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JULY 2017 | Published by RCPA

Issue #071

Breath and blood alcohol have a volatile relationship

Breath and blood alcohol have a volatile relationship

That alcohol can be measured by a blood test is generally understood. It’s not so obvious that it can be measured reliably in the breath, so we asked an expert to explain the relationship between blood and breath alcohol.

Associate Professor Anthony Moynham, Forensic Physician at the Sydney Forensic Medicine & Science Network and a Fellow of the RCPA’s Faculty of Clinical Forensic Medicine, says a key part in understanding this relationship is realising that alcohol is a very small and volatile molecule.

“Volatile in this context means it has the capacity to vaporise, and because the molecule is very small it can go straight through tiny pores in the walls of all of the body’s cells.”

The basic principle of being able to measure alcohol in the breath is that alcohol travels via the circulation within the lungs where it easily evaporates from the circulating blood into the air in the lungs each time we take a breath. We then exhale that air which contains the vaporised alcohol.

A/Prof Moynham says the reason other types of drugs such as cannabis can’t be measured in the breath is because they are not made up of very small molecules, and they are not volatile. This means they can’t partake in this gaseous exchange in the lungs. Alcohol is also rapidly absorbed and readily distributed throughout the body because of its small size.

“It doesn’t need an active transport medium. It basically drives itself around the body, including being able to cross the thin permeable membrane between the circulating blood and the alveoli in lungs,” explains A/Prof Moynham.

If we attached a GoPro to an alcohol molecule it would record an epic journey. Once ingested, it travels through walls of the digestive system (especially the stomach and upper part of the small intestine) into the blood, and then goes through the liver before returning to the heart. It is then pumped through the lungs and heart (again) before making its way around the rest of the body including the brain.

Alcohol is removed from the bloodstream by a combination of metabolism, excretion and evaporation. A/Prof Moynham says most alcohol is metabolised by the liver, about 5% is excreted via the urinary system, a tiny amount is sweated out, and about 1% is evaporated then expired through the lungs. This is the alcohol measured by a breath alcohol test.

“The breath that is deep in the lungs is the most accurate measure of breath alcohol because that is the air that has the closest relationship with the blood. Most breath tests require about one litre of expired air to ensure this deep lung air is captured,” explains A/Prof Moynham.

The effects of alcohol (which is a depressant drug) on the body are well known, and A/Prof Moynham says being able to measure breath alcohol concentration through roadside tests has markedly reduced motor vehicle fatalities.

“Breath testing is also more user friendly than pulling people over and requesting a blood sample,” he explains. We certainly can’t argue with that, but it doesn’t completely replace a blood test.

“The evidential breath analysis carried out by the police is considered very accurate, however if there are any doubts the option of a blood test can be taken up as a blood analysis for alcohol is the final accurate test.”

The effects of drink driving are covered in the December 2015 edition of ePathWay.




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