question: How do breathalyzers determine blood alcohol content from air in your lungs? And are they actually accurate?
comment: First alcohol gets absorbed into your blood. Your blood carries it through your body and into your brain and lungs. When you exhale, traces of alcohol get mixed up with your breathe. This is also the reason why you can smell alcohol from someone who had a few drinks. Breathalyzers estimate these traces of alcohol. New devices are made to increase its accuracy. But its still possible to make false reading with certain factors such as gargling an alcohol (without actually consuming it) will leave traces of alcohol in your mouth. Some also claim that mouth wash with alcohol can possibly make false positive reading from the device.
passages: Variance in how much one breathes out can also give false readings, usually low. This is due to biological variance in breath alcohol concentration as a function of the volume of air in the lungs, an example of a factor which interferes with the liquid-gas equilibrium assumed by the devices. The presence of volatile components is another example of this; mixtures of volatile compounds can be more volatile than their components, which can create artificially high levels of ethanol (or other) vapors relative to the normal biological blood/breath alcohol equilibrium.
  
  Section::::Common sources of error.:Mouth alcohol.
   Breath alcohol is a representation of the equilibrium of alcohol concentration as the blood gases (alcohol) pass from the (arterial) blood into the lungs to be expired in the breath. Arterial blood distributes oxygen throughout the body. Breath alcohol concentrations are generally lower than blood alcohol concentrations, because a true representation of blood alcohol concentration is only possible if the lungs were able to completely deflate. Vitreous (eye) fluid provides the most accurate account of blood alcohol concentration.
  
  Section::::Common sources of error.:Testing during absorptive phase.:Drinking after driving.
   One of the most common causes of falsely high breath analyzer readings is the existence of mouth alcohol. In analyzing a subject's breath sample, the breath analyzer's internal computer is making the assumption that the alcohol in the breath sample came from alveolar air—that is, air exhaled from deep within the lungs. However, alcohol may have come from the mouth, throat or stomach for a number of reasons. To help guard against mouth-alcohol contamination, certified breath-test operators are trained to observe a test subject carefully for at least 15–20 minutes before administering the test.
   Some semiconductor models are designed specifically to allow the sensor module to be replaced without the need to send the unit to a calibration lab.
  
  Section::::Common sources of error.:Non-specific analysis.
  
  One major problem with older breath analyzers is non-specificity: the machines identify not only the ethyl alcohol (or ethanol) found in alcoholic beverages but also other substances similar in molecular structure or reactivity.
   Breath analyzers assume that the subject being tested has a 2100-to-1 partition ratio in converting alcohol measured in the breath to estimates of alcohol in the blood. If the instrument estimates the BAC, then it measures weight of alcohol to volume of breath, so it will effectively measure grams of alcohol per 2100 ml of breath given. This measure is in direct proportion to the amount of grams of alcohol to every 1 ml of blood. Therefore, there is a 2100-to-1 ratio of alcohol in blood to alcohol in breath. However, this assumed partition ratio varies from 1300:1 to 3100:1 or wider among individuals and within a given individual over time. Assuming a true (and US legal) blood-alcohol concentration of .07%, for example, a person with a partition ratio of 1500:1 would have a breath test reading of .10%—over the legal limit.
   BULLET::::- Infrared spectroscopy: Infrared breathalyzers allow a high degree of specificity for ethanol. Typically evidential breath alcohol instruments in police stations will work on the principle of infrared spectroscopy.
  
