In our previous study of Henry’s Law we learned when a volatile chemical (ethanol, for instance) is dissolved in a liquid (blood) and brought into contact with a closed air space (the lungs), an equilibrium is formed andBlood Breath Partition Ratio in DWI Cases there exists a fixed ratio between the concentration of ethanol in the air space and the concentration in the liquid. (at a given temperature and pressure) In a closed system the amount of ethanol in the airspace above the liquid is proportional to the amount of ethanol in the liquid. We also learned Henry’s Law can only provide an approximation of the ethanol in human breath since the law applies to a closed system at a given temperature and pressure. The lungs are not a closed system.

The Intoxilyzer 5000 is designed to analyze a sample of a person’s breath based on Henry’s law. Moreover, the Intoxilyzer is designed to produce a breath alcohol result exactly corresponding to a simultaneous blood alcohol sample. The occurs because of the scientific assumption embodied in Section 49.01(1) of the Texas Penal Code. This assumption implies that 210 liters of breath contains the same amount of alcohol as 100 milliliters of blood. The assumption about human physiology required to support this involves the partitioning of alcohol from the blood. But is this assumption a defensible scientific fact?

The problem for the Intoxilyzer is assuming the partitioning of alcohol from the blood into the breath is 2100:1. That is, for every 2100 parts of ethanol in the blood there is one (1) part ethanol in the breath. Again, section 49.01(1) of the Texas Penal Code embodies this assumption. However, not all persons partition alcohol from their blood based upon this ratio.

Studies show partition ratios as low as 834:1. Other studies show partition ratios ranging between 1555:1 and 3005:1. (Dubowski, 1985) The important issue for the criminal defense lawyer is the Intoxilyzer 5000 OVERESTIMATES the alcohol concentration of someone blowing with a partition ratio less than 2100:1. According the the Dubowski study, a person with a partition ratio of 1555:1 would cause the Intoxilyzer 5000 to overestimate by 26% the breath alcohol concentration compared to that of a simultaneous blood sample.

The Intoxilyzer 5000 does not measure a person’s blood:breath partition ratio. It is calibrated with the assumed value of 2100:1. So, we never know what a person’s actual partition ratio is at the time of a breath test. Consequently, the Intoxilyzer becomes an incorrect indicator of a person’s actual blood alcohol concentration when the partitioning of alcohol from their blood deviates from the assumed 2100:1.

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