The Problem with The 2100:1 Blood to Breath Ratio

By Stanley J. Broskey, PhD

Forensic Scientist, Bucks County, Pa.


DWI prosecutions depend upon the assumption that everyone’s blood-breath ratio is the same. This assumption is enacted into law, along with a specific blood-breath ratio.

In some countries, such as the United States, that ratio is 2100:1; in some European countries, such as the Netherlands and Belgium, it is 2300:1, while other European countries such as France and Austria have decreed a blood-breath ratio of 2000:1.

But the question is: If experts from each country were to testify under oath that their country’s ratio was, in fact, the correct one, who would be correct?

These selected blood-to-breath ratios are only the so called adopted ‘Average’ Blood-to-Breath conversion factor, based on the shape of a Normal or Gaussian Statistical Curve. The top of this Normal curve would be the “average” [or] most common number to be used.   But people vary quite a bit, and one’s blood-to-breath really depends greatly on the client’s alveolar Ventilation/ Perfusion Ratio when the client blew into a police evidential breath tester.

“The assumption is that all of the suspect’s alveoli are ventilated equally, and that the blood flow through his capillaries is the same for each alveolus.   However, even normally to some extent, and especially in many lung diseases, some areas of the lungs are well ventilated, but have no blood flow; whereas, other areas of the lung have excellent blood flow, but little or no ventilation.

“In either of these two conditions, gas exchange through the respiratory membrane is seriously impaired…

this imbalance between alveolar ventilation and alveolar blood flow is called one’s ‘ventilation/ perfusion ratio’ expressed as one’s VA/Q quotient. …   Abnormal VA/Q in the upper & lower normal lung: In a normal person, in the upright position, both pulmonary capillary blood flow and alveolar ventilation are considerably less in the upper part of the lung than in the lower part of the lung:   However, blood flow is decreased considerably more than ventilation is.

At the other extreme, in the bottom of the lung, there is less ventilation in relation to blood flow … a smaller fraction of the blood fails in gas exchange, not to be normally, oxygenated [or conducting gas exchange]….

“A very abnormal VA/Q exists in Chronic Obstructive Lung Disease: those who smoked for many years [or exposed to occupational fumes]…many of their alveoli become unventilated for gaseous exchange, which becomes as little as 1/10th normal” See Guyton’s & Hall, Medical Physiology Textbook, 11th Ed, pp. 499 to 501.    

The 2100:1 number was chosen in the USA, in the early 1950’s, by an Ad Hoc Committee of five persons — Harger, Borkenstein, Greenberg, Jetter, and Forrester — with very limited data at the time. Each of these five persons had a vested interest in their own breath test device being marketed to law enforcement.

They all chose the “Simulator” as their reference point, with a dilute solution of ethanol in distilled water in a 500 ml jar, even though they knew that ethanol in human whole blood in the Simulator would be a more accurate choice, as was indicated in J. Lab & Clin. Med., vol. 26, pp 1527 to 41, [1941] by Haggard & Greenberg et al. Distilled water and human whole blood do not have the same chemical and physical properties.   These two media are not the same.

The “Simulator” only has about 62 sq. cm. of surface area on the top of the jar, whereas according to Handbook of Human Toxicology by Dr. Edward Massaro the human lung has 1,430,000 +/- 120,000 sq. cm of surface area. This means that many occupational or “do it yourself” volatiles on human breath will not show up in a “Simulator” containing a distilled water medium that has only 62 sq cm of surface area.

The human lung’s surface area of 1.4 million sq. cm would be equivalent to an airfield runway approximately 256 feet. long and 6 feet. wide.

One could argue it is highly unethical and scientifically incompetent to assume that everybody’s blood-to-breath ratio is exactly the same. That may make for easy DWI convictions, but we know that humans can differ quite widely.

It is common knowledge that DNA varies dramatically from one person to another. Since one’s DNA controls organ function, it is likely that lung function will vary from subject to subject.

The practical implication of this is that  if my blood-to-breath ratio was 1400:1, but the state had mandated a 2100:1 conversion ratio, a breath test would produce a 50% higher percent BrAC readings than what I should have achieved. It is fair to ask if that is a “reliable” test?

Whether it is blood typing, signatures, or fingerprints, governmental forensic scientists always seek to prove one’s uniqueness in courtrooms.   Only in DWI prosecutions do the prosecution’s forensic scientists argue that everyone is the same.

The problem is that for indirect testing, such as breath tests, one should account for human variability, and this has not been done.

There are other problems with indirect testing.

For example, government experts do not consider the differences in the two sexes, where different set of hormones are always at play, nor the subject’s body temperature when the sample is taken.

The problem is that the higher one’s temperature happens to be, the more Ethyl Alcohol and/or some other volatile will be able to evaporate from one’s alveoli.

If that is not enough, Table 1-1 in Sandritter’s ‘Color Atlas & Textbook of Macro-Pathology’, 4th Ed., page 4, lists the following gram lung weights for males: 15 years of age & 120 lb body wt.[ 550 gm]; 20 yrs are different than male lung weights. Obviously, if lung weights vary, that means not everyone has the same number of alveoli.

The United Kingdom has acknowledged these concerns and set its limit at 35 micrograms of alcohol per 100 ml of breath.   No breath-to-blood conversion factor is utilized.

But in the UK, DWI prosecution does not take place until 40 micrograms or above is registered on a UK breath test device.

If the breath test registered between 40 to 50 micrograms/100 ml of breath, the UK DWI suspect has a right to have the breath test checked by direct analysis of his blood.   See Forensic Medicine, 9th Edition., page 324, by Dr. Keith Simpson, Home Office Pathologist, London, Edw. Arnold Publrs, Baltimore, MD, USA.

The United Kingdom has pointed the way toward development of a more accurate and fairer breath-test standard. It is regrettable that U.S. legislators have failed to follow suit. One can only hope that the material in this article will provide defense attorneys enough information to challenge the applicability of the 2100:1 ratio to all DWI defendants.

About the Author

Prof. Stanley Broskey has been an analytical scientist for 53 years, as of the year 2001. A former Forensic Chemist/ Toxicologist for the City of Philadelphia, Pa,, and the New Jersey State Police. His office is at 26 East Robin Road, Holland, Bucks Co., PA 18966. Phone: 215-355-8051; email:; website:

This article was originally published in 25 DWI Journal: Law & Science 1-3, September 2011.

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