  BULLET::::- Fuel cell: Fuel cell gas sensors are based on the oxidation of ethanol to acetaldehyde on an electrode. The current produced is proportional to the amount of alcohol present. These sensors are very stable, typically requiring calibration every 6 months, and are the type of sensor usually found in roadside breath testing devices.
   Breath analyzers do not directly measure blood alcohol content or concentration, which requires the analysis of a blood sample. Instead, they estimate BAC indirectly by measuring the amount of alcohol in one's breath. In general, two types of breathalyzer are used. Small hand-held breathalyzers are not reliable enough to provide evidence in court but reliable enough to justify an arrest. Larger breathalyzer devices found in police stations can then be used to produce court evidence.
   The oldest breath analyzer models pass breath through a solution of potassium dichromate, which oxidizes ethanol into acetic acid, changing color in the process. A monochromatic light beam is passed through this sample, and a detector records the change in intensity and, hence, the change in color, which is used to calculate the percent alcohol in the breath. However, since potassium dichromate is a strong oxidizer, numerous alcohol groups can be oxidized by it, producing false positives. This source of false positives is unlikely as very few other substances found in exhaled air are oxidizable.
   The hand held Intoxalock device requires the driver of the vehicle to submit a breath sample using, inhale, blow pattern into the device. After the device calculates the drivers Blood alcohol content, the device displays a message telling the driver to start the vehicle. A series of "rolling retetests" occur periodically as the vehicle is being driven and breath samples are given with same process used to initially start the vehicle.
   Calibration is the process of checking and adjusting the internal settings of a breath analyzer by comparing and adjusting its test results to a known alcohol standard. Law enforcement breath analyzers need to be meticulously maintained and re-calibrated frequently to ensure accuracy.
  
  There are two methods of calibrating a precision fuel cell breath analyzer, the Wet Bath and the Dry Gas method. Each method requires specialized equipment and factory trained technicians. It is not a procedure that can be conducted by untrained users or without the proper equipment.
   Two breathalyzer technologies are most prevalent. Desktop analyzers generally use infrared spectrophotometer technology, electrochemical fuel cell technology, or a combination of the two. Hand-held field testing devices are generally based on electrochemical platinum fuel cell analysis and, depending upon jurisdiction, may be used by officers in the field as a form of "field sobriety test" commonly called PBT (preliminary breath test) or PAS (preliminary alcohol screening) or as evidential devices in POA (point of arrest) testing .
   Extrapolation is calculated using five factors and a general elimination rate of 0.015/hour.
   The problem with mouth alcohol being analyzed by the breath analyzer is that it was not absorbed through the stomach and intestines and passed through the blood to the lungs. In other words, the machine's computer is mistakenly applying the partition ratio (2100:1, see above) and multiplying the result. Consequently, a very tiny amount of alcohol from the mouth, throat or stomach can have a significant impact on the breath-alcohol reading.
   Other substances that might reduce the BrAC reading include a bag of activated charcoal concealed in the mouth (to absorb alcohol vapor), an oxidizing gas (such as NO, Cl, O, etc.) that would fool a fuel cell type detector, or an organic interferent to fool an infrared absorption detector. The infrared absorption detector is more vulnerable to interference than a laboratory instrument measuring a continuous absorption spectrum since it only makes measurements at particular discrete wavelengths. However, due to the fact that any interference can only cause higher absorption, not lower, the estimated blood alcohol content will be overestimated.
   In 1954 Robert Frank Borkenstein (1912–2002) was a captain with the Indiana State Police and later a professor at Indiana University Bloomington. His Breathalyzer used chemical oxidation and photometry to determine alcohol concentrations. Subsequent breath analyzers have converted primarily to infrared spectroscopy, though this method is subject to invalid results depending on ambient air temperature, the temperature of the device, and the body temperature of the subject, depending on specificity of the readings and how they correlate with one's BAC measured via a voluntary blood draw. The invention of the Breathalyzer provided law enforcement with an orally-invasive test providing immediate results to determine an individual's breath alcohol concentration at the time of testing, based on, according to this article, consistently faulty samples.
   Section::::Diagnosis.
  
