Henry’s Law and DWI Testing


Testing blood alcohol concentration (BAC) based on breath samples has been a staple of roadside law enforcement since the late 1930s, when Prof. Rolla H. Harger of Indiana University developed a device called the “drunkometer.” Drivers who fail the tests face prosecutions for violating laws that prohibit driving with a certain BAC, which are referred to as “per se” statutes.

Per se DWI statutes do not require proof that the motorist's ability to drive was affected by alcohol: simply that at the time they operated a motor vehicle they had a BAC above a set limit.

But this indirect measurement of sampling breath to determine alcohol concentration in the blood unnecessarily subjects the prosecution's case to attack by defense counsel. In the case of People v. Stephen Lopresti, a DWI prosecution brought against a former New York City Assistant District Attorney who was arrested and given a breath test which indicated a BAC level of.17, I was able to win an acquittal by attacking the science behind the Intoxilyzer 5000, which is used by the police in New York City in DWI arrests.

Although there was much attention paid by the media to the “ticket fixing scandal” that surrounded that case, the acquittal would not have been possible without the ability to challenge the science behind the breath testing device.

These types of attacks are mounted by defense counsel on a regular basis any time a breath testing instrument is the subject of a DWI trial.

Of all the tests used to measure BAC, the simplest and most cost-effective is a breath test. The scientific principle underlying the acceptance of this indirect measurement is “Henry’s Law,” named for the chemist who defined it in 1803, William Henry.

Simply put, Henry's Law predicts that if a volatile substance such as alcohol is dissolved in some solvent, (such as blood) the concentration of the alcohol in the vapor above the liquid (i.e., your breath) will be proportional to its concentration in the blood. Henry's Law requires a closed system where there is a constant pressure and a constant temperature.

Expelled deep lung air is supposed to accurately reflect what a blood test might reveal and has historically been considered sufficiently reliable to be admitted in court. However, the government should be less concerned with whether the results of breath testing may be admitted and more concerned with whether there are more reliable methods of analysis which will strengthen their case.

While breath tests can reliably detect the presence of alcohol in the human body, it is less clear that breath testing can determine an accurate quantitative analysis of alcohol in the blood.

The requirements of Henry's Law; a closed system with constant pressure and constant temperature, simply do not apply in the lungs. Variables in blood-to-breath ratios have historically provided a fertile area of attack for defense counsel. Current scientific research, published in respected peer-reviewed journals, now shows that results from breath alcohol analysis or breath testing are not sufficiently reliable for purposes of quantitative analysis.

In an effort to counter these attacks, states such as Washington have amended their per se statute to penalize a certain breath alcohol level. This has taken us far from the original goals of DWI laws, which were to prevent drivers from getting behind the wheel while impaired.

That is because breath-alcohol concentration may not always reflect the concentration of alcohol in blood and in turn may not correlate to impairment.

To understand why, it is important to understand the physiological effects of alcohol. After consumption and digestion, alcohol is absorbed into the blood and carried throughout the body. When alcohol reaches the brain, effects associated with intoxication become observable: the more alcohol in the blood, the greater the effects.

If the government wishes to prohibit DWI through per se statutes that prescribe a limit for blood alcohol concentration, then laws should be enacted that give law enforcement tools for testing blood, not breath.

Several states have already started to move in this direction. In Michigan and Oregon, the driver is taken to a facility where a qualified medical practitioner (physician, nurse, emergency medical technician, or phlebotomist) draws a blood sample, or a qualified person is called to the police station to draw the sample.

In Arizona and Utah, a number of law enforcement officers have been trained and certified as phlebotomists and are authorized to draw blood samples. In Wyoming, the police are permitted to get a warrant for blood in certain cases where a motorist refuses a breath test.

The war on drunk driving has clearly saved lives. However, as in any war, the government should actively make an effort to reduce its casualties and use the strongest weapons available to achieve its primary goals.

The testing of blood may be more invasive then the indirect measurement through breath, but those accused of DWI have the right to an accurate measure of BAC. If the government wants the quick and convenient results provided by breath testing, then those devices should be used for screening or as a second test, making their evidence stronger and more forensically sound.

Steven B. Epstein is a founding partner at Barket, Marion, Epstein & Kearon, LLP. Mr. Epstein has extensive experience in the field of DWI defense. He is a member of the National College for DUI Defense, and a leading instructor on the issue of DWI offenses for New York City's public defenders and has been invited by a number of bar associations as a guest lecturer. In addition, he is an adjunct professor at Pace University School of Law and has been published on the topic of “Strategies for Defending DWI Cases in New York.” He welcomes reader comments.

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