As I mentioned in an earlier post, "Why Breathalyzers Don’t Measure Alcohol", breath machines are actually designed to report the presence of any compound containing the methyl group in its molecular structure, not just alcohol. They cannot distinguish the difference between alcohol and, say, acetaldehyde.
Acetaldehyde? That’s a compound produced in the liver in small amounts as a by-product in the metabolism of alcohol. Unfortunately, alcohol moving from the blood into the lungs has been found to metabolize there as well — and, thus, to produce acetaldehyde there. The amount of acetaldehyde produced in the lungs varies from person to person.
However, scientists have found one interesting fact: acetaldehyde concentrations in the lungs of smokers are greater than for non-smokers — far greater.
Translated: smokers are more likely to have falsely high readings on a Breathalyzer. "Origin of Breath Acetaldehyde During Ethanol Oxidation: Effect of Long-Term Cigarette Smoking", 100 Journal of Laboratory Clinical Medicine 908.
End result: because breathalyzers can’t tell the difference between alcohol and acetaldehyde, a higher blood-alcohol reading. And if you are a smoker, a much higher reading. (Note: Some manufacturers of breath machines, such as the Intoxilyzer 5000, began offering an option some time ago: an "acetaldehyde detector". Few law enforcement agencies have chosen to purchase the option.)