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Alcohol acts as a depressant in the body. It causes eye muscles to function more slowly resulting in altered eye movement and perception. The consumption of alcohol also affects drivers’ ability to judge distances and speed on the road and their hand/foot eye coordination. Because of the detrimental effects that alcohol has on the functioning of organs (eyes) and systems (nervous) critical to driving, most countries have laws with specific regards to alcohol consumption while driving.
These laws state what level of concentration of alcohol in a person’s blood is legal when driving. The concentration of alcohol in a person’s blood is known as the blood alcohol concentration or blood alcohol level (BAL). This number is usually given as a percentage and expresses the number of grams of alcohol per 100 grams of blood. For example a BAL of 1 percent represents about 1.0 drop of pure alcohol per 100 drops of blood. Currently, Trinidad and Tobago is trying to adopt legislation which would set a legal limit for the BAL of drivers. Worldwide, the legal limit for BAL for divers usually lies between 0.00 and 0.10 percent.
The more alcohol a person consumes, the slower his or her reaction time becomes. Reaction time of a drunk driver may be reduced by 10-30 percent as compared to a sober driver. A drunk driver’s vision is blurred and the judgment of distance, speed, and hazards is diminished. Studies have indicated that with a BAL of 0.05 percent, a driver is twice as likely as a driver with a BAL of 0.00 to be involved in a crash. With a BAL of 0.08 percent, the likelihood increases ten times and a BAL of 0.15 percent increases the risk by a factor in the hundreds. In addition, crashes involving alcohol are more likely to result in severe or fatal injuries. Evidence indicates that this may be due to the fact that inebriated drivers are less likely to use seatbelts and more likely to exceed speed limits, significantly increasing their risk of severe crash-related injury.
It is generally accepted that the liver can process 0.5oz (approximately 15ml) of pure alcohol in an hour and thus is the unit of measure when counting the number of drinks consumed. This alone however, cannot be used to determine BAL. Factors such as weight, gender and metabolism can all affect BAL. Generally, a heavier person will have a lower BAL after consuming the same volume of alcohol. Women, generally can consume less alcohol than men due in part to their smaller size and higher body fat content, in addition women absorb 15 to 20 percent more alcohol than men. Also, persons metabolise (break down) alcohol at different rates. Someone who metabolises alcohol more quickly will be able to consume more alcohol with a smaller rise in BAL. The presence of food in the stomach also affects the absorption of alcohol as it causes the alcohol to remain longer in the stomach where it is not absorbed as effectively as in the intestine.
Table Describing effects of alcohol at various BALs.
| Number of Drinks | Approximate BAL | Expected effects in 150 pound person |
| ONE 12 oz glass of a liquid of at least 4.5 percent pure alcohol | 0.03 percent | Drinker will feel unfocused |
| TWO 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.06 percent | Drinker will experience a decrease in the ability to perform simple physical tasks |
| THREE 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.09 percent | Drinker’s reaction time will be slowed, they will experience poor muscle control, slurred speech and wobbly legs |
| NOTE: NO country in the world allows you to drive a motor vehicle when your BAL is higher than 0.10 percent | ||
| FOUR 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.12 percent | Drinker’s judgment will be clouded, and their ability to make logical decisions will be impaired. |
| FIVE 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.15 percent | Drinker’s vision will be blurred, speech will be unclear, walking will be unsteady, and coordination will be impaired. |
| SIX 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.18 percent | All of drinker’s behavior will be impaired and drinker will find it difficult to stay awake |
| TEN to TWELVE 12 oz glasses of a liquid of at least 4.5 percent pure alcohol | 0.30 percent | Drinker will be in a coma-like state. Most people are unable to stay awake with a BAL higher than 0.30 percent. |
| NOTE: If the BAL reaches higher than 0.50 percent the drinker will be in a deep coma and in danger of death. As it approaches 1.0 percent in the blood (that is 1g of alcohol for every 100g of blood!) the breathing center in the brain becomes paralyzed and death occurs. | ||
It is important to remember that the numbers in this table are just an average and represent the effects that would be expected for a typical individual. The rate of consumption of alcohol will affect these numbers as will the other differences between individuals discussed above.
An informative guide to
understanding blood alcohol concentration and alcohol impairment can be
found at the U.S. National Highway Traffic Safety Administration website,
The ABCs of BAC, at:
http://www.nhtsa.dot.gov/people/injury/alcohol/stopimpaired/ABCsBACWeb/page2.htm
References:
Science and Road Safety, Science
and Technology in Action, Third Edition, The National Car Testing Service of
Ireland.
http://www.sciencetechnologyaction.com/lessons/53/NCT.pdf
Module 15: Drinking and Driving,
ICAP (International Center for Alcohol Policies),
http://www.icap.org/portals/o/download/all_pdfs/blue_book/Module_15_Drinking_and_Driving.pdf
Module 16: Blood Alcohol
Concentration Limits, ICAP (International Center for Alcohol Policies),
http://www.icap.org/portals/o/download/all_pdfs/blue_book/Module_16_Blood_Alcohol_Limits.pdf
Alcohol Effects FACTS ON: The
Effects of Alcohol by Gail Gleason Milgram, Ed.D
http://www.dui.com/dui-library/studies/alcohol-effects
Written: March 2009
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