Zero tolerance means that it is illegal anyone under 21 to drive with any alcohol in his or her system. Zero tolerance laws are in effect in every state. However, strictly speaking, “zero tolerance” is a bit of a misnomer in many states. The BAC level that many zero tolerance laws set is not actually .00. In many states, anything below .01 or .02 will not result in a citation or arrest. An important reason why relatively small amounts of alcohol are tolerated in many states is that lawmakers are aware that alcohol testing instruments are unreliable and can have a significant margin of error. Moreover, breathalyzers can register things other than alcohol consumption. However, the irony is that adults are therefore given less leeway: anything above .08 automatically means a DUI for people over 21.
For more information on the problems with breathalyzers, read the article “Driving Under the Influence of Bread and Binaca.”