This will not be a simple answer to a seemingly simple question. Being charged with operating under the influence and being prosecuted for it are two separate issues. Impairment charges based on what the officer observes in the field may not be accepted by a district attorney for prosecution if the blood alcohol concentration (BAC) is below a statutory level. Every person reacts to the consumption of alcohol differently and individuals who don't drink regularly likely will show greater effects of impairment at lower BAC levels and in many cases, any combination of drugs will increase the effects. A prosecution of this nature really depends on the officer's detailed observations of the subject's behavior. In most cases, impairment charges at lower BAC levels result in reduced charges. So the partial answer is yes, a person may be charged with impairment at lower BAC levels, but prosecutorial outcomes may differ.
If the question is regarding an evidentiary BAC, the charges and subsequent prosecution really depends on the relationship of the BAC to the time of operation. If the blood alcohol concentration was obtained several hours after the incident, as is the case in some instances, a retrograde analysis (using a standard natural rate of elimination over a known period of time to calculate the BAC) would establish an estimated BAC at the time of operation that could provide an evidentiary basis to support impairment and associated charges. Typically, if a BAC of .02 was obtained at the time of operation absent observations of obvious impairment, the level of alcohol in the blood would not be sufficient to support a charge of impairment but could provide evidence for and be contributory to other charges such as careless or reckless operation, or contributory to other violations of rules of navigation. In many states, if the operator was under 21, that BAC or in some instances, any measurable BAC could lead to alcohol related charges following an incident. In my experience, presenting BUI training to law enforcement officers across the nation, I know of no jurisdiction or state that would pursue a charge of operating under the influence if an evidentiary BAC of .02 was obtained and no observations of obvious impairment were documented, or no drugs were present at the time of operation.
That being the reality of prosecution, both the National Highway Traffic Safety Administration (NHTSA) and National Transportation Safety Board (NTSB) have concluded that discernable alcohol impairment exists at lower blood alcohol levels. Scientific research on this topic concluded that alcohol impairment (i.e. relaxation and altered emotional states) begin with the absorption of the first alcoholic beverage-- and increases its effects and symptoms with each subsequent drink. Additionally, the studies indicated that states proposing legislation to lower the statutory levels below .08 BAC would be supported by scientific research in doing so.
Hope that helps, sincerely,
Maj. Tim Baumgarten (ret.)
Arizona Game and Fish Department