How Breathalyser Evidence Works in Drink Driving Cases
Police use two types of breath test in drink driving investigations. The roadside preliminary breath test is a screening device only and cannot be used as evidence in court. The evidential breath test, conducted at the police station using an approved device such as the Intoxilyser EC/IR or the Evidenzer, produces the reading that forms the basis of the prosecution case.
The device takes two breath samples and the lower of the two readings is used. Under section 5 of the Road Traffic Act 1988, the legal limit in England and Wales is 35 micrograms of alcohol per 100 millilitres of breath. A reading above this threshold is sufficient for a charge, but the reliability of that reading can be challenged.
Breathalyser evidence is not infallible. The machines operate on infrared spectroscopy technology that measures alcohol molecules in deep lung air. Any factor that interferes with this process can produce an inaccurate result, and identifying such factors is a key part of a specialist drink driving defence.
Common Breathalyser Errors
Calibration Issues
Evidential breath testing devices must be calibrated regularly in accordance with the manufacturer's specifications. The Intoxilyser EC/IR, for example, runs an automatic self-calibration check before each test cycle. If the device fails this check or there are gaps in the calibration records, the accuracy of the reading is open to challenge.
Defence solicitors can request the calibration history and maintenance logs for the specific device used. If the records reveal missed calibration dates, failed checks or irregular servicing, this can form the basis of a robust defence argument that the reading should not be relied upon.
Mouth Alcohol Contamination
Mouth alcohol is one of the most common causes of falsely elevated breath readings. If alcohol remains in the mouth from recent drinking, vomiting, belching or gastric reflux conditions such as GERD, the device may measure mouth vapour rather than deep lung air. This can produce a reading significantly higher than the true blood alcohol level.
Police are required to observe the suspect for a period before conducting the evidential test, typically 20 minutes, to ensure nothing enters the mouth that could contaminate the sample. Medical conditions such as acid reflux, hiatus hernia or diabetes can cause ongoing contamination that the observation period may not resolve.
Operator Errors
The police officer operating the breath testing device must follow a strict statutory procedure laid down by the manufacturer and Home Office guidance. This includes correctly entering the suspect's details, ensuring the device has completed its purge cycle and verifying that the subject provides a continuous breath of sufficient volume and duration.
Failure to follow the correct operating procedure can compromise the reliability of the result. If the officer was not properly trained on the specific device model, or made errors during the testing process, a solicitor can argue that the evidence should be excluded.
Challenging the Intoxilyser Machine