  Definitive diagnosis relies on a blood test for alcohol, usually performed as part of a toxicology screen. Law enforcement officers in the United States and other countries often use breathalyzer units and field sobriety tests as more convenient and rapid alternatives to blood tests. There are also various models of breathalyzer units that are available for consumer use. Because these may have varying reliability and may produce different results than the tests used for law-enforcement purposes, the results from such devices should be conservatively interpreted.
   Infrared instruments are also known as "evidentiary breath testers" and generally produce court-admissible results. Other instruments, usually hand held in design, are known as "preliminary breath testers" (PBT), and their results, while valuable to an officer attempting to establish probable cause for a drunk driving arrest, are generally not admissible in court. Some states, such as Idaho, permit data or "readings" from hand-held PBTs to be presented as evidence in court. If at all, they are generally only admissible to show the presence of alcohol or as a pass-fail field sobriety test to help determine probable cause for arrest. South Dakota had previously relied solely on blood tests to ensure accuracy, but has implemented evidential blood alcohol breath tests since Sep-2011.
   Section::::Common sources of error.
  
  Police in Victoria, Australia, use breathalyzers that give a recognized 20% tolerance on readings. Noel Ashby, former Victoria Police Assistant Commissioner (Traffic & Transport), claims that this tolerance is to allow for different body types.
  
  Section::::Common sources of error.:Calibration.
   A 1927 paper produced by Emil Bogen, who collected air in a football bladder and then tested this air for traces of alcohol, discovered that the alcohol content of 2 litres of expired air was a little greater than that of 1 cc of urine. However, research into the possibilities of using breath to test for alcohol in a person's body dates as far back as 1874, when Francis E. Anstie made the observation that small amounts of alcohol were excreted in breath.
   Many handheld breath analyzers sold to consumers use a silicon oxide sensor (also called a semiconductor sensor) to determine the blood alcohol concentration. These sensors are far more prone to contamination and interference from substances other than breath alcohol. The sensors require recalibration or replacement every six months. Higher end personal breath analyzers and professional-use breath alcohol testers use platinum fuel cell sensors. These too require recalibration but at less frequent intervals than semiconductor devices, usually once a year.
   Other false positive of high BAC and also blood reading are related to Patients with proteinuria and hematuria, due to kidney metabolization and failure.
  
  The metabolization rate of related patients with kidney damage is abnormal in relation to percent in alcohol in the breath. However, since potassium dichromate is a strong oxidizer, numerous alcohol groups can be oxidized by kidney and blood filtration, producing false positives.
   Section::::Common sources of error.:Retrograde extrapolation.
   Breath test is a widespread method for quickly determining alcohol intoxication. A breath test measures the alcohol concentration in the body by a deep-lung breath. There are different instruments used for measuring the alcohol content of an individual though their breath. Breathalyzer is a widely known instrument which was developed in 1954 and contained chemicals unlike other breath-testing instruments. More modernly used instruments are the infrared light-absorption devices and fuel cell detectors, these two testers are microprocessor controlled meaning the operator only has to press the start button.
   The breath analyzer test is usually administered at a police station, commonly an hour or more after the arrest. Although this gives the BrAC at the time of the test, it does not by itself answer the question of what it was at the time of driving. The prosecution typically provides an estimated alcohol concentration at the time of driving utilizing retrograde extrapolation, presented by expert opinion. This involves projecting back in time to estimate the BrAC level at the time of driving, by applying the physiological properties of absorption and elimination rates in the human body.
   Section::::Chemistry.
  
  When the user exhales into a breath analyzer, any ethanol present in their breath is oxidized to acetic acid at the anode:
  
  CHCHOH(g) + HO(l) → CHCOH(l) + 4H(aq) + 4e
  
  At the cathode, atmospheric oxygen is reduced:
  
  O(g) + 4H(aq) + 4e → 2HO(l)
  
  The overall reaction is the oxidation of ethanol to acetic acid and water.
  
  CHCHOH(l) + O(g) → CHCOOH(aq) + HO(l)
  
  The electric current produced by this reaction is measured by a microcontroller, and displayed as an approximation of overall blood alcohol content (BAC) by the Alcosensor.
  
  Section::::Law enforcement.
  