What is BAC?

Originally featured in the 2014 fall edition of MADDvocate.

It’s becoming more common to find breath testing devices used to measure blood alcohol concentration (BAC) in bars and in private homes thanks to modern technology. But what is BAC and should you rely on these types of devices to determine whether you are sober enough to drive home?

BAC refers to the amount of alcohol in a person’s blood. It is measured as weight of alcohol per unit of volume of blood. Typically this measurement is converted to a percentage such as .10 percent, which means that one-tenth of a percent of a person’s blood is alcohol. The legal BAC limit throughout the United States is .08, although laws and penalties vary among states. Alcohol, which is quickly absorbed into the bloodstream, can be measured within 30 to 70 minutes after a person has had a drink.

According to Jim Fell, senior research scientist with the Pacific Institute for Research & Evaluation, how quickly your BAC rises depends on a variety of factors including the number of drinks you have, how fast you drink, your gender, your age, your weight and whether you have food in your stomach.

Still, the more you drink the more steadily your ability to safely drive a motor vehicle decreases. According to the National Highway Traffic Safety Administration, the risk of a driver with BAC between .08 and .10 having a fatal single-vehicle crash is at least 11 times greater than for drivers with a BAC of zero.

Still the Safest Course: Planning Ahead

Given the danger of driving under the influence, should you rely on personal BAC testing devices to determine whether you are below the legal limit? While the idea may seem like a good one, MADD does not support the use of personal alcohol tests to help drivers make decisions about their ability to drive after consuming alcohol.

Here’s why.

First, many personal BAC testing devices may not be accurate. Less expensive breath testers use semiconductor sensors rather than the more expensive fuel cell sensors in most commercial-grade breath testers used by law enforcement. This has caused some worry about reliability.

Additionally, commercial -grade devices (used by the police) are checked and maintained regularly, something that may not be happening with personal testers.

Second, if you measure your BAC in a bar and the reading comes up .06, you may think you’re fine to have one more drink, which can end up being a big mistake. Impairment begins with the first drink, so it’s always dangerous to try to stay “one sip under the limit.”

Finally, BAC climbs with time after consuming alcohol until you stop. If you consume several shots of alcohol and test your BAC immediately after having those drinks, the reading could be under the limit, but in 30 minutes, the likelihood is that the reading will be much higher. Making choices about drinking and driving based on personal alcohol testing is dangerous even if the intention is good. That’s why planning ahead is vitally important. If you’re going to drink, the only safe course of action is to plan for a safe way home before you start drinking, whether it’s a non-drinking designated driver, a cab, a car service or public transportation.