What Is Blood Alcohol Content (BAC)?

Feb 15, 2017

Drunk and impaired driving remain serious problems affecting the entire country. According to National Highway Traffic Safety Administration, 10,265 people died in alcohol-impaired driving crashes in 2015 alone. That accounted for about 29 percent of all traffic-related deaths in the country. Today, an estimated 28 people die every day in incidents involving alcohol-impaired drivers. That equates to a preventable death every 51 minutes.

The U.S. has enacted various laws and measures to discourage drunk driving, but how does one determine or measure drunkenness? That’s where blood alcohol content comes in, giving law enforcers an easy means of quantifying the alcohol in your system and hopefully preventing accidents, damage to property, and deaths caused by impaired driving. Read on to learn more about blood alcohol content.

Understanding Blood Alcohol Content

So, what is blood alcohol level and how does it affect you? Simply put, blood alcohol content or concentration (BAC) is the measure of alcohol present in your blood. It is usually presented as a percentage that is equivalent to a specific number of grams of alcohol per 100 milliliters of blood. For example, a blood alcohol concentration of 0.04 percent equates to 0.04 grams of alcohol for every 100 milliliters of blood. Your BAC can be measured through your blood, breath, and urine.

When you drink an alcoholic beverage, the alcohol is metabolized in the liver. As a general rule of thumb, your liver can process one standard alcoholic drink in about an hour. Consuming more than this will result in the excess alcohol getting stored in your blood and body tissue until the liver can eventually metabolize it. This is why drinking several shots or otherwise consuming more drinks in a short amount of time can result in a higher blood alcohol concentration for several hours.

What Constitutes One Drink?

It’s easy to underestimate just how much alcohol constitutes one drink, especially considering there isn’t always a single standard drink measurement. Generally, one drink equals:

  • 12 fluid ounces of beer (about 5 percent alcohol by volume)
  • Up to 9 fluid ounces of malt liquor (about 7 percent alcohol by volume)
  • 5 fluid ounces of wine (about 12 percent alcohol by volume)
  • 1.5 ounces of 80-proof distilled spirits or liquor (about 40 percent alcohol by volume)

Factors That Affect Your BAC

Even understanding that it takes about one hour to process a drink, it’s important to remember that it’s only a general guideline. Two people can drink the same drink and still end up with different blood alcohol concentrations. Although your BAC is mainly affected by the amount that you drink and the time you take in all those drinks, everyone processes alcohol at different rates based on a wide range of physical factors, including:

  • Genetics – The enzymes in the liver that break down the alcohol differs from person to person. These enzymes can work at different speeds and are defined by your genes, so some people can naturally break down alcohol faster than others.
  • Age – The younger you are, the faster your metabolism. This often means that younger people will reach higher blood alcohol concentrations faster than older people.
  • Gender – Men and women have different body compositions. Women tend to have a higher ratio of body fat to lean muscle, which often results in them absorbing alcohol and reaching a higher BAC more quickly than men.
  • Body size – Smaller people will tend to reach a high BAC faster as the alcohol is concentrated within a smaller body mass.
  • Liver health – A liver that has previously been damaged or is otherwise unhealthy will have a difficult time efficiently processing alcohol, resulting in a higher blood alcohol concentration.
  • Food – Eating a meal prior to drinking can slow down the rate of absorption, which can allow for a slower, steadier increase in your BAC over time. Drinking on an empty stomach can lead to a more immediate increase in BAC. However, eating before consuming alcohol will not prevent you from getting drunk.

You may have several drinks and top out at over 0.08 BAC, but your male friend who is larger, older, and who maybe ate before drinking only has a BAC of 0.05. It’s important to understand that BAC varies based on a wide range of factors, and the only true way of knowing your own drunkenness is to measure your BAC.

How Is Blood Alcohol Content Measured

You can find several charts and calculators online that can help you determine your BAC, but these are only approximations that often overestimate the strength of your drinks and ignore most personal factors, including your metabolism, recent medications, and the last meal you had. While you can absolutely use them as a general gauge, they will not give you an accurate reading.

The most reliable and accessible way to estimate your BAC is using a breath testing unit, or breathalyzer, which uses a breath sample to determine how much alcohol is in your system. When you drink alcohol, it is not immediately digested nor does it break down chemically when it enters your bloodstream. As your blood flows from your lungs, a fraction of the alcohol evaporates into the membranes of your lungs air sacs, known as alveoli. The alcohol in the alveoli can be detected when you exhale.

The concentration of alcohol in your breath is directly related to the amount of alcohol actually in your blood at a ratio of about 2,100 to 1. That means that about 2,100 milliliters of alcohol in your alveolar air equals about 1 milliliter of alcohol in the blood. Given the right tools, you can then get a fairly accurate estimate of your BAC based on a single breath.

Most widely available car breathalyzers use an ethanol fuel cell sensor. The fuel cell is composed of two platinum electrodes with an acid-electrolyte material sandwiched between them. As you exhale into the device, the platinum electrodes oxidize the alcohol in your breath, producing electrons, protons, and acetic acid.

The protons combine with electrons and oxygen to create water, while the stray electrons travel through a wire that connects the two electrodes, creating an electrical current. The more alcohol that the platinum electrodes oxidizes, the stronger the current. Microprocessors can then measure the strength of the current and calculate it into your BAC.

How Blood Alcohol Content is Measured and Affects You

Alcohol has some severe effects on your mood, mind, and physical health based on your BAC levels. So then, what is high blood alcohol level and how is it measured? Take a look below.

  • 0.02-0.03 – A slight intensification in mood but few obvious effects
  • 0.05-0.06 – Exaggeration of emotions and behaviors, feelings of warmth and euphoria, impaired judgment, a decrease in reaction and fine-motor control
  • 0.07-0.09 – Physical imbalance, noticeable speech impairment, impaired motor coordination, impaired hearing and vision, increased confidence, feelings of depression or euphoria
  • 0.11-0.12 – Greater difficulties with coordination and balance, impairment of judgment and mental faculties
  • 0.14-0.15 – Significant impairment of physical and mental control, complete lack of motor skills, slurred speech, blurry vision
  • 0.20 – Complete loss of motor control, mental confusion, inability to walk or stand
  • 0.30 – Severe intoxication, possible loss of consciousness

At a BAC of about 0.14 and above, you will likely require medical evaluation. However, remember that these are just averages. Some people may experience intoxication at lower levels.

BAC and Driving

In all states, the legal driving limit is a BAC of 0.08, meaning that it is illegal to drive a car or operate any vehicle if your blood alcohol concentration is 0.08 or above. However, further studies are showing that even lower BAC levels can lead to problems while driving. Just one standard drink can increase your BAC by 0.02. With a BAC of 0.05 (which is still under the legal limit), you are twice as likely to crash a car as someone who hasn’t had any drinks.

Alcohol acts as a depressant, slowing down your brain’s overall activity. That can have some serious effects on your driving as it can lead to:

  • Impaired vision
  • Reduced concentration
  • Reduced reaction time
  • Difficulty processing sensory information
  • Feeling more drowsy or relaxed, potentially resulting in you falling asleep at the wheel
  • Failure to obey traffic rules
  • Overconfidence, which can lead to risk-taking
  • Difficulty performing several tasks at once (like staying in your lane while driving at the posted speed limit and paying attention to any traffic around you)

Many people who drink alcohol believe that, as long as they are careful, they can drive safely, but that in itself is an effect of alcohol. Alcohol can affect how you view yourself and your reality, making you more confident in your own abilities. That means you may ultimately be unaware of how much your driving abilities have changed with any amount of alcohol in your system.

Driving Safely

The best way to stay safe if you intend to drive home is to not drink at all. You can still by all means drink and enjoy yourself without putting yourself and others in danger.

Keep track of your drinking

If you decide to drink, keep a mental note of how much you drink per hour and try to limit yourself to just one drink per hour. Some people may need to drink even less to keep down their BAC. This allows your body to break down the alcohol without allowing it to accumulate.

Limit your own drinking

Avoid mixed drinks and try to stick mainly with low-alcohol beverages. It’s often difficult to measure how much alcohol is present in a mixed drink.

Avoid drinking in shots or rounds

  • Avoid topping up your glass. Constantly refilling your drink can make it harder to keep track of how much you’ve had.
  • Start with non-alcoholic drink and then have a non-alcoholic drink after every second or third beverage.
  • Drink in sips to slow down your consumption.

Allow your BAC to go down before you drive

Depending on how you have been drinking your BAC can continue to rise up to three hours after your last drink. The only way to actually lower your BAC is time, so don’t expect coffee or cold showers to sober you up. The more alcoholic beverages you consumed, the longer it will take, but understanding that it generally takes an hour to break down one drink, wait for at least one hour per drink you’ve consumed before you start driving.

Always have a backup plan

If you have had too much to drink and cannot wait it out, make sure you have a backup plan, whether that means having a designated driver or calling a cab.

While BAC is a hugely important factor in measuring your sobriety, much of it comes down to you. No one knows your body better than yourself. Regardless of your BAC, if you do not feel fit to drive, do not force it. It’s always better to be safe than sorry.

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