If you are arrested for DUI and a breath test shows a blood alcohol concentration (BAC) of .08% or higher, you are guilty. It does not matter, of course, whether you are a man or a women: the laws do not discriminate. Maybe they should…
Researchers at the University School of Medicine in Trieste, Italy, found that the stomach lining contains an enzyme called gastric alcohol dehydrogenase that breaks down alcohol, and that women have less than men. To determine the relative effects of the enzyme, they gave alcohol both orally and intravenously to groups of alcoholic and non-alcoholic men and women. They found that women reached the same levels of blood alcohol as men after drinking only half as much; with weight differences taken into account, they found that women reached BAC levels illegal in a DUI case after drinking 20 to 30 percent less alcohol than men.
The scientists’ conclusion: legislatures may need to consider sex differences in drunk driving laws when defining safe levels of drinking for driving motor vehicles. Frezza and Lieber, "High Blood Alcohol Levels in Women: The Role of Decreased Gastric Alcohol Dehydrogenase Activity and First-Pass Metabolism", 322(2) New England Journal of Medicine 95 (1990).
Yet another study has found that women have lower partition ratios of blood to breath. What kind of ratios? Well, all breath machines in DUI cases measure the amount of alcohol in a person’s breath. But the what we really want to know is the amount of alcohol in the person’s blood. So how do we get that? Simple: a small computer in the Breathalyzer multiplies the amount of alcohol it detects in the breath sample by 2100 times. This is based upon the theory that, on average, there are 2100 units of alcohol in the blood for every unit of alcohol in the breath. (Note: that’s an average — but it varies from person to person.)
According to the study, women have a significantly lower partition ratio. Jones, "Determination of Liquid/Air Partition Coefficients for Dilute Solutions of Ethanol in Water, Whole Blood and Plasma", Analytical Toxicology 193 (July/August 1983). And the lower the ratio, the higher the reading — even though the true BAC does not vary. Example: a woman with a true BAC of .06% and a ratio of 1500:1 (rather than the presumed 2100:1) will get a reading on the machine of .09% — above the legal limit.
Put another way, the breath machine will show an average man accused of drunk driving to be innocent — but a woman with the same blood alcohol level to be guilty.
And then there’s the problem of birth control….
Scientists in Canada have found that "women taking oral contraceptive steroids (O.C.S.) appeared to eliminate ethanol significantly faster than women not taking O.C.S." Papple, "The Effects of Oral Contraceptive Steroids on the Rate of Post-Absorptive Phase Decline of Blood Alcohol Concentration in the Adult Woman, 15(1) Canadian Society of Forensic Science Journal 17 (1982). That means that women will reach peak BAC faster, and return to lower levels more quickly. This, of course, can create serious problems in a DUI case when attempting to estimate BAC at the time of driving based upon a breath test administered one hour later.
Making the problem worse, researchers have also discovered that women who were taking birth control pills or who were pregnant had higher levels of acetaldehyde on their breath, due to the decreased ability to metabolize the enzyme as the level of sex steroids increases. So what? Well, most breath machines use infrared analysis in measuring the breath sample of a DUI suspect. But these machines don’t really measure alcohol, rather they measure any compound which contains the "methyl group" in its molecular structure. And acetaldehyde is one of these compounds. Result: a higher "blood alcohol" reading on the breathalyzer. Jeavons and Zeiner, "Effects of Elevated Female Sex Steroids on Ethanol and Acetaldehyde Metabolism in Humans", 8(4) Alcoholism: Clinical and Experimental Research 352 (1984).
It’s always a problem when the law, in its infinite wisdom, assumes that all of us are exactly the same. (See "Convicting the ‘Average’ DUI Suspect".)