How Accurate are Breathalyzers?
With more than a little federal coercion, all states have now passed laws making it a criminal offense to drive with .08% alcohol in your blood. And most people suspected of violating that law are given breath tests to determine their blood-alcohol concentration (BAC). The breathalyzer will take a small sample of the suspectâ€™s breath and estimate how much alcohol is in it â€” and then estimate how much may be in the blood. And what that machine says is pretty much the end of it. There will be no second tests. There will be no cross-examination of the machine.
Just how accurate and reliable are these machines that we have permitted them to become judge and jury?
To begin with, scientists universally recognize an inherent error in breath analysis, generally of plus or minus .01% in the reading. That means that if everything is working perfectly (an unlikely scenario), a .10% breathalyzer test result can be anywhere from .09% to .11%. This has been acknowledged by courts across the country (see, for example, People v. Campos, 138 Cal.Rptr. 366 (California) and Haynes v. Department of Public Safety, 865 P.2d 753 (Alaska); in State v. Boehmer, 613 P.2d 916 (Hawaii), the courts recognized an even larger .0165% inherent error).What does that tell us about the accuracy of these breathalyzers? Well, letâ€™s again take a test result of .10%. Taking inherent error into consideration â€” and assuming the machine was working perfectly, the officer administers the test correctly, and the suspectâ€™s physiology is normal and perfectly average â€” the true BAC could be anywhere from .09% to .11%. In other words, the true BAC can be 10% in either direction â€” or, put another way, anywhere within a 20% margin of error.
These machines have a 20% margin of error?
Thatâ€™s right. A person accused of driving with over .08% BAC can be convicted by a machine which, if everything else is perfect, has a built-in 20% margin of error. Would you be comfortable with an airline pilot who worked with a 20% range of error? A brain surgeon? A bank teller? How about the evidence in a criminal case where guilt must be proven beyond a reasonable doubt?
But it gets worseâ€¦.
Most states have standards for breath testing. Although some states only provide for a single breath test, most require that two breath tests be given â€” and the results must be within a given range. In North Carolina, for example, there must be two test results within any group of three which fall within .02% of one another; if they are .10, .07 and .13, for example, the officer must start over. In California, the officer can continue giving tests for as long as it takes until he gets two consecutive results within .02%; results of .18, .10 and .12, for example, would be a valid result.
Think about that for a moment. Letâ€™s take that California example. And let’s say the officer gets a .10% reading on a test. What must the result of the next test be to be acceptable? Well, it can be .08, .09, .10, .11 or .12 â€” that is, anywhere between .08% and .12%.
In other words, the acceptable range of error is 40%! And based entirely upon this, a defendant can be convicted of driving with over .08% blood-alcohol â€” and, further, legally presumed guilty of the separate offense of driving under the influence.
In a country where the legal standard is â€œproof beyond a reasonable doubtâ€, the legal standard in drunk driving cases is â€œproof with a 40% margin of errorâ€.
Close enough for government work.