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Alcohol Alert

National Institute on Alcohol Abuse and Alcoholism No. 25 PH 351 July 1994

Alcohol-Related Impairment

Alcohol consumption is associated with a wide range of accidents and injuries resulting from the impaired performance of complex mental and motor functions. The relationship between alcohol and motor vehicle crashes is well known; alcohol also has been implicated in many railroad, boating, and aircraft accidents. The subtlety and complexity of the skills required to operate these vehicles make them susceptible to impairment by low doses of alcohol (1). This Alcohol Alert examines the ways in which alcohol impairs complex mental and motor functions and discusses approaches to impairment testing. The discussion is limited to the acute impairment that results from an episode of intoxication, rather than the chronic impairment that may develop after years of heavy drinking.

Extent of the Problem

Epidemiologic studies reveal the extent of alcohol's effect on transportation safety in the United States. First, 40 percent of all traffic fatalities (the leading cause of accidental death) are alcohol related (2). Second, although alcohol has not been directly implicated in U.S. commercial airline crashes, typical estimates of alcohol involvement by pilots in fatal general aviation crashes range from 10 to 30 percent (3). Third, a recent review of Coast Guard reports suggests possible alcohol involvement in 60 percent of boating fatalities (including persons who fell overboard) (4). Finally, in postaccident testing of railroad employees in 1990, 3.2 percent tested positive for alcohol or other prohibited drugs (5). The percentage of alcohol or other drug involvement may be higher when a fatality is involved (6).

Studies of Alcohol-Related Impairment

The epidemiologic evidence linking alcohol and transportation accidents is supported by experimental studies of alcohol's effect on specific driving-related skills. Although the following discussion concentrates on highway traffic safety, most of the skills involved pertain to other forms of transportation as well. These skills may be divided into cognitive skills, such as information processing, and psychomotor skills (those involving eye-brain-hand coordination). Impairment is related to alcohol in terms of its concentration in the bloodstream. For reference, a blood alcohol concentration (BAC) of 0.04 percent might be achieved by a 150-pound man consuming two drinks in 1 hour (7).

The brain's control of eye movements is highly vulnerable to alcohol. In driving, the eyes must focus briefly on important objects in the visual field and track them as they (and the vehicle) move. Low to moderate BAC's (0.03 to 0.05 percent) interfere with voluntary eye movements, impairing the eye's ability to rapidly track a moving target (8-10).

Steering is a complex psychomotor task in which alcohol effects on eye-to-hand reaction time are superimposed upon the visual effects described above. Significant impairment in steering ability may begin as low as approximately 0.035 percent BAC and rises as BAC increases (11).

Alcohol impairs nearly every aspect of information processing by the brain (3). Alcohol-impaired drivers require more time to read a street sign or to respond to a traffic signal than unimpaired drivers; consequently, they tend to look at fewer sources of information (12). Research on the effects of alcohol on performance by both auto-mobile and aircraft operators shows a narrowing of the attentional field beginning at approximately 0.04 percent BAC (13).

The most sensitive aspect of driving performance is the division of attention among component skills. Drivers must m aintain their vehicles in the proper lane and direction (a tracking task) while monitoring the environment for vital safety information, such as other vehicles, traffic signals, and pedestrians. Alcohol-impaired subjects who are required to divide their attention between two tasks tend to favor one of them. Therefore, alcohol-impaired drivers tend to concentrate on steering, becoming less vigilant with respect to safety information. Results of numerous studies indicate that divided attention deficits occur as low as 0.02 percent BAC (12).

The ability to divide attention is especially critical in aviation. Morrow and colleagues (14) noted that radio communication during simulated flight was impaired significantly by divided attention deficit at BAC's as low as 0.04 percent.

The combined effects of these individual deficits on overall performance have been studied under simulated vehicle-operating conditions. A review of six ground-traffic simulator studies demonstrated consistently poorer performance at BAC's of 0.048 percent and above (15). In a typical study of the effects of pilot impairment, aircraft pilots completed eight sessions of simulated flight between San Francisco and Los Angeles in a Boeing 727-232 simulator (16). Planning and performance errors, procedural errors, and failures of vigilance each increased significantly with increasing BAC. Serious errors increased significantly at the lowest BAC, 0.025 percent, compared with performance at 0 percent BAC.

Results of epidemiologic and experimental studies permit certain conclusions to be drawn. First, the degree of impairment depends on the complexity of the task involved as well as the BAC (1). Second, the magnitude of alcohol-induced impairment rises as BAC increases and dissipates as alcohol is eliminated from the body. Third, at a given BAC, some skills are more impaired than others (12). Finally, investigators have not found an absolute BAC threshold below which there is no impairment of any kind. Certain skills important for driving are impaired at 0.01 to 0.02 percent BAC, the lowest levels that can be measured reliably by commonly used devices (17).

