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------------------------------------------------ Alcohol concentration when measured by a breath sample is expressed in?
1) Milligrams per deciliter (mg/dL)
2) Parts per million (ppm)
3) Percentage (%)
4) Liters per minute (L/min)

Alcohol concentration in a breath sample is most commonly expressed as a percentage (%), and this is known as blood alcohol concentration (BAC). It is a legal measure of intoxication levels. Other units like milligrams per deciliter (mg/dL) and parts per million (ppm) are used for various concentrations, but percentage is standard for BAC.

Explanation:

Alcohol concentration when measured by a breath sample is most commonly expressed as a percentage (%). This measure, known as blood alcohol concentration (BAC), compares units of alcohol to units of blood. It is used legally to assess intoxication levels and is a measure of the amount of alcohol in a person's bloodstream. More than 90 percent of ingested alcohol is metabolized by the liver, with the remainder being excreted through breath, urine, saliva, and sweat — which is how devices like Breathalyzers are able to determine a person's BAC.

While other units like milligrams per deciliter (mg/dL), parts per million (ppm), and parts per billion (ppb) are used in various settings for measuring concentration, percentage is the standard for measuring BAC. The term 'parts per million' refers to the ratio of the mass of the substance to the total mass being considered, multiplied by one million. However, when referring specifically to BAC, percentage is the norm.