Breath test devices can determine a blood alcohol concentration (BAC) because there is a scientific relationship between the amount of alcohol in the blood and the amount of alcohol in the breath. All devices used in Canada presume that 2100 parts of deep lung air contain the same amount of alcohol as one part of blood. But a problem arises if there is residual alcohol in a person’s mouth when they provide a sample of breath. The alcohol from the mouth adds to the alcohol in the breath and increases the apparent BAC. It’s an apparent BAC because it is a falsely high measurement, sometimes by a significant amount. The problem is not with the device, but with the breath sample.
A mouth alcohol effect can be caused anytime alcohol is consumed, or it can occur with burping, belching, or regurgitating alcohol-laden fluids into the mouth. Mouth alcohol does not last a long time — no more than 15 minutes — but if it is present it can create a very high false reading.
Operators of evidentiary instruments in the police office are required to observe a driver for at least 15 minutes before each breath test to ensure there is no burping, belching or consumption of alcohol that could produce a mouth alcohol effect.
ASD operators are not required to conduct a 15 minute observation period before an ASD test. However they are required to determine when alcohol was last consumed and will ask the driver for this information. If there has been drinking within the previous 15 minutes, an officer should add an appropriate amount of time so that there are 15 minutes from the time of the last drink to the time of the test.