Impairment Testing

Accurate measurement of impairment is essential for traffic safety and law enforcement. Unfortunately, the devices used in the laboratory to measure impairment are not practical for roadside use. Therefore, all States have enacted per se laws by which a BAC above a specified limit is sufficient evidence of impairment for legal purposes. These laws are based on the assumption that BAC alone is an accurate indicator of impairment (1).

For traffic enforcement, the alcohol concentration in the subject's breath is determined by a hand-held device. When the procedure is performed correctly, the meas-ured breath-alcohol concentration accurately reflects the BAC and can be correlated with alcohol-induced impairment without interference from alcohol vapors contaminating the mouth (18).

Objections have been raised to the validity of BAC or breath alcohol as an indicator of impairment. The degree of impairment associated with a given BAC is not constant and may vary among individuals. This may be explained in part by the phenomenon of tolerance. Tolerance is a decrease in the magnitude of an effect of a given dose of a drug after repeated exposure to the drug. Thus, more experienced drinkers show less impairment than less experienced drinkers across a range of BAC's in tests of motor coordination, sensory perception, and intellectual function (19,20).

The relationship between BAC and impairment also may vary according to the subject's age. According to Moskowitz and colleagues (17), drivers below the age of 25 and above the age of 69 would be expected to have higher crash rates at a given BAC than the remainder of the population. Laboratory data show that increasing age magnifies the adverse effect of low doses of alcohol on tracking (11); simulated flight experiments show that older pilots are more impaired than younger pilots at equal BAC's (14,20).

Alcohol poses a more serious risk for younger drivers because they have comparatively little experience with alcohol (tolerance) or with driving (2,21). Therefore, many States have established lower per se limits for minors (as low as 0.02 or 0.0 percent BAC versus 0.1 or 0.08 percent BAC for adults (22)).

It is not always possible to determine precisely how high a person's BAC had been when BAC measurement is delayed. For example, BAC determination often is postponed until hours after an accident or arrest, when the subject is in custody or in the hospital. The BAC may then be significantly lower than at the time of the earlier event. Common practice has been to estimate the subject's earlier BAC based on standard equations for the rate at which alcohol leaves the body. However, the factors that influence the elimination of alcohol vary greatly. Calculations based on average elimination rates for populations can give rise to considerable error when applied to individuals (23,24).

A similar problem arises when a BAC determination must be made after death (25). The interpretation of alcohol in post mortem specimens is complicated by the presence of alcohol produced by microbial fermentation. This problem is minimized by analysis of the vitreous humor, the liquid that fills the eyeball behind the lens. Because this liquid is reasonably protected from microbial activity and remains relatively constant after death, the alcohol concentration in vitreous humor is used commonly to confirm post mortem BAC determinations (26).

The above-mentioned disadvantages of BAC as a per se indicator of impairment have prompted research on devices that reflect impairment directly. For example, both the National Highway Transportation Safety Administration and private industry have developed ignition interlock systems that prevent use of a vehicle by an impaired person. These systems require the performance of a brief (10- to 30-second) task to test reaction time, short-term memory, visual tracking, or other measures of psychomotor coordination. Performance on these tasks has been shown to correlate reasonably well with actual (or simulated) driving performance (3) without reference to the cause of the impairment.

Alcohol-Related Impairment--A Commentary by
NIAAA Director Enoch Gordis, M.D.

There are many opportunities for alcohol-impaired persons to cause injury and death to themselves and to others. Driving, flying airplanes, boating, using complicated machinery such as printing presses or other industrial equipment, performing surgical procedures, participating in recreational activities such as swimming and skiing, and many other activities become more risky when the people engaging in them are impaired by alcohol use. The substantial numbers of people who are injured or who die from alcohol-related accidents every year attest to the seriousness of the problem.

By understanding how alcohol impairs complex mental functions, we can develop ways to prevent many alcohol-related injuries by negating alcohol's ability to interfere with cognitive and motor function. As neuroscience research matures, this may become possible. We also can prevent alcohol-related injuries by developing easily deployed devices to measure impairment. Setting a legal threshold for determining impairment by BAC (per se laws) has disadvantages that might be avoided by use of an inexpensive, convenient field device for determining impairment directly. Such a device would have the advantage of identifying persons who are unfit to drive or engage in other potentially dangerous activity for any reason, such as extreme fatigue, illness, infirmity, emotional states, or the use of alc ohol and other drugs and medications. Whether the public would endorse the use of such devices rather than the current widely accepted and supported per se laws is certain to be a matter of significant public policy debate.


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All material contained in the Alcohol Alert is in the public domain and may be used or reproduced without permission from NIAAA. Citation of the source is appreciated.

Copies of the Alcohol Alert are available free of charge from the Scientific Communications Branch, Office of Scientific Affairs, NIAAA, 6000 Executive Boulevard, Suite 409, Rockville, MD 20892-7003. Telephone: 301-443-3860.

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Updated: October 2000