Even if you have a blood alcohol concentration of less than 0.08 percent, you can still be charged with a DUI or DWI under certain circumstances.
Generally, anyone with a BAC of 0.08 percent or more is considered "per se intoxicated," which means there's no other evidence necessary to prove a driver's impairment. But just because you're not at the 0.08 percent threshhold doesn't make "buzzed" driving OK.
Here are situations that can trigger a DUI or DWI when you have a BAC of less than 0.08 percent:
Zero Tolerance Laws
All states carry "per se" or "zero tolerance" DUI laws that focus on drivers under the legal drinking age. These laws set forth criminal punishments for people under 21 for driving with any trace of alcohol in their systems (i.e., a BAC above zero), or with negligible BAC levels such as 0.01 or 0.02 percent.
In many states, zero tolerance laws can also apply to drivers who are driving drunk with kids in the car. For example, if you are transporting children as part of a day-care business, you could face a DUI or DWI even with a BAC well below the legal limit.
In most states, "intoxicated" driving applies not only to alcohol but to drugs as well. This even includes prescription drugs, many of which can impair your ability to drive.
For example, if you were driving under the influence of prescription pills and topped it off with a glass of wine that caused you to become impaired, then you could be on the hook for a DUI or DWI, even if your BAC is below 0.08 percent.
As a general rule of thumb, you may face a DUI or DWI if you were "noticeably impaired" while you were driving.
Passage of Time
If your BAC was less than 0.08 percent, it can be tricky to establish that you were impaired. However, if your BAC was hovering near 0.08 percent -- like 0.07 percent -- when you were pulled over, the prosecutor could argue that your BAC was actually above the legal limit of 0.08 percent when you were driving, and fell below that level before the blood or breath sample was taken.
It's also worth remembering that juries aren't terribly sympathetic when it comes to DUIs, so even a drunken driving charge stemming from a BAC below the legal limit could very well lead to criminal punishment. If you're facing this situation, it may be wise to consult an experienced DUI/DWI lawyer in your area.