Accuracy of Breath Testers: A Threat to Law-Abiding Drivers

About half of all alcohol related traffic crash deaths involve drivers with BACs of at least 0.16 or higher. (BAC is alcohol breath concentration.) That is twice the legal limit. Many of these high-BAC drivers are hard core drunk drivers. That is, they repeatedly abuse alcohol and drive while drunk. So hard core drunk drivers are a major threat to safety. So breath analyzers are a major tool in convicting such dangerous offenders. Therefore the accuracy of breathalyzers is important. Without it, the guilty can go free. On the other hand, the innocent can end up with a conviction.


I.   Accuracy of Breathalyzers.

II.  Solutions to Breathalyzer Inaccuracy.

III. Resources.

Breath testers have various brand names. Examples are Breathalyzer, Intoxilyzer, Alcosensor, and Alcoscan. However, these don’t actually measure BAC. That requires the testing a blood sample. Instead, they only estimate BAC. And they do so indirectly.

I. Accuracy of Breath Testers

Different types of machine use different techniques. Larger machines tend to yield better estimates than hand-held models. That’s why some states don’t permit estimates from hand-held machines in court. South Dakota does not even permit evidence from any type or size breath tester. To the contrary, it uses blood tests. That is to ensure accuracy and protect the innocent.

A major problem with some machines is simple. They identify the ethanol found in alcohol beverages. No problem there. But they also identify other substances similar in molecular structure. That is, any compound containing the methyl group structure. Over 100 compounds are in the human breath at any one time. And 70 to 80 percent of them contain methyl group structure. So machines can falsely report them as ethyl alcohol. The more different ethyl group substances the machine detects, the higher will be the false BAC estimate.


Dieters and diabetics can have acetone levels hundreds and even thousand of times higher than that in others. Some breath machines can falsely report acetone and many other substances as alcohol.

One investigator found that alcohol-free subjects can generate BAC readings of about .05 after eating bread. In fact, many foods contain alcohol. In addition, many meds contain alcohol.

accuracy of breathalyzersAlso substances in the environment can lead to false BAC readings. For example, an alcohol-free subject applied a pint of contact cement to a piece of plywood. Then he applied a gallon of oil-base paint to a wall. The total activity lasted about an hour. Twenty minutes later the subject blew into a breath tester. It registered a BAC of 0.12. That is 50% above the legal limit.

Similarly, a painter with a protective mask spray painted a room for 20 minutes. Although a blood test showed no alcohol, a breath tester falsely reported his BAC as .075.

Any number of other products found in the environment can cause erroneous BAC results. These include compounds found in lacquers, paint removers, celluloid, gasoline, and cleaning fluids.

Other common things that can reduce the accuracy of breathalyzers. They include electrical interference from cell phones and police radios, tobacco smoke, dirt, and moisture.


Breath testers can be very sensitive to temperature. As a result, they can give false readings if not adjusted for air temperatures. The temperature of the subject is also very important. Each one degree of body temperature above normal will cause a substantial elevation (about 8%) in apparent BAC.

Many testers assume a 2,100-to-1 ratio in converting alcohol in the breath to estimates of alcohol in the blood. But this ratio varies from 1,900 to 2,400 among people. It also within a person over time. This variation will lead to false BAC readings.

Physical activity and hyperventilation can lower apparent BAC levels. One study found that the BAC readings of subjects decreased 11 to 14% after running up one flight of stairs. They decreased and 22-25% after doing so twice. Another study found a 15% decrease in BAC readings after vigorous exercise.


Some breath analysis machines assume a hematocrit (cell volume of blood) of 47%. However, hematocrit values range from 42 to 52% in men and from 37 to 47% in women. A person with a lower hematocrit will have a falsely high BAC reading.

Failure of officers to use the devices properly is a source of error. So is failure to have the machines properly maintained and re-calibrated as required.

Research shows that breath tests vary at least 15% from actual blood alcohol concentration. At least 23% of all individuals tested will have a BAC reading higher than their actual BAC. That’s about one of every four.

One expert has written this.

