FVC and its Critical Relevance to Breathalyzer Test Defence for the Criminal Lawyer

In order to provide a proper breath sample into a breath testing instrument, a forced vital capacity (FVC) of approximately 2.5 litres is required (WOA30301-3). FVC is the volume of breath that can be exhaled after a maximum inhalation and is an indication of lung size and health. This fundamental physiological requirement is the cornerstone of many defences against an alleged breathalyzer test refusal. In addition to the deliberate and conscious refusal of breath testing, criminal lawyers should be aware that the following factors could cause a person not to deliver a proper breath sample, creating a valid legal argument:

Lung size or FVC varies according to the height, age and gender of the subject. A young tall man (age 20 years, height 6’2”) has a predicted FVC of approximately 6.2 litres and should, generally, have no problem providing a breath sample into a breath testing instrument. Whereas a 60 year old woman 5’0” in height, has a predicted FVC of approximately 2.5 litres and, thus, could have genuine difficulty providing a suitable breath sample for a breathalyzer test. This isn’t a matter of non-cooperation but a simple biological limitation.

Poor instructions by the police may contribute to a person being unable to provide a breath sample (WOA30305). Police Officers should encourage the subject to “blow longer and keep blowing, keep blowing, keep blowing” as opposed to directing the subject to “blow harder”. For most individuals, blowing harder often results in the lungs being emptied too soon, causing a failure to complete the breathalyzer test. I have been involved in cases where the police officer actually conducted a breath test themselves in order to demonstrate to the subject how to blow properly. (Yes all the police breath test results were zero!)

If the mouthpiece has a hole in it or a piece of plastic is stuck in it from the manufacturing process or during the removal of the mouthpiece from the plastic bag, airflow may be blocked. A criminal lawyer will want to ensure that the mouthpiece is examined closely in cases of a refusal. A faulty device is a common and often overlooked reason for a failed breathalyzer test, shifting blame from the driver to the equipment.

A proper breath sample may not be provided if a person has trouble forming a proper lip seal as a result of local dental anesthesia (WOA30310) or a medical condition like Bell’s Palsy which reduces feeling and control in the lips. This physical inability to create an airtight seal can invalidate the entire breathalyzer test process.

Upper dentures which are slipping can decrease the ability to provide a proper breath sample and present another legitimate reason for a breath sample refusal. The instability of dental hardware prevents the necessary seal and pressure from being maintained for the duration of the breathalyzer test. To learn more about the effect of upper dentures on breath sampling, please click on this paper on Slipping Dentures.

Not all refusals are the result of the deliberate and conscious refusal of breath testing. An alleged failure of a breathalyzer test can stem from a myriad of physical and technical issues. Anyone involved in a refusal case should immediately contact a skilled DUI, DWI or criminal lawyer for consultation to explore these crucial defences. The outcome of your case depends on a lawyer’s deep understanding of these complexities.

*All the references cited may be found in WOA 2E in the appropriate section according to the number.”