When people talk about the legal alcohol limit being 0.08 percent for drivers, they're talking about your blood alcohol concentration. The police can use breath tests and blood tests to measure it and see how much you've had to drink. If your BAC goes over 0.08 percent, you can get a DUI -- though you can also get one if you show clear impairment under 0.08 percent.
But does that BAC really show how impaired you are? Is it a reliable indicator?
In many ways, it is not. As noted above, you can be impaired even if you're still under 0.08 percent, and the police know it. Alcohol impacts different people in different ways.
The opposite is also true. You can hit 0.08 percent or even go over it and not be as impaired as someone else who is still under that threshold.
One important factor is how often you drink. If you almost never drink, you may be significantly impaired at 0.06 percent. You're just not used to the way that alcohol affects your body. Many young drinkers fall into this category because they haven't had any time to build up a tolerance.
If you drink often, though, your body adapts to it. You get used to it and you can often function even after drinking. You could reach 0.08 percent and fail the breath test even though your ability to drive isn't actually impaired much at all.
Of course, the technical level of impairment does not really matter if you're over the legal limit. They can still give you a DUI based on that result, even if you argue that you're safe to drive, so you need to know what legal options you have.