The difference between impaired and over 80 is best illustrated by an example…
Some people simply have no tolerance for alcohol, due perhaps to their youth, inexperience or even health issues. Even one or two drinks will dramatically affect their sobriety. If they are pulled over in such a state they would be arrested immediately for impaired driving and taken to the police station. They may even blow under the legal limit but they will nevertheless face a charge of impaired driving because, despite the relatively minor amount of alcohol the effect on their ability to drive is obvious and pronounced.
Conversely, some people such as heavy drinkers and alcoholics can consume inordinate amounts of alcohol and betray no signs of impairment whatsoever. If they are pulled over the investigating officer may simply detect an odour of an alcoholic beverage on their breath and thereupon demand that they provide a sample of their breath into a roadside screening device.
Once they have registered a fail on that machine the officer then has reasonable and probable grounds to arrest them for suspicion of being over the legal limit. I have had cases such as these where the person subsequently blows more than two or three times the legal limit but wasn’t even charged with impaired driving. They will simply face the initial charge of over 80.
The difference in these two cases is obviously one of tolerance. Generally, there is a correlation between one’s blood-alcohol level and impairment but that is not necessarily the case. The real issue is whether the amount of alcohol consumed by a particular individual impaired their ability to operate a motor vehicle.
This charge is typically proven using a certificate of analysis which is a document signed by the breath technician attesting to the breath samples provided and the time they were given. This is the evidence that the defence would seek to exclude on the basis of some investigative lapse or constitutional violation. Given that a detainee risks self incrimination through police interrogation and the legal compulsion to provided breath samples, various constitutional issues are engaged including a detainee’s Charter rights to be secure from arbitrary detention, their right to be informed about the reason for their detention, their right against unreasonable search and seizure and the right to counsel.
Although it is understandable that most who are charged with this offence may feel that their guilt is a foregone conclusion because they “blew over” at the station, factual guilt must be distinguished from legal responsibility where the Crown’s case fails in the face of a well-crafted constitutional argument that serves to exclude the breath readings and leaves the prosecution with no evidence at trial.
A good illustration of an investigative lapse leading to an acquittal is the case of R. v. B.H.  O.J. No. 1390 [link] who blew nearly 3 times the legal limit but was acquitted because the Crown failed to lead evidence that his tests were taken “pursuant to a demand”. As the judge on appeal concluded,
“Without the presumptions or calling as witnesses the breathalyzer technician and a toxicologist, the prosecution can neither prove the accused’s blood alcohol level nor relate it back to the time of the driving. Clearing the evidentiary hurdles posed by the presumptions is wholly the Crown’s responsibility. In this case, failure to adduce evidence that there was a proper breath demand prevented their operation.”