Legal Wizard

Back Calculations

The procedure whereby forensic science can calculate the amount of alcohol in the body at the time of riding.

Alcohol & the body

Where Ethanol contained in an alcoholic beverage is drunk absorption is initially through the mouth, gullet and stomach. However, the major absorption is in the small intestine. Absorption rate is about the same as water. The rate limiting factor is gastric emptying into the small intestine. Consequently ethanol absorption is more rapid when alcohol is consumed on an empty stomach. Strong alcohol, when the stomach is empty, behaves differently since (if strong enough) absorption is delayed by a defence mechanism.

Once in the bloodstream, alcohol rapidly equilibrates throughout all body fluids. The process takes 30-90 minutes. Elimination of alcohol from body fluids begins with 10% being lost in sweat, faeces, urine or breath. The remaining 90% is metabolised as food and destroyed chemically.

There are different types of alcohol including methanol, ethanol, propanol and so on. Each has different properties and each has to be taken into account when undertaking back calculations. Alcohols are absorbed depending on a whole range of factors including the contents of your stomach and your physiology. The rate of elimination depends on your physiology, the presence/absence of certain chemicals and on your previous drinking history. In general terms it is eliminated at a rate of 1 8milligrams per 100ml of blood per hour (ie 7.8% micrograms per 100ml of breath per hour).