Breath testing, as currently used, is a very inaccurate method for measuring BAC. Even if the breath testing instrument is working perfectly, physiological variables prevent early reasonable accuracy…. Breath testing for alcohol using a single test instrument, should not be used for scientific, medical or legal purposes where accuracy is important.1

II. Solutions to Breathalyzer Inaccuracy

There are good ways to virtually eliminate an unfairly conviction for DUI or DWI. One is to choose not to drink. Another is to pace the rate of drinking and follow other tips for maintaining a low BAC. A third is to have a designated driver. If you use a designated driver, you never have to worry about the accuracy of breathalyzers.

1. Don’t Drink

If you choose not to drink, you’ll find that it’s not a big deal to decline a drink. Here are some possibilities.

    • No thanks.
    • No thanks, but I’d love to have a Pepsi.
    • Sorry, but I’m serving as designated driver.
    • I can’t. I’m doing brain surgery in the morning.
    • No thanks; I feel good enough already.
    • Sorry, my analyst won’t let me.
    • I can’t because I get high on grape juice.
    • Not now. I’m testing my willpower.
    • No thanks. I want a clear head to appreciate you fully.
    • Not right now — the party’s wild enough as it is.
    • Sorry, I never drink on Friday (or whatever day it is).
    • Sorry, I promised my parents (spouse, date, etc.) that I wouldn’t.
    • No thanks. I prefer to watch.
    • No thanks. Suzie (or whoever) can’t stand me when I drink.
    • Sorry — I’m taking meds.

You can also “lose” alcohol drinks. Or order water, orange juice, or other beverages that look like an alcohol beverages. In fact, most people don’t really pay much attention to what are drinking. You might find that you actually enjoy yourself without becoming intoxicated. And there’s no hangover!

2. Keep Your BAC Low

If you choose to drink you can keep your BAC low.  Here are some tips.

accuracy of breathalyzers
Standard Drinks
    • Standard drinks have the same amount of pure alcohol. It’s six-tenths of an ounce.
    • A general rule of thumb is to have no more than one drink per hour.
    • Eat or munch while drinking. This helps keep BAC low.
    • Sip your drinks.
    • Avoid drinking games or contests.
    • Have non-alcohol drinks betwen alcoholic ones.
    • Stick to standard drinks. Be careful when drinking punches or drinks in strange size containers. That’s because its’s hard to know how much alcohol they contain. Thus, it’s hard to pace them.

3. Use or Serve as a Designated Driver

Consider either using or serving a designated driver. A designated driver agrees to drink no alcohol and drive others home. The others are free to drink or not as they choose. Many bars provide free non-alcohol beverages to designated drivers.

Designated drivers have probably saved 100,000 lives and spared many more thousands of people from suffering injury from druink driving.2 Over nine out of 10 Americans who attend social events would like to see more designated drivers.3 And the proportion of people either using or serving as a designated driver has increased dramatically over time. Over 93,000,000 Americans either served as or benefitted from a designated driver.4

A designated driver helps others

    • Avoid embarrassment.
    • Keep their driver’s licenses.
    • Avoid fines.
    • Stay out of jail.
    • Prevent needless injury and death.5

The designated driver non-drinker has a respected role at social functions. There is no pressure to drink. Serving as designated driver can also help legitimate a personal choice not to drink.6

Tips for designated driving.
    • Identify the designated driver ahead of time.
    • Consider taking turns serving as designated driver. Look after your friends and family and they can look after you.
    • Larger groups should have more than one designated driver.7

Don’t Worry about the Accuracy of Breath Testers.

Breath testers may be inaccurate and lead to unjust problems. But you needn’t worry about the accuracy of breath tester. Simply choose not to drink. Or to drink but keep a low BAC. Or have a designated driver.

III. Resources: Accuracy of Breathalyzers

Popular Books
Technical readings on Accuracy of Breath Testers

1. Hlastula, M. Physiological Errors Associated With Alcohol Breath Testing.

2. Designated Driver Campaigns Working. Houston Chron, Nov 30, 2017, p. A14.

3. A Guide to Community-Based Designated Driver Programs.

4. Designated Driver Campaign Working.

5. Project CHEERS to the Designated Driver.

6. A Guide, ibid.7. Alberta Tran & Util. Designated Driver – A Smart Choice.