It’s late at night, and the city’s bars have just closed. You’re a police officer patrolling your regular area when you see a car weaving erratically across the road. You follow the car into a residential neighborhood. The driver sees you and pulls over. Shining your flashlight into the car, you notice that the driver is a man in his thirties, about 160 pounds.
As he hands over his license and registration, the man slurs his speech. He is uncoordinated and seems to have trouble getting his license out. You ask him if he’s been drinking. He admits that he had two beers at a nearby bar.
You decide to give the driver a breathalyzer test. This test shows the man’s current blood alcohol content (BAC) as 0.12%, which is above the DUI limit of 0.08%. You arrest him for DUI.
Calculating BAC
BAC can be estimated in the following way:
BAC calculation equation. Blood alcohol content equals mass of alcohol consumed in grams, divided by volume of water in the body in milliliters, multiplied by 100
Use this formula and your math skills to answer the questions below.
A regular 12-oz beer is about 5% alcohol. This works out to about 14.03 grams of alcohol per beer. If the driver drank two beers, how many grams of alcohol did he consume?
The driver weighs about 160 lbs. What is his body weight in kg? What is his body volume in mL? (1 lb = 0.45 kg) (1 kg = 1000 mL)
For most males, 68 percent of the body is water. What is the volume of water in the driver’s body in mL?
Use the above information to calculate BAC.
The measured BAC was 0.12%. Was the driver telling the truth about how much he drank? Calculate the difference between the two BAC percentages.
If the driver had really consumed only two beers, would he have been arrested for DUI? Explain.
In reality, BAC can vary widely among individuals. The math above is just a rough estimate—that’s why forensic toxicologists and police officers use tests like the breathalyzer to accurately determine a person’s blood alcohol level.
Signs of Impairment
The level of a person’s impairment depends on their BAC. Below are some signs of impairment that you might observe based on BAC.
TABLE 1 Signs of impairment based on BAC Level
BAC Level (%) Behavior Signs
0.020 – 0.039
Euphoria, relaxation
0.040 – 0.059
Lower inhibition, impaired judgment, lack of caution
0.060 – 0.099
Slightly impaired balance, speech, coordination
0.100 – 0.129
Noticeably impaired coordination and reactions, slurred speech
0.130 – 0.159
Extreme impairment, blurred vision, dysphoria
0.160 – 0.199
Nausea, sloppiness
0.200 – 0.249
Unable to walk, vomiting, possible blackout
0.250 – 0.399
Unconsciousness, alcohol poisoning
0.400 and above
Coma, possible death
Based on this table and the description in Part I, did the driver’s behavior match his measured BAC of 0.12 percent? Explain. (1-2 sentences)
If the driver had consumed only two beers as he claimed, what kind of behavior might he show? (1-2 sentences)
The legal limit for driving is 0.08 percent BAC. Can a driver show effects of impairment below this level? What effects might be present? Do you think it is safe to drive with these effects? Explain your answer. (2-3 sentences)
