Theo Nash
When it comes to enforcing laws related to alcohol consumption, such as drinking and driving, law enforcement officers rely on various tools and measurements to determine an individual's level of intoxication. One of the most commonly used methods is the breath alcohol test, often referred to as a breathalyzer test. This test measures an individual's Blood Alcohol Content (BAC) or Blood Alcohol Concentration, which is the percentage of alcohol present in their bloodstream. BAC is the standard unit of measurement used by law enforcement to define intoxication and determine an individual's level of impairment, especially when operating a motor vehicle.
| Characteristics | Values |
|---|---|
| Name of the unit of measurement | Blood Alcohol Content (BAC) |
| What it measures | Percentage of alcohol in a person's bloodstream after drinking |
| How it is measured | Blood, saliva, urine, or breath sample |
| How it is used in law enforcement | To define intoxication and provide a rough measure of impairment |
| Devices used by law enforcement | Breath test devices, including Preliminary Breath Test Devices (PBTs) and Evidential Breath Test Devices (EBTs) |
| Devices used for screening | Preliminary Alcohol Screening (PAS) tests |
| Devices used for evidence in court | Evidential Breath Test Devices (EBTs) |
| Factors affecting accuracy | Other compounds in the breath, temperature, person's health, human error, volume of air exhaled |
| Legal limit in the US | 0.08% |
| Legal limit in Utah | 0.05% |
| Legal limit in Sweden, Denmark, Norway, Finland, Germany, and Switzerland | Expressed in mass-mass concentrations |
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What You'll Learn
Breath test devices
Law enforcement officers use breath test devices to determine an individual's intoxication level and to measure the concentration of alcohol in a person's system. These devices are also known as breath analyzers or breath alcohol testers. The two main categories of breath test devices are evidential breath test devices (EBTs) and preliminary breath test devices (PBTs).
Evidential Breath Test Devices (EBTs)
EBTs are State-approved and conform to established specifications. They can be portable or stationary, and the results from EBTs can be used as evidence in court. These devices are larger and more reliable, and the police typically keep them at the jail or station. EBTs may also be used to conduct alcohol screening tests.
Preliminary Breath Test Devices (PBTs)
PBTs, also known as screeners, are hand-held devices used at the roadside by officers to determine if there is probable cause for an arrest. These devices are small and reliable enough to justify an arrest but are not always accurate or calibrated frequently enough to be used as evidence in court. California is an exception, allowing PBT results as evidence of the presence of alcohol.
Passive Alcohol Sensors (PASs)
PASs are another type of breath test device used by law enforcement. These are usually integrated into flashlights or clipboards and measure alcohol presence in the air where individuals are breathing. PASs are particularly effective at detecting impaired drivers at checkpoints, but some officers dislike using them as they require proximity to the driver and attention to the device.
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Blood alcohol content (BAC)
Breath test devices and PAS units are commonly used by law enforcement to measure BAC levels. These devices can be used as soon as they are purchased, and officers are trained in their use and maintenance. Breath test devices are especially valuable for two classes of drivers: those with high alcohol tolerance and those under 21 who may be in violation of zero-tolerance laws. A breath test device can also be useful at crash scenes where a driver is injured and unable to perform a field sobriety test.
There are two main types of breath analyzer tests: preliminary alcohol screening (PAS) tests and evidential breath tests (EBTs). PAS tests are small handheld devices that police may use in the field to determine a person’s BAC. EBTs are larger, stationary machines that are more reliable, usually kept at the jail or police station. A PAS machine may be used prior to an arrest, and an EBT may be used after the arrest to confirm the results.
Breathalyzers use a chemical reaction to calculate BAC. The alcohol vapor in a person’s breath reacts with an orange solution called potassium dichromate, turning it green. This color change creates an electrical current, which the breathalyzer converts into a value to determine the BAC. The BAC limit in most U.S. states is 0.08%, but in Utah, the limit is 0.05%. For an adult, a BAC of 0.08% may correspond to consuming four or more drinks in 2 hours.
BAC is used by law enforcement to define intoxication and provide a rough measure of impairment. It is legally useful because it can be measured objectively, making it difficult to contest in court. Most countries forbid the operation of motor vehicles, heavy machinery, boats, and aircraft above prescribed BAC levels. Some jurisdictions also regulate bicycling under the influence.
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Preliminary Breath Test (PBT)
A Preliminary Breath Test (PBT) is a tool used by law enforcement to determine probable cause for a DUI arrest. It is a hand-held device used at the roadside by officers to assess whether a driver is under the influence of alcohol. The device measures the concentration of alcohol in a person's breath, also known as their BAC (blood alcohol content) or BrAC (blood alcohol concentration). This reflects the percentage of alcohol in a person's blood, and authorities can use BAC levels to gauge a person's level of intoxication.
