Right now anyone with a blood alcohol concentration above .08 percent is considered too drunk to drive. The National Transportation Safety Board is recommending states lower that limit to .05 percent. Research shows that some people are impaired at .05 percent; science also tells us that how drunk you get when you drink depends on a variety of factors.
The National Institutes of Health looked at more than one hundred studies on the subject. It found at .08 percent most people showed significant signs of impairment. But even at .05 percent, some struggled with a simulated driving test. Researchers documented changes in eye movement, visual perception and reaction time. The effects were stronger for sleep deprived and younger drivers.
How you get to a blood alcohol concentration of .05 percent depends on a lot of factors, says Paul Doering, Emeritus Professor at the University of Florida's Department of Pharmacotherapy and Translational Research.
For one thing, size matters. The bigger you are, the more blood you have to dilute that drink, which means your blood alcohol concentration will be lower.
Another factor is gender, says Doering.
"Women take less alcohol to get to the same level as a man might because of their distribution of fat versus muscle," he says.
Women tend to have a higher ratio of fat to muscle. Fat doesn't absorb alcohol, but muscle does. So when a woman has a drink, proportionally less of it is taken into her body tissue, leaving more in the bloodstream.
Food in your belly can slow down how fast alcohol enters your blood, giving the liver more time to process it. Cocktails with protein, like milk, can also slow absorption.
Even genetics can play a role, says Doering.
"Certain ethnic groups, for example Asians, lack an enzyme that helps them break down alcohol," he notes. The enzyme is aldehyde dehydrogenase. A University of San Diego study found that about half of all people of Asian descent have a mutated version of the gene responsible for this enzyme. As a result, they produce less of it and metabolize drinks slower than people without the mutation.
Despite all of this variability, most people should be safe to drive if they stick to only one drink an hour.
Lowering the legal limit to drive, as recommended by the NTSB, can make people safer, says Michele Simon, who runs Eat Drink Politics, a watchdog group that tracks the food and beverage industries. But Simon argues that there are other policies governments can enact that are more effective.
"Increasing price decreases consumption," she says. "Making sure that there aren't too many places to access alcohol, that reduces harm in a neighborhood, and then making sure the industry isn't marketing to youth. Those are the three most important factors."
States will decided whether to adopt the lower legal limit. Already, the recommendation has run into opposition from restaurants and beverage companies, and both Simon and Doering don't expect laws to change anytime soon.
Still, when the NIH studied drinking, it found people showed impairment at levels far below even .05 percent. With a blood alcohol reading of just .02 percent, subjects showed a weakened ability to focus on more than one thing at a time, a skill important for driving. So no matter how little you drink, it's worth it to be cautious when you get behind the wheel.