So a couple of local celebrities' brush with the law for drink driving have stirred up media interest on the supposed prevalence of drink driving in Singapore. I think many drivers insist on driving even after they have downed a couple of beers, is that they feel they are still sober and are able to handle a car. However, breathalyzer tests measure Blood Alcohol Content, it does not measure a person's level of intoxication. This varies from person to person depending on one's alcohol tolerance. So even if you feel perfectly fine after 3 glasses of red wine after dinner, you can still fail a breathalyzer test. (makes you wonder how anyone can drive in France or Germany because all they drink is wine and beer!)
Like all tests that rely on a proxies (ie. your breath), externalities affecting the proxy can have significant effects on the measurement of the target metric (your BAC). One of the most common sources of error in breathalyzer measurement is testing too early. When the body is absorbing alcohol, the distribution of alcohol in the body is not uniform. It can take 45 minutes to more than 2 hours after drinking for the distribution to be uniform. During this time, some parts of the body will have higher blood alcohol content then others. During peak absorption, arterial BAC, which is what breathalyzers measure, can be as much as 60% higher than venous BAC. So taking a breathalyzer test before alcohol absorption is complete will tend to overstate your BAC. Whereas blood tests, usually done by drawing venous blood, will measure a lower BAC during peak absorption.
Another common source of error is mouth alcohol. Because breathalyzers assume that the breath sample is alveolar air (from the lungs), it will not compensate for breath mixed with alcohol from the mouth, throat or stomach. A tiny amount of alcohol from these areas can have a significant impact on breathalyzer readings. It may take 15-30 minutes for alcohol in the mouth to dissipate through the normal rinsing effects of saliva. Even then, it might not fully wash away alcohol trapped in dentures or gums (caused by periodental diseases). This is probably where a cup of Chinese tea might help wash mouth alcohol away. (Unfortunately, it does not lower one's BAC)
Even belching or burping will get you in trouble. Eructation causes liquids and gas from the stomach (including alcohol) to rise up into the soft tissue of the esophagus and oral cavity, where it will stay until it is dissipated. So if you happen to burp just before blowing into the breathalyzer, better rinse your mouth thoroughly and drink some Chinese tea!
Finally, you may also be incorrectly penalised by breathalyzers if you suffer from gastroesophageal reflux disease, commonly known as Acid Reflux. This is when the stomach valve becomes herniated, allowing alcohol to rise and permeat the esophagus and mouth, where it is then exhaled into the breathalyzer.
So, if you drink, don't drive. If you are sober and really have to drive, protect yourselves from our ever zealous authorities, take the necessary precautions to ensure breathalyzers do not overstate your BAC.