To determine a person's blood alcohol concentration, you must know how much the person weighs.

• A. The time since drinking
• B. The type of alcohol
• C. The person's height
• D. How much the person weighs.

Answer: (D)

Blood alcohol concentration is determined by how much alcohol is distributed throughout the body's water. A person’s weight affects how much body water there is to dilute the alcohol: with more body water, the same amount of alcohol becomes more diluted, leading to a lower BAC, while a lighter person has less dilution and a higher BAC. So knowing weight is essential for estimating how concentrated the alcohol will be in the blood. Other factors do matter—time since drinking changes BAC as the liver metabolizes alcohol, and sex and body composition can influence distribution—but weight is a primary factor used to gauge how strong the BAC will be for a given amount of alcohol. Height isn’t a direct determinant, and the type of alcohol doesn’t change BAC once the amount of ethanol is fixed.