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What’s the Difference Between Impairment and Intoxication?

May 11th, 2018 DUI OWI What’s the Difference Between Impairment and Intoxication?

There are several ways to look at this question.  The National Highway Traffic Safety Administration defines the two terms this way. Notice the results-based language contained in these definitions:

  • Impairment—Refers to the deterioration of an individual’s judgment and/or physical ability. Physiological and cognitive impairment begin at BAC levels below those that are associated with intoxication. As a legal standard, impairment and DWI laws are based on a person’s physical or mental impairment as judged on the basis of BAC level, performance in standardized field sobriety tests, or observed behavior. Although this report is focused on impairment from alcohol, impairment can result from other substance use, distracted driving, and other reasons.
  • Intoxication—Refers to the condition of having physical or mental control markedly diminished by the effects of alcohol or drugs. This is usually based on a subjective determination (one feels the sensation or observes a behavior in another person). Physiological impairment begins at BAC levels below those that are associated with intoxication. Intoxication is not a legal standard.

Scientists have produced charts such as this one[i], which much more carefully explain the differences:

Blood Alcohol Concentration Stage of alcoholic influenceClinical Signs and Symptoms
0.01 – 0.05subclinicalInfluence of effects not apparent or obvious.  Behavior nearly normal by ordinary observation.  Impairment detectable by special tests.
0.03 – 0.12euphoriaMild euphoria, sociability, talkativeness.  Increased self-confidence, decreased inhibitions.  Diminution of attention, judgement and control.  Some sensory-motor impairment.  Slowed information processing.  Loss of efficiency in critical performance tests.
0.09 – 0.25excitementEmotional instability, loss of critical judgment. Impairment of perception, memory, and comprehension.  Decreased sensatory response, increased reaction time, Reduced visual acuity, peripheral vision and glare recovery.  Sensory-motor incoordination, impaired balance.  Drowsiness.
0.18 – 0.30confusionDisoriented, mental confusion, dizziness.  Exaggerated emotional states (fear, rage, grief, etc.).  Disturbances of vision (diplopia, etc.) and of perception of color, form, motion, dimensions.  Increased pain threshold.  Increased muscular incoordination, staggering gait, slurred speech, Apathy, lethargy.
0.25 – .40stuporGeneral inertia, approaching loss of motor functions.  Markedly decreased response to stimuli.  Marked muscular incoordination, inability to stand or walk.  Vomiting, incontinence of urine and feces.  Impaired consciousness, sleep or stupor.
0.35-0.50comaComplete unconsciousness, coma, anesthesia.  Depressed or abolished reflexes.  Subnormal temperature.  Impairment of circulation and respiration.  Possible death.
0.45+deathDeath from respiratory arrest.


The legal definition is still different. In Michigan, the law says that it is unlawful to operate a motor vehicle while intoxicated. This is called OWI or operating while intoxicated.  A person is intoxicated if they have a bodily alcohol level at or above .08% and/or where alcohol has materially and substantially lessened their ability to operate a motor vehicle in a normal way.  The standard jury instruction for impairment says this:

To prove that the defendant operated while visibly impaired, the prosecutor must prove beyond a reasonable doubt that, due to the drinking of alcohol the defendant drove with less ability than would an ordinary careful driver. The defendant’s driving ability must have been lessened to the point that it would have been noticed by another person.


i[] See, Jones, Garriott’s Medicolegal Aspects of Alcohol, Pharmacology and Toxicology of Ethyl Alcohol, Lawyers and Judges Publishing Company, Fifth Ed., pg 28, 2007. (chart copyright by Kurt M. Dubowski, University of Oklahoma College of Medicine, Oklahoma City, OK, 1997).