The legal BAC limit for adults 21 and over is .08 percent. For drivers under 21, the limit drops to .02 percent under the Junior Operator Law, which effectively means any measurable alcohol. Commercial drivers operating under a CDL are held to a .04 limit while driving a commercial vehicle. Keep in mind the .08 number is a per se threshold - you can still be convicted of OUI at a BAC below .08 if the Commonwealth proves impairment through officer observations, field sobriety tests, and circumstantial evidence. We have seen convictions at .05 when the driver was involved in a crash and the officer testified to obvious impairment. A Massachusetts DUI lawyer will examine both the per se and impairment theories when building your defense.