Blood Alcohol Content BAC Calculator
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How Does A Breathalyser BAC Calculator Work?

How can a person's breath show how much that person has had to drink?

Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream. Alcohol is not digested upon absorption or chemically changed in the bloodstream. As the blood goes through the lungs, a physiologically predictable amount of the alcohol will moves across the lung membranes and into the lungs themselves.

Once in contact with the air in the lungs, it evaporates and is exhaled. The concentration of the alcohol in the air in the lungs is directly related to the concentration of the alcohol in the blood.

The ratio of breath alcohol to blood alcohol is 2100 to 1 (and called the partition ratio), so the alcohol content of 2100 milliliters of exhaled air will be the same as for 1 milliliter of blood. The maths are simple from there and leads to blood alcohol readings expressed as a percentage of alcohol in the blood.

The partition ratio can vary between 1700 and 2400 depending upon the individual and local environmental conditions, leading to a breath analysis reporting either a higher or lower calculated blood alcohol reading.

Since it is a physiological response, the partition ratio can be raised or lowered by:

  • Body Temperature: The widely used blood-to-air partition ratio of 2100 is based on a normal body temperature of 98.6°F. A higher body temperature of the individual will overestimate the actual BAC because of the higher volatility (or vapor pressure) of liquids like alcohol at a higher temperature. An elevation in body temperature of 1°C (1.8°F) results in a 7% higher value in the result since the air in the lungs will contain an artificially higher amount of evaporated alcohol. Therefore, a person with a body temperature of 100.4°F, and with an actual blood alcohol of 0.0935%, will register a value of 0.10% by the breath test. Be careful if you have a cold or the flu.

  • Cellular Composition Of Blood: Blood contains suspended cells (e.g. red and white cells) and proteins, and is therefore only a partial liquid. The partition ratio of 2100 is based on an average cell volume 47%. Some of this cell volume is tissue, and at 47% cell volume, 81.5% of the blood volume is liquid in which the alcohol is actually dissolved. A person with a lower cell volume will have a falsely elevated blood alcohol level based on a breath test since the amount of alcohol in 2100ml of lung breath will be dissolved in a slightly higher amount of liquid, and hence have a lower concentration. Cell volume values range from 42% to 52% in males, and 37% to 47% in females. This variability will only have a small impact on BAC (ranging from -2% to +5%) so don't rely on it in court.

In most countries the partition ratio is presumed to be 2100:1 unless the defendant presents evidence that his/her personal ratio should be reasonably be assumed to be significantly higher or lower.

Factors Affecting Breathalyser Accuracy

Breathalysers only estimate the BAC, not physically measure it, so several factors can cause the breathalyser to give either an artificially higher or lower reading. The most common are:

  • Body temperature and blood composition as outlined above.
  • Some breathalysers can't differentiate between ethyl alcohol and other compounds of a simple chemical nature. Methyl compounds such as acetone and ketones can be present in the breath of diabetics and those taking dietary supplements, causing the breathalyser to over-estimate the BAC. Similarly vinegar can cause confusion with some of the older or dumber breathalysers. Envronmental substances in the vicinity of the breathalyser including compounds found in lacquer, paint remover, celluloid, gasoline, and cleaning fluids can also effect the reading in older breathalysers.
  • Breathalysers assume that the alcohol concentration in the breath will be the same as in the blood, and that the breath is sourced from deep in the lungs, but the breath alcohol concentration can be increased by vomit or blood in the mouth, acid reflux, or simply having had a drink recently with alcohol residue from that drink still being in the mouth at the time of testing. Mouthwash or breath freshener often contain alcohol, so don't use these in an attempt to disguise the smell of alcohol when being pulled over.
  • Electrical interference from cell phones and police radios.
  • By law, police breathalysers are required to be calibrated for a particular ambient operating temperature, and nearly all are. The problem comes in areas that experience cold winters - while the breathalyser may be calibrated for the correct outside temperature, if its taken out of the nice warm patrol car and used immediately it will not have had enough time to cool down to its calibrated operating temperature and will give an artificially high reading.
  • Absorption of alcohol into the blood may lag actually drinking it by as much as 1-2 hours so even though you may have alcohol in your breath, if you've only consumed it recently it may not be in your bloodstream in the concentration that testing your breath indicates.

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This web site does not endorse drink driving or the excessive consumption of alcohol. Blood alcohol calculations are to be used as a guide only.