The PBT is a screening device, and in most states, these devices cannot be used as evidence in court. The exception is California, where PBT results can be used as evidence of the presence of alcohol, particularly to enforce zero-tolerance laws for drivers under 21. In Maryland, for example, the PBT result is not admissible as evidence in court, but an officer may use a person's refusal to take the test as a reason for arrest.
The PBT is a valuable tool for law enforcement as it provides a quick and non-invasive way to assess a person's intoxication level. It is especially useful for drivers who may appear to perform normally but have a high alcohol tolerance or are under 21. The use of PBTs can help increase DWI arrests and reduce alcohol-related crashes.
While the PBT is a useful initial assessment tool, it is not as precise as other methods of measuring intoxication levels, such as blood tests or the Intoximeter. The National Highway Traffic Safety Administration recommends that PBTs be used as an alternative or complement to traditional roadside psychomotor tests and in situations where reasonable grounds exist for believing a driver is under the influence of alcohol.
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Evidential Breath Test (EBT)
An Evidential Breath Test (EBT) is a breathalyser device that can produce test results that are admissible in a court of law. EBTs are state-approved and conform to established specifications. They can be portable or stationary.
EBTs are used by law enforcement officers to determine an individual's intoxication level via breath analysis. These tests calculate the concentration of alcohol in a person's breath, which reflects the percentage of alcohol in their blood. This is known as BAC (blood alcohol content). BAC is measured in grams of alcohol per 100 ml of blood. In the US, the BAC limit is 0.08%, while in Utah, it is lower at 0.05%.
EBTs are considered more reliable than preliminary breath test devices (PBTs) or preliminary alcohol screening (PAS) tests, which are small handheld devices used by officers in the field. While PBTs and PAS tests can be used to determine probable cause for an arrest, EBTs provide more concrete evidence of alcohol impairment that can be presented in court.
To be approved for use in evidential breath testing, EBT devices must undergo evaluation by the National Highway Traffic Safety Administration (NHTSA) and appear on the NHTSA Conforming Products List (CPL) for evidential breath testers. These devices must also have certain capabilities, such as providing printed triplicate results, assigning unique numbers to each test, and being able to distinguish alcohol from acetone at the 0.02 alcohol concentration level.
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Field sobriety testing
Field sobriety tests (FSTs) are a set of tests used by police officers to determine if a person suspected of impaired driving is intoxicated with alcohol or other drugs. FSTs are primarily used in the United States and Canada to meet "'probable cause for arrest' requirements, which are necessary to sustain an alcohol-impaired driving (DWI or DUI) conviction based on a chemical blood alcohol test.
FSTs are considered "divided attention tests" that assess a suspect's ability to perform the type of mental and physical multitasking that is required to operate an automobile. These tests evaluate balance, coordination, and attention. The three tests that constitute the Standardized Field Sobriety Tests (SFSTs) are:
- The Horizontal Gaze Nystagmus Test: This test involves following an object with the eyes (such as a pen) to determine characteristic eye movement reaction. When a person is intoxicated, the jerking of the eyes becomes more exaggerated and occurs at lesser angles. If four or more clues appear between the two eyes, the driver is likely to have a blood alcohol content (BAC) of 0.10 or greater.
- The Walk-and-Turn Test: This test is designed to measure a person's ability to follow directions and remember a series of steps while dividing attention between physical and mental tasks. The suspect is asked to walk heel-to-toe in a straight line.
- The One-Leg Stand Test: The officer asks the driver to stand with one foot about six inches off the ground and count from 1,001 until the officer instructs them to put the foot down. During the next 30 seconds, the officer looks for indicators such as swaying, using arms to balance, hopping, or putting the foot down.
It is important to note that, according to the National Highway Traffic Safety Administration (NHTSA), a suspect does not "pass" or "fail" a field sobriety test. Instead, the police determine whether clues are observed during the test. If a driver fails any of the field sobriety tests, the officer may then request a breath test or a chemical test to confirm their blood alcohol content (BAC).
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Frequently asked questions
Law enforcement uses Blood Alcohol Content (BAC) to check for alcohol. BAC is the percentage of alcohol in a person's bloodstream after drinking.
BAC is typically measured through a breath test, which requires a person to blow into a device known as a breath analyzer or breathalyzer. Other ways to measure BAC include blood, saliva, or urine samples.
There are two main types of breath analyzers: preliminary alcohol screening (PAS) tests and evidential breath tests (EBTs). PAS tests are small handheld devices used by police in the field, while EBTs are larger, stationary machines that are more reliable and kept at police stations or jails.
The BAC limit in the US is generally 0.08%, but this varies by state. For example, in Utah, the limit is 0.05%. Other countries have different BAC limits, ranging from zero tolerance to 0.08%.
