What is the DWI Legal BAC Limit?

The DWI legal BAC limit is .08. This means a driver is guilty of DWI if his blood alcohol concentration (BAC) is .08 g/210L or greater at the time of driving. Courts call this the per se theory of DWI. But this theory creates issues.

For instance, the law requires drivers arrested for DWI to cooperate. In other words, the driver must willingly give a breath test. This test measures the driver’s BAC. And prosecutors use the test results to convict drivers of DWI. As a result, many drivers decline to provide a breath test.

Also, BAC tests raise accuracy issues. A person is DWI when he is intoxicated “while operating a motor vehicle in a public place.” Thus, a driver must be drunk when he is driving and not later at the time of the test. Otherwise, he is not guilty of DWI.

But there is a time gap between the time of arrest and the time of the BAC test. This time gap varies. Sometimes it is only twenty minutes. Other times it is longer than 1.5 hours. As a consequence, the test may not accurately reflect the driver’s BAC level at the time of driving.

These are two recurring issues in DWI cases. However, to see how these problems arise, we first need some context. First, we look at how DWI laws in America evolved. Second, we review why drivers give breath samples. From there, we can talk about DWI defense options. So let’s get started.

History of DWI and Legal BAC limits in America.

The problem of drunk driving started at the dawn of the vehicle age. Consequently, New Jersey passed one of the earliest DUI laws in America. But this law was crude. And it did not contain a BAC limit. Nonetheless, other states passed similar DUI laws.

Unfortunately, people continued to drive drunk. Accordingly, states refined their DUI laws. In 1939, Indiana passed the first DUI law with a legal BAC limit. The limit was .15. But at the same time, this limit was only a presumption. This meant defendants could challenge the limit at trial. In the same manner, other states passed BAC limit laws.

Later, states lowered the BAC limit from .15 to .10. Then they lowered it again. This time it went from .10 to .08. In addition, states moved from a presumption to a per se definition. And this is the current law in Texas.

Yet, these BAC laws raised two questions. Can a driver decide not to give a breath test? And, what happens if a driver does not give a breath test? To answer these questions, we need to talk about Implied Consent laws.

What are Implied Consent laws?

Implied consent laws require drivers arrested for DWI to give a breath sample. Above all, the Implied Consent laws require suspected drunk drivers to cooperate with BAC testing. If a driver does not provide a breath sample, DPS or the DMV will suspend his or her license.

The Texas Implied Consent law follows this pattern. It requires drivers arrested for DWI to give a breath test. Additionally, it requires police to give drivers over the age of 21 the following warnings:

  • If the driver does not provide a breath test, DPS will suspend his license for at least 180 days.
  • If the driver provides a breath test over the limit, then DPS will suspend his license for at least 90 days.
  • Police also tell the driver that if he or she refuses, police will get a warrant to draw blood.

Most important of all, these warnings show why drivers give breath tests. Drivers who cooperate get lighter suspensions. At the same time, these drivers make it easier for prosecutors to convict them on a DWI charge. This is also why many drivers do not give a breath test. In this situation, police apply for a warrant.

DWI and DUI Blood Draw Testing.

In blood draw cases, a medical professional such as a nurse draws blood from a suspected drunk driver. The nurse injects a syringe needle into the driver’s vein and then draws the blood into a vial. The nurse places the blood vial into an evidence bag. And police then send the vial to a lab for testing. Finally, the lab tests the blood for alcohol using a gas chromatograph with a headspace sampler.

The science for blood draws is a little technical. But in these cases, the defense must make sure the blood samples were properly collected. This includes making sure the blood does not clot in the tube. And making sure the vial is not contaminated with Candida albicans. Candida albicans (CA) is a common yeast. CA can combine with sugar to create alcohol. Thus, if the vial is contaminated with CA, it can create alcohol in the tube. This will cause a BAC level that is falsely high.

One important note on CA. The blood vials used for blood draws are laced with sodium fluoride (NaF). The scientific literature states that sodium fluoride prevents CA from creating alcohol in blood vials. But sometimes the vials are defective. Or, the nurse does not properly mix the blood with the sodium fluoride in the vial after the blood draw. At any rate, these are known issues. And the defense must explore them before trial.

DWI and DUI Breath Alcohol Testing.

Texas uses the Intoxilyzer 9000 for breath testing. It requires the driver to blow into a tube. The machine then takes this sample and looks for alcohol molecules. To do this, the machine uses infrared technology.

Hand drawing of Ethanol Molecule. San Antonio DWI Defense.
The DWI and DUI Intoxilyzer measures the infrared energy from the ethanol molecule. Further, this measurement will establish the blood alcohol concentration of the driver sometime after he is arrested. This is strong evidence that helps the prosecutors convict suspected DWI drivers.

Specifically, alcohol contains three atoms. Hydrogen, oxygen, and carbon. Chemical bonds link these atoms together. More importantly, these bonds absorb infrared light in a unique pattern. In essence, this pattern becomes a fingerprint. This allows the machine to find the alcohol and then measure it. Once the machine completes the test, it then prints out the results. These results contain the driver’s BAC level.

Breath Alcohol Testing vs. Blood Draws

Breath alcohol testing and blood draws do the same thing. They both measure the BAC of drivers arrested for DWI. But they differ in several important ways. In particular, the biggest difference is that blood draws are more invasive than breath tests.

Primarily, blood draws require a medical professional to put a needle in the driver’s vein. And blood draws gather more information than breath tests. As an example, a blood draw can show if the driver used cocaine, marijuana, methamphetamine, or pot. In contrast, breath tests require the driver to blow into a machine. This test captures a breath sample that does not contain the same amount of information as a blood sample.

