What is Henry's Law in relation to DWI cases?

Henry's Law is a scientific principle stating that the amount of alcohol dissolved in a liquid (blood) is proportional to the amount of alcohol in the gas (breath) above it. Breathalyzers rely on this law to estimate Blood Alcohol Content (BAC) from a breath sample.

How does a breathalyzer calculate blood alcohol content?

Breathalyzers do not measure blood alcohol directly. Instead, they use a standard 'partition ratio' of 2100:1, assuming that for every 2,100 parts of alcohol in the blood, there is 1 part in the breath. This conversion is based on Henry's Law.

Is the 2100:1 ratio accurate for everyone?

No. The 2100:1 ratio is an average, but individual human physiology varies greatly. Body temperature, hematocrit levels, and breathing patterns can all alter a person's actual ratio, potentially leading to falsely high BAC readings.

Can body temperature affect a breath test result?

Yes. Henry's Law requires consistent temperature. If a person has a fever or elevated body temperature, the amount of alcohol in their breath increases, which can artificially inflate the breathalyzer reading.

How can a lawyer use Henry's Law to defend a DWI?

An experienced DWI attorney can argue that the standardized 2100:1 ratio does not apply to the specific defendant. By highlighting physiological variances (like fever or lung capacity), the defense can cast doubt on the accuracy of a borderline BAC result.

What factors make a breath test inaccurate?

Factors include the partition ratio assumption, body temperature, breathing patterns (hyperventilation vs. holding breath), mouth alcohol, and radio frequency interference. All of these rely on the presumed stability of Henry's Law conditions.

Do I need a specific lawyer for a breath test defense?

Yes. Challenging the science behind Henry's Law requires an attorney who understands forensic toxicology and the technical specifications of breath test machines like the Datamaster or Intoxilyzer.

Can I refuse a breath test in New York?

You can refuse, but New York is an 'implied consent' state. Refusal typically leads to an automatic license revocation of at least one year and a civil fine, even if you are later found not guilty of the criminal DWI charge.

What Does Henry’s Law Have To Do With The Breath Alcohol Test

The science underlying the breath test machine can be complicated, and it helps to have an DWI lawyer who is experienced at challenging the breath test following an arrest for drinking and driving.

Henry’s Law, named after chemist William Henry, is an integral part of how the breath test machine functions.

The breath test machine does not directly measure blood alcohol content.

Instead, it determines the amount of alcohol in a breath sample and uses that value to determine the amount in a person’s blood.

To do so, it relies on Henry’s Law, which – in general terms – states that the amount of a substance (such as alcohol) dissolved in a liquid (e.g., the blood) can be determined based on the amount of that substance in a gas (e.g., the air in the lungs) immediately above the liquid.

The ratio of the substance between the gas and liquid depends on the substances involved, temperature and pressure.The ratio of blood alcohol to breath alcohol used by breath test machines is 2,100 : 1.

Unfortunately, the use of a fixed ratio of 2,100 : 1 does not accurately reflect variations in human anatomy.

As mentioned above, the proper application of Henry’s Law requires consistent values for temperature and pressure, and these values are not consistent when dealing with real people.

While the ratio may reflect the average ratio, very few people actually will have a blood alcohol to breath alcohol ratio of 2,100 : 1, and there is no practical way of determining the actual ratio of a given individual for the purpose ofmaking the breath alcohol test more accurate.

As a result, particularly in cases where the breath test result is only slightly above the legal limit, an experienced DWI lawyer may explore the application of Henry’s Law during plea negotiations or at trial to argue that the test result is not entirely accurate.

If you need a DWI lawyer who understands the breath test, call us at 585-484-7432.