Section 2.Calculating Alcohol Concentration
  (1) Theoretical alcohol concentration

  Water constitutes about 70% of a human's weight. Assuming that all this water is blood, and that blood is equivalent to pure water, what will happen to a man weighing 75 kg if he drinks a 350 ml can of beer with 5% alcohol content?²

  Alcohol permeates throughout the man's body water, which amounts to 52,500 ml. Disregarding physiological phenomena, constitutional makeup, physical condition, and how fast he drank the beer, the theoretical alcohol concentration in his blood will be 0.264 mg/ml (consumed alcohol/body water)

  Consumed alcohol
  350 ml (amount of beer) × 0.05 (alcoholic content) × 0.792 (alcoholic density) = 13.860 g
  Theoretical blood alcohol concentration
  13,860 mg / 52,500 ml = 0.264 mg/ml

  The legal threshold for driving under the influence of alcohol is "0.3 mg alcohol per 1 ml blood or 0.15 mg per 1 liter of exhaled breath". Theoretically, a 350 ml can of beer is sufficient to raise the blood alcohol to that level, making the breath smell of alcohol, to boot.³


  (2) Blood alcohol concentration in the body

  The alcohol absorbed by the body dissolves in the blood and circulates through the entire body, readily passing through biomembranes to wherever water exists. The speed of permeation differs considerably among individuals, depending on gender, age, constitutional makeup, weight, amount of subcutaneous fat, enzymes, physical condition, amount of alcohol consumed, type of beverage, how quickly the alcohol is consumed, how much time has elapsed since drinking, and many other conditions. Blood and breath alcohol concentration is determined by the complex combination of these factors.
  There are formulae to calculate blood alcohol concentration from the above factors. In court, Ueno's formula is adopted for calculation, with Widmark's formula also coming into use. On the Internet, there are web pages where you can calculate your blood alcohol concentration by simply entering your weight, amount of alcohol consumed, time elapsed since drinking, etc.

Blood alcohol concentration
(3) Ueno's formula

  Both Ueno's formula and Widmark's formula are based on the same principles: Alcohol concentration in the body is calculated in view of body water volume, from which the decline in alcohol concentration along with time is deducted to obtain the blood alcohol concentration at the time. Maximum and minimum values are calculated, as the loss of alcohol within the body and the rate of decline in alcohol concentration varies with individuals.
  Shown below is an example of Ueno's calculation of blood alcohol concentration for a man weighing 75 kg, two hours after drinking 1,000 ml of beer.

Formula
  a. Volume of water in the body is obtained by multiplying the body weight by the distribution rate (0.7 for male) in Ueno's method.
  b. Deducted from the theoretical blood alcohol concentration is the loss of alcohol including: that eliminated from the body through perspiration and breath, that broken down, and that dissolved in a different way depending on the type of body fluid. Two volumes are obtained here: one calculated with maximum alcohol loss (0.8) and one with minimum alcohol loss (0.7).4

  c. As alcohol concentration decreases at a rate of 0.12 − 0.19 mg/ml every hour, this value, multiplied by the elapse of time after drinking, is deducted from the values obtained in b.5
  Maximum value
      0.754 mg/ml × 0.8 - 0.12 mg/ml × 2 hours = 0.363 mg/ml
  Minimum value
      0.754 mg/ml × 0.7 - 0.19 mg/ml × 2 hours = 0.147 mg/ml

  Blood alcohol concentration of the 75 kg male, two hours after drinking 1,000 ml of beer, comes out somewhere between 0.147 and 0.363 mg/ml, a considerably high figure. However, the values obtained here are just calculated values, and the actual blood alcohol concentration may be outside this range depending on physical conditions and other factors.


(4) Change in actual breath alcohol concentration and Ueno's formula

  Each of the 16 test subjects (males aged 25-51) was asked to consume an amount of alcoholic beverage that would result in a breath alcohol concentration of 0.2 mg/l, 20 minutes after drinking, taking into account weight, physical condition, amount of food in the stomach, etc., in order to see the change in breath alcohol concentration.
Measured values of breath alcohol concentration and values calculated by Ueno's formula are compared in the following charts.
Test subject 1
Test subject 2 Test subject 3 Test subject 4
Test subject 5 Test subject 6 Test subject 7
Test subject 8 Test subject 9 Test subject 10
Test subject 11 Test subject 12 Test subject 13
Test subject 14 Test subject 15 Test subject 16
  In the majority of test subjects, measured values were within the range of calculated values (between maximum and minimum values), showing that breath alcohol concentration can be calculated with a fair amount of accuracy.
But calculated values are just calculated values, and are not 100% precise. Some individuals indicate values outside the calculated ranges. Accordingly, it should never be assumed that one's alcohol concentration is low enough just because sufficient time has passed since drinking.
  Individuals show significant differences in the peak value of breath alcohol concentration, the time when the value peaks out, and the rate of change in breath alcohol concentration. In this test, subjects consumed an amount of beer ranging from 850 cc at the least to 1,500 cc at the most, spending about half an hour for drinking. Most of the subjects indicated breath alcohol concentration of 0.15 mg/l or more, or at least close to that level, after two hours.
Even after three hours, half the subjects recorded 0.10 mg/l, with a few still showing close to that level after four hours. Data after four hours was not taken, but it is safe to assume that a fairly high breath alcohol concentration would still be detected four to five hours after consuming alcohol.

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Institute for Traffic Accident Research and Data Analysis (ITARDA)