For these reasons, the Supreme Court treats these two tests differently. Chiefly, police do not need a warrant to get a breath test. Strikingly, it is not always clear when police need a warrant for a blood draw. But the Supreme Court provided guideposts to help on this issue.

DWI Blood Draws, Search Warrants, and the Fourth Amendment.

In a normal DWI case that does not involve an accident, police must get a warrant. But if the DWI arrest involves a car accident, then police do not need a warrant. Similarly, police probably do not need a warrant if the driver is unconscious.

On the surface, these cases seem to conflict. They do not appear to make sense. The reason is the Fourth Amendment. Or more precisely, an exception to the Fourth Amendment. The Supreme Court calls this exception exigent circumstances. It means if there is an emergency, then police do not need a warrant for the blood draw. Otherwise, they must get a warrant for the blood draw.

In short, this is how prosecutors get the BAC level of the suspected drunk driver. But what happens if the test shows the driver is over the limit? Does this mean the driver is guilty? And what if the driver disagrees with the test results? What are his or her options? Let’s turn to this topic now.

I am over the limit. Does this mean I am automatically guilty of DWI?

No. Breath and blood tests are powerful evidence. However, a BAC-test result, by itself, does not prove the driver was drunk at the time of driving. To convict a driver of DWI with a BAC test that is over the limit, the prosecutor must show:

  1. The BAC test provides trustworthy proof of alcohol in the driver’s breath or blood; and
  2. The driver’s BAC at the time of driving was over the limit based on the test.

If the prosecutors cannot show these two points at trial, then the driver is not guilty of DWI. See Kirsch v. State, 306 S.W.3d 738, 745 (Tex. Crim. App. 2010).

Now, let’s see how we can use this information to present a defense to a DWI charge.

BAC curves and DWI Legal BAC limits.

Alcohol consumption follows a known pattern. First, the driver absorbs alcohol into the body. This happens when the driver drinks a beer, takes a shot of whatever, or sips a glass of wine. Second, the alcohol is distributed into the body. Finally, the body eliminates the alcohol. As the diagram below shows, these three phases create the BAC curve.

Texas DUI and DWI Blood Alcohol Concentration Graph.
The BAC Curve graph shows the three phases of alcohol distribution. This includes the absorption phase, the peak phase, and the elimination phase. These phases help determine the blood alcohol concentration of a suspected drunk driver at the time of driving.

Moreover, the three cars in the diagram show a driver maybe anywhere along the BAC curve when he is pulled over. This can affect the accuracy of the BAC test that is taken at the police station. If the time gap between the stop and the breath test is significant, then the BAC test will not accurately represent the driver’s BAC at the time of the stop and arrest.

Furthermore, the driver’s body absorbs alcohol almost immediately after drinking. The alcohol passes from the mouth to the stomach and intestines. From there, it goes into the blood and reaches the brain. At this point, the driver will start showing signs of intoxication. But the onset of intoxication depends on several factors. Let’s see how this works.

BAC Absorption Factors

The rate of absorption varies from driver to driver. And it depends on many factors, including:

  1. Food in the stomach;
  2. Type of food in the stomach;
  3. Gender;
  4. Weight;
  5. Age;
  6. Mental state;
  7. Drinking pattern;
  8. Type of alcohol (beer, whiskey, or wine);
  9. Amount consumed; and
  10. Time period of drinking.

In short, these factors can distort the BAC curve. As a consequence, the BAC curve may not be as smooth as our diagram. And this distortion can lead to a falsely elevated BAC test result.

BAC Elimination Rate Factors

The absorption phase continues until the person stops drinking. Then, at some point, the BAC curve reaches its peak. From there, the BAC curve will begin to fall as the body eliminates the alcohol.

The elimination rate is slow but consistent. Many government lab techs state the rate is .015 per hour. In other words, the body eliminates alcohol at .015 per hour. But many scientists disagree with this number. Their point is that elimination rates vary from person to person. This too can affect the BAC curve and the accuracy of the breath test.

For instance, if police test the driver during the absorption phase, then his BAC at the time of the test will be higher than at the time of driving. This could lead to a wrongful conviction. Lawyers call this the rising-alcohol defense.

On the other hand, if police test the driver during the elimination phase, then his BAC at the time of the test is lower than at the time of driving. And it is difficult to tell from a single breath test if the driver is in the absorption or elimination phase at the time of the test.

The Bottom Line on DWI Legal BAC Limits

The larger the time gap between the DWI arrest and the test, then the more likely the test does not accurately measure the driver’s BAC at the time of driving. This is a critical issue in DWI cases. The law makes it a crime for the driver to be drunk when he was driving, not when he was tested. This can be a strong defense in close cases. For example, when the driver’s BAC test is moderately over the limit at the time of testing. In these situations, the BAC test may not accurately measure the driver’s true alcohol level at the time of driving.

Affordable DWI and DUI Defense Attorney

Alcohol is fun. That is why people drink when they hang out with friends. Normally, this is not a problem. Unfortunately, the police DWI dragnet catches many drivers with alcohol in their systems. Some of these drivers are drunk. Others are not. For those drivers who are drunk, a plea-bargain deal is the way to go. But for drivers wrongfully arrested for DWI, they should fight their DWI charge. And therein lies the rub–it is not always clear if a driver is sober or intoxicated at the time of driving from a single BAC test.

Genaro R. Cortez

Law Office of Genaro R. Cortez, P.L.L.C.

730 West Hildebrand Avenue
Suite 2,
San Antonio, Texas 78212
Phone: 210-733-7575
Fax: 210-733-7578
Email: genaro.cortez@cortezlawyer.org