Most people know that alcohol can affect judgement and reaction time, but fewer understand how long it can actually be detected after drinking. A breathalyser can usually detect alcohol for several hours and in some cases up to 24 hours after the last drink. The exact time depends on the amount consumed, the body’s metabolism, and other personal factors.
This matters because alcohol may still register on a test long after the effects seem to fade. Someone might feel sober, yet a breathalyser could still show alcohol in their system. That gap between how the body feels and what the test shows often leads to mistakes, especially before driving or returning to work.
By looking at how alcohol breaks down in the body and what influences detection times, it becomes easier to understand why results vary so much from person to person. This knowledge helps avoid surprises and makes it clear why caution is often the safer choice.
How Long Can a Breathalyser Detect Alcohol?
A breathalyser can detect alcohol for a limited period that depends on how much a person drinks, how their body processes alcohol, and the type of device used. The detection range may last only a few hours for light drinking but extend much longer after heavy intake.
Typical Detection Windows for Breath Tests
Most breath tests detect alcohol for 2 to 24 hours after drinking. A single standard drink may remain detectable for about 2–3 hours. Moderate drinking across several hours can extend detection to 6–12 hours. In cases of heavy or prolonged drinking, alcohol may still register for up to 24 hours.
The body eliminates alcohol at a fairly steady pace, usually about one drink per hour. However, factors such as sex, body weight, and food intake alter how long alcohol stays in the system.
Disposable breathalyser test kits often provide shorter detection ranges than police-grade devices. They are designed for quick checks and can help people assess whether alcohol may still be present before driving or working.
How Breathalysers Estimate Blood Alcohol Concentration
Breathalysers measure alcohol in exhaled air and then estimate blood alcohol concentration (BAC). Alcohol enters the bloodstream after drinking and passes into the lungs. As blood circulates through the lungs, small amounts of alcohol move into the air sacs.
When a person exhales, the device analyses this air sample. Many devices use electrochemical sensors that react with alcohol and produce an electrical signal. This signal is then converted into a BAC reading.
Although breathalysers provide quick results, they estimate BAC rather than measure it directly. Factors such as calibration, device quality, and proper use influence accuracy. For personal checks, people can use disposable breathalyser test kits, while workplaces or law enforcement rely on more advanced models.
Earliest and Latest Points of Alcohol Detection
Alcohol can appear on a breath test within minutes of drinking. The exact time depends on how quickly alcohol is absorbed into the bloodstream. Food in the stomach slows absorption, so detection may take longer compared to drinking on an empty stomach.
The latest point of detection varies more widely. For light drinking, the body clears alcohol within a few hours. After a heavy session, measurable alcohol may still be present for much of the next day.
Individual metabolism plays a large role in these variations. Age, genetics, and overall health all affect how long alcohol lingers. This is why two people who drink the same amount may test positive for very different lengths of time.
Alcohol Impairment and Legal Implications
Even after a person feels sober, alcohol may still appear on a breathalyser. This creates risks for those who need to drive or operate machinery. A BAC above the legal limit can lead to penalties such as fines, licence suspension, or even criminal charges.
It is important to understand that impairment often begins before the legal threshold is reached. Reaction times slow, judgement weakens, and coordination declines even at lower levels of alcohol.
Because of these risks, many people use personal breath tests to check their status before driving. However, only official devices used by authorities determine legal outcomes. Personal tests should be seen as a guide, not a guarantee of safety.
Key Factors That Influence Detection Times
The length of time alcohol stays detectable depends on how the body processes ethanol, how quickly it absorbs and eliminates it, and how personal traits such as weight, sex, and genetics shape these processes. Each of these factors can shorten or extend the detection window measured by a breathalyser.
Alcohol Metabolism and the Role of the Liver
The liver plays the main role in alcohol metabolism. It uses enzymes to break ethanol into smaller compounds that the body can remove. The first step involves alcohol dehydrogenase (ADH), which converts ethanol into acetaldehyde, a toxic substance.
Another enzyme, aldehyde dehydrogenase (ALDH), then turns acetaldehyde into acetate. Acetate is less harmful and leaves the body as water and carbon dioxide. This process happens at a fairly steady rate, usually equal to about one standard drink per hour.
Because the liver can only process alcohol at this fixed pace, any alcohol consumed beyond that rate builds up in the bloodstream. As a result, the breathalyser can detect alcohol for longer periods after heavier drinking.
In addition to ethanol breakdown, the body produces alcohol metabolites such as ethyl glucuronide (EtG). Although EtG is more often measured in urine, its presence shows that alcohol remains in the body long after the main effects wear off.
Alcohol Absorption and Elimination Rates
Alcohol enters the bloodstream through the stomach and small intestine. The speed of absorption depends on factors such as whether food is present. A full stomach slows absorption, while drinking on an empty stomach allows alcohol to reach the blood more quickly.
Once absorbed, alcohol spreads through body fluids and tissues. The elimination phase then begins, with the liver steadily breaking it down. Unlike many substances, alcohol does not speed up clearance as blood levels rise. Instead, the body removes it at a constant rate.
For example, a person who consumes one drink may test positive for two to three hours. Someone who consumes several drinks in a short time may remain positive for 12 hours or more. This difference reflects the balance between rapid absorption and slow elimination.
Individual Differences: Weight, Gender, and Genetics
Body weight strongly affects alcohol concentration. A larger person has more body water, which dilutes alcohol. In contrast, a smaller person reaches a higher blood alcohol concentration after the same number of drinks.
Sex also influences metabolism. On average, women have less body water and lower levels of ADH in the stomach and liver. This means alcohol often remains in their system longer compared to men of the same weight.
Genetics adds another layer of variation. Some people inherit enzyme patterns that break down acetaldehyde more slowly, leading to higher and longer-lasting alcohol levels. Others may have faster enzyme activity, which shortens detection times.
These biological differences explain why two people who drink the same amount may show very different breathalyser results hours later.
Conclusion
A breathalyser can usually detect alcohol for a range of several hours, often up to 12–24 hours, depending on how much a person drank and their individual metabolism.
Factors such as body weight, gender, food intake, and overall health influence how long alcohol remains measurable. Therefore, detection times often vary widely from one individual to another.
Understanding these timeframes helps people recognise that alcohol does not leave the body quickly and that safe limits depend on more than just the number of drinks consumed.
