When alcohol is consumed, it is absorbed into the venous portion of the circulatory system. It is through the venous system that blood returns to the heart and lungs to be purified and recirculated as arterial blood throughout the body via a network of capillaries.
Breath tests measure arterial blood alcohol content, whereas blood samples removed for analysis are usually taken from the venous system.
Higher alcohol content during absorption
Since absorption occurs "up-stream" of the lungs, arterial blood will have a higher alcohol content than venous blood during active absorption, that is, before elimination becomes the dominant phase. Because the capillary blood network supplies blood to the tissues and organs of the body, alcohol will be stored in these locations.
It has been shown that the alcohol content of the arterial and venous compartments can differ considerably during the absorption phase. As a result, breath sample analysis may underestimate or overestimate the true (venous) blood alcohol level depending on whether the blood alcohol curve is rising or falling.
Factors affecting absorption
As a result, in order to provide an accurate interpretation of the breath results obtained, some information should be available suggesting the status of the subject both at the time of interest and the time at which the breath tests were performed. Factors that affect the time required to reach the fully post-absorptive state may include the following:
Henry's Law, which states that the quantity of gas that dissolves in a liquid at standard temperature and pressure is directly proportional to the partial pressure of that gas in the gas phase, makes it possible to quantify the relationship between the breath alcohol content and the blood alcohol content. This relationship is defined as the "apparent blood:breath ratio."
The blood:breath ratio
This ratio describes the relationship between the alcohol content of breath and the alcohol content of blood at a given point in time. It defines the quantity of breath that would contain the same amount of alcohol as a given quantity of blood.
Numerous average values and ranges have been reported in the literature. For many years, the value of 2100:1 was accepted as the population average. This ratio indicates that 2100 parts of breath contain the same quantity of alcohol as 1 part of blood. Alternatively, 1 millilitre of blood contains 2100 times more alcohol than 1 millilitre of breath. This means that 80 milligrams of alcohol in 100 millilitres of blood are equivalent to 80 milligrams of alcohol in 210 litres of breath.
More recent studies, however, indicate an average venous blood:breath ratio of approximately 2300:1 for subjects in the post-absorptive state (range approximately equal to 1700-3000:1). In addition, it is clear that the blood to breath ratio is not constant within and between drinking sessions.
Breath testing equipment in Canada
Breath testing equipment, as used by police and other authorities in Canada, is calibrated using a ratio of 2100:1. Individuals with ratios greater than 2100:1 would have their levels underestimated, whereas those with ratios less than 2100:1 would have their levels overestimated.
This consideration when combined with the knowledge that the average blood:breath ratio is approximately 2300:1 implies that the results obtained in any given breath test may be underestimated by about nine per cent when testing an average person in the post-absorptive state. However, this does not preclude the overestimation of a true blood alcohol level in certain cases.
The difficulties associated with the conversion of breath to blood values would be eliminated if legislation were changed to reflect the alcohol content of breath only.