Sometimes I hear or read someone on the news referring to a criminal defendant as having a certain amount of alcohol in his or her system with some variation on the phrase “the legal limit.” For example, a news anchor might say, “Johnson allegedly was tested and found to have a blood alcohol level more than twice the legal limit.” To be precise, this is shorthand for “the legal limit for operation of a motor vehicle,” although sometimes it’s used in contexts where blood-alcohol level has no “legal limit.”

Setting aside any superficial quibble about vocabulary, I think it’s important that people realize (or keep in mind) that this is a misleading phrase. Operating under the influence, in general*, requires proof of operation of a motor vehicle either with blood-alcohol content of at least 0.08% or “under the influence of intoxicants.”

First, you should know that intoxicants can include drugs other than alcohol, which means some people are convicted of operating under the influence when they have no alcohol in their system but are under the influence of a prescription (like Valium) or a black-market drug (like heroin). Getting a 0.00 on a breathalyzer test was not enough to keep these people out of jail.

Second, you should know that a reading of 0.08 or above is enough for an OUI conviction, but it’s a myth that a reading of 0.07 or below means the possibly impaired motorist is good to go. A driver could have a reading anywhere from 0.01 to 0.07 and still get charged with OUI in Maine because operation “under the influence” can be proven if the driver was impaired at all by an intoxicant. Thus, this number doesn’t constitute a magical line that separates drunken drivers from sober drivers. And that brings up my last point.

Third, because breathalyzer tests are not as accurate as their proponents may claim, it is possible to avoid a conviction for OUI despite a test reading 0.08. The margin for error is sometimes described as +/- 0.01, which means a defendant can raise a reasonable doubt about his or her guilt at trial with a level of 0.08. Furthermore, because of the way that a breathalyzer machine measures the alcohol content of a person’s breath, there is some room to doubt intoxication in rare cases when the test is even higher. That probably won’t prevent the BMV from suspending the person’s license, though.

In summary, 0.08 is not a “legal limit” to drive in the sense that a driver with blood-alcohol level below it is automatically legal to drive. It is a “legal limit” in the sense that, in all but a few rare cases, someone who is caught driving with that blood-alcohol content will be found guilty of OUI.

*I’m writing for the typical case, but it should be noted that the alcohol content for per se OUI that involves a commercial vehicle is 0.04%, and for a driver under the age of twenty-one it’s any measurable amount of alcohol.