toxicology Federation and expert at the Court of Appeal of Versailles.

Before getting behind the wheel, everyone has the opportunity to blow into a breathalyzer to assess his blood alcohol level. These devices are no longer mandatory in private vehicles since 2020, but they make it possible to “give a signal” according to Professor Alvarez. “Beyond 0.25 milligrams per liter of air, you are considered positive”, he explains. In this case, avoid driving.

For traffic checks, police and gendarmes have more sophisticated and more precise approved devices: breathalyzers. Blowing into this device is enough to indicate your blood alcohol level. A blood test is not necessary to confirm the result. “If the breathalyzer indicates that you are positive with more than 0.25 milligrams per liter of exhaled air, you will have a fine; at 0.4 milligrams per liter of air you no longer have the right to drive your car again”specifies the toxicologist.

In the event of an accident, a forensic analysis is carried out in an approved laboratory such as that of Professor Alvarez. Thanks to the technique of chromatography, the analysis carried out using the driver’s blood makes it possible to measure the concentration of ethanol very precisely. Measured in blood, the blood alcohol limit allowed on the road is 0.5 grams per litre.

For the same quantity of alcohol ingested, the blood alcohol level varies from one person to another. “It all depends on the volume of distribution, ie the amount of water in the body. Women generally have a lower volume of distribution than men. explains Professor Alvarez. Their blood alcohol level will therefore rise higher than that of men.

The weight of a person can also have an impact on their blood alcohol level, but this factor is less important than the volume of water in the body.

The type of alcohol also plays. “The stronger the alcohol, the faster it passes through the blood, the higher its concentration. For the blood alcohol level to rise lower, it is better to consume less strong alcohol during a meal. Similarly, it is preferable to drink water between each glass, this will help to limit the blood alcohol level”, observes the toxicologist.

water and food will dilute and slow the absorption of ethanol in the body. The blood alcohol level will therefore be lower, but the same amount of alcohol will pass into the blood, with varying effects depending on the individual and their tolerance to alcohol.

The same concentration of alcohol in the blood, therefore the same blood alcohol level, will not produce the same effects from one person to another. Thus, a regular drinker whose body is accustomed to alcohol will keep control of his faculties longer. “In the extreme, one could say that at 0.5 grams of alcohol in the blood, a chronic alcoholic is less dangerous than an occasional drinker who has the same blood alcohol level. But the law is the same for all. recalls Professor Alvarez.

If you are driving with a blood alcohol level greater than 0.5 grams of alcohol in the blood, you are subject to a fine. Why this limit? “Studies have shown that up to 0.4 grams of alcohol in the blood, there is no increased risk of an accident. Above, a driver is potentially dangerous,” he explains. Driving, a blood alcohol level above 0.8 grams is considered a misdemeanor.

Depending on the individual, two standard glasses of alcohol of any kind (wine, beer or other) can be enough to exceed the blood alcohol limit allowed while driving, i.e. 0.5 grams of alcohol in the blood.

Indeed, these standard glasses, which correspond to the volume served in bars and nightclubs, contain the equivalent of 10 grams of pure alcohol. A 10 cl flask of wine at 12 degrees contains as much alcohol as a 2.5 cl glass of whiskey at 40° or a 25 cl beer at 5°.

“For a man, drinking the equivalent of a standard drink corresponds to a blood alcohol level of approximately 0.2 grams per liter of blood, but for a woman this blood alcohol level can rise to 0.25 grams. With two glasses, it is therefore possible to exceed the authorized blood alcohol level”, observes Professor Alvarez.

Beware of drinks served with family or friends. Very often, they are fuller than the standard glasses of establishments authorized to serve alcohol. Under these conditions, blood alcohol levels rise faster and higher.

“When you drink a glass of alcohol, the concentration in the blood peaks after about 30 minutes. Afterwards, we lose 0.15 to 0.2 grams per liter per hour. For example, if you have a blood alcohol level of 0.7 grams per litre, you will have to wait two hours to drop below 0.5 grams per litre, the limit below which you are allowed to drive. If you have 2 grams of alcohol in your blood, you will have to wait ten hours”, replies Professor Alvarez.

We see tips and tricks circulating on social networks to lower your blood alcohol level: drinking coffee, sucking lemons or coins, sweating, coating your stomach with oil or vinegar… But the only really effective measure is patience. “We cannot falsify the measurement of the blood alcohol level. You have to wait the time necessary for the alcohol to be eliminated,” insists the toxicologist.

Exercising after drinking too much is not a bad idea. But this will not be enough to normalize his blood alcohol level: “By playing sports, we exhale more, so we eliminate more ethanol. But that won’t bring the blood alcohol level down low enough to drive. »

Can you assess your blood alcohol level yourself? Websites and smartphone apps offer calculators. But for Professor Alvarez, “their reliability has not been assessed”. “The difficulty, when trying to assess one’s own blood alcohol level, is to know how much alcohol one has actually ingested. These apps are based on the consumption of standard glasses and not on the glasses we use at home”, he remarks. The reliability of these calculators is therefore not guaranteed.