Written by attorney Patrick Mahaney

A Simplified Theory of the Blood to Breath Ratio

The underlying theory behind all breath test devices is that there is a uniform and standard ratio of breath alcohol to blood alcohol in the ratio of 2100 to 1. In theory, 2100 cubic centimeters of exhaled breath will contain the same per cent by weight of alcohol as one cubic centimeter of whole blood. This relationship is commonly expressed as the 2100:1 ratio and is also variously referred to by the terms "partition ratio," "conversion ratio," and "breath to blood ratio." All breath testing devices currently employed in the United States use the 2100:1 ratio to determine blood alcohol. The underlying basis for this ratio is predicated on a phenomenon known as Henry's Law which states that the concentration of a volatile substance dissolved in a liquid is directly proportional to the vapor pressure of the volatile substance above the liquid. Henry's Law is accepted as basic science, much as the laws of gravity and magnetism are accepted as facts of science. The contested issue in breath testing is not the fact of Henry's Law, but deciding the proper breath to blood ratio for use as an evidentiary test. Breath testing is based on the measurement of alcohol contained in deep-lung or "alveolar" air. As blood flows into the lungs to exchange carbon dioxide for oxygen, a part of the alcohol flowing in the blood stream is also exchanged and exhaled. Henry's Law provides the basis for measuring the exact amount in grams per cent by weight of alcohol in the blood stream by measuring the amount of alcohol in exhaled breath. The 2100:1 ratio is more akin to a statistical mean based on the population as a whole. The 2100:1 ratio is based on a 1972 study by the National Safety Council that determined 2100 cubic centimeters of lung air at 34 degrees centigrade will closely equal the amount of alcohol present in one cubic centimeter of blood. However, there are variances between each person, although the statistical variance is usually slight. Although the 2100:1 ratio has been systematically challenged nationwide by members of the criminal defense bar, no U.S. state court has yet struck down the statutorily mandated presumptive ratio. For the state of Alabama, the 2100 to 1 ratio is the statutory standard: "Percent by weight of alcohol in the blood shall be based upon grams of alcohol per 100 cubic centimeters of blood or grams of alcohol per 210 liters of breath." Code of Alabama, 1975, section 32-5A-194 (a)(5). This means the legislature has mandated all Alabama trial courts to accept as a statutory requirement the statistical ratio of 2100 to 1 as a fact, and not subject to objection.

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