On its way around the body the blood picks up alcohol from any drink the person may have consumed as it passes through the walls of the digestive organs, notably the stomach and the upper part of the small intestine. This blood then passes through the liver on its way back to the heart, where it is then pumped through the lungs before flowing back to the heart to be distributed around the rest of the body [the arterial supply].
In the lungs the exchange of oxygen from the air into the blood, and of the waste product carbon dioxide in the reverse direction, proceeds continuously during the process of breathing. Now, just as carbon dioxide evaporates from the blood into the breath, then so does a small, representative portion of any alcohol that is also present. This process is known as gaseous exchange.
The actual quantity of alcohol that evaporates into the breath depends on its concentration in the blood. This is known as Henry’s Law.
For a better understanding of Henry’s Law, consider what happens when you pour some whisky into a glass and then smell the air above it: there is a strong smell of alcohol. If you now add some water to the whisky then the concentration of alcohol in it decreases, and so therefore does the smell of alcohol in the air above it.
So measuring the strength of alcohol in the air above a drink enables us to determine what concentration of alcohol is actually present in the drink. It is exactly this process that is occurring deep in the lungs, where the incoming air and the blood are in intimate contact.
So the level of alcohol in the breath depends on its concentration in the blood.
The relationship between the blood and breath alcohol concentrations in equilibrium is well-defined, and the value of the actual concentration ratio is close to 2:300:1. This in effect means that breath is about 2,300 times weaker in alcohol concentration than the blood it was in close contact with.
The alcohol evaporation process in the lungs is practically instantaneous, just as in respect of a drink. We do not have to wait for the smell of alcohol above a drink to develop: nor does that smell get any stronger as time passes.
But we would never smell a drink by holding it at arm’s length. Although we would be able to detect the presence of alcohol in it that way, we could never quantify its concentration; as we would be sampling air that had not been in intimate contact with that drink. Likewise with breath, in order to obtain a reliably accurate alcohol measurement, we must sample and analyse breath that has been in close contact with the blood. This intimate contact occurs most efficiently in the small tubes deep in the lungs, the alveoli, which are separated from the circulating blood only by a thin, permeable membrane. In effect, therefore, the incoming air in the alveoli comes into intimate contact with the circulating blood, and is then exhaled.
One important thing though, this all means that when conducting a breath analysis we must sample deep lung air. The level of alcohol in the specimen we analyse will then properly reflect what is present in the circulating blood, and so causing impairment in the brain.
Apart from one instrument, all Lion breathalyser products are designed to measure the alcohol level only in deep lung air, and to alert the operator if the subject attempts to provide any other type of specimen for analysis.
The one exception here is the AlcoBlow®. This Lion instrument is specifically designed to sample air from the top of the subject’s breath stream – rather like smelling a drink at arm’s length – and so simply to give a fast but reliable and objective assessment of whether that subject has any alcohol in their breath – and hence their blood – at all. But because the AlcoBlow® does not analyse deep lung air, it is not possible to obtain a quantified measurement with this instrument.