Many in may think that if one has a breath test indicating that they were intoxicated while driving, there is little reason to dispute it. This comes from the assumption that breath testing devices produce results accurate enough to indicate guilt with every use. Yet that simply is not the case. Indeed, research information shared by the National Motorists Association shows that breath testing devices can have a margin of error as high as 50%. 

Understanding how breath test results can often be inaccurate requires a comprehension of the process of how alcohol ends up on one’s breath. According to the Alcohol Pharmacology Education Partnership, the form of alcohol included in most alcoholic beverages is ethanol. Because ethanol is a water-soluble compound, it is able to permeate (via passive diffusion) the linings of the organs composing the gastrointestinal tract after being ingested. This allows the ethanol to enter the bloodstream, where veins then carry it throughout the body and eventually the lungs. 

Oxygen in the lungs then causes some of the ethanol molecules in the blood to vaporize. That vaporized ethanol is then expelled from the body when one breathes. As this happens, the balance between gaseous ethanol in the lungs and liquid ethanol in the blood must remain in equilibrium. Thus, with every breath, the ethanol content of the blood lowers. 

This means that as one breathes, their blood-alcohol content is slowly lowering. This makes getting an accurate measurement with a breath testing device difficult. These devices assume a baseline blood-to-breath ratio, yet in reality one’s actual ratio might be very different due to their physical traits. These factors all contribute to breath test results often being debatable in DUI cases.