Everyone knows drinking and driving is dangerous. Right? So instead of reminding you not to drink and drive on St. Patrick’s Day (or any other day) we thought we’d explain how blood alcohol concentration works. Consider it Alcohol 101.

Related: make sure you’re protected all year round with reliable car insurance from Esurance

Warning: we’re about to get scientific. Don’t run away. We’ll do our best to make it interesting.

What does “blowing a .08” actually mean?

The legal drinking limit in the U.S. is .08. That means, if someone blows a .08 or higher when given a breathalyzer test, they’re considered too drunk to drive. But that number’s a bit abstract if you don’t know what it’s measuring.

The number on the breathalyzer refers to your blood alcohol concentration (sometimes referred to as blood alcohol content or BAC). When a person drinks, the alcohol is absorbed directly through the walls of the stomach and small intestine. From there it moves into the bloodstream and travels throughout the body to the brain. Blood alcohol concentration is measured by the percentage of alcohol in a certain volume of blood. So, if someone has a BAC of .10, a sample of their blood would contain .10 percent alcohol.

How can a breathalyzer test measure alcohol concentration in blood?

Good question — just how are our breath and blood related?

As alcohol-infused blood travels through the lungs, some of that alcohol moves across the lungs’ membranes (the thin layer of tissue) and into their air sacs, or alveoli. The concentration of alcohol in the alveolar air correlates with the concentration of alcohol in the blood. So when a person exhales, the alcohol in the alveolar air can be detected by the breathalyzer. (Sure beats drawing blood on the side of the road!)

What factors affect blood alcohol concentration?

You may notice that the same number of drinks can affect you differently at different times. That’s because there are a few additional factors that could also affect your BAC.

  • The alcohol content of the drink. It doesn’t take a scientist to deduce that a martini has a higher alcohol concentration than a beer.
  • How fast you drink. In general, the quicker you drink, the higher your peak BAC will be. In other words, doing shots will hit you harder than sipping.
  • How much you’ve eaten. Alcohol is absorbed more slowly into the blood stream if there’s food in your stomach because the food dilutes the alcohol.
  • How much water you drink. Like food, water dilutes the alcohol. Have a glass of water for every alcoholic drink you imbibe to avoid become overly inebriated.

Just remember, though you may feel in control, your BAC could be at or above the legal limit.

Does size matter? (We’re talking about blood alcohol concentration here)

There’s ongoing debate as to whether size matters, but when it comes to BAC, bigger is definitely better. The more you weigh, the more water is present in your body. This water dilutes the alcohol and lowers the BAC. Women tend to have less water and more body fat per pound of body weight than men. Since alcohol can’t penetrate fat cells as easily as other cells, more alcohol remains in women’s blood than in men’s. So, generally speaking, if a woman and a man drink the same amount, the woman will have a higher BAC.

Okay, we’re going to preach a little: don’t drink and drive

About one third of crash-related fatalities can be attributed to drunk driving. Though we all know the dangers of driving drunk when we’re sober, common sense often goes out the window when we’re inebriated. So plan ahead. It could end up saving your life.

How can you avoid drinking and driving?

  • Before the festivities begin, designate a sober driver. Draw straws, offer payment, or promise to be the designated driver (DD) for a month. Just make sure someone’s committed to being the DD before you head out on the town.
  • If you’re going to a house party, make arrangements to stay the night. If you’re going out to the bars, find a hotel within walking distance.
  • Get a ride home (from a sober driver) or call a rideshare. 
  • Find out if your city has a sober ride program. They’re often free and many run all night.
  • Pace yourself. Drink one alcoholic beverage per hour and supplement with water. It’ll keep your BAC low and also help you avoid a hangover in the morning.
  • If you’re hosting a party, remind your guests to plan ahead and designate a sober driver; offer alcohol-free beverages; and make sure all guests leave with a sober driver (or offer up your couch and a sleeping bag).

We hope everyone has a fun and safe St. Patrick’s Day this year. May the luck of the Irish be with you! And feel lucky all year long with surprisingly painless car insurance from Esurance



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about Jessica

During her time as senior copywriter at Esurance, Jessica wrote about everything from automotive trends to insurance tips to driving dogs (it’s a thing!). In her free time, you can find Jessica hiking with her dog (who cannot drive), devouring a good mystery, or very slowly learning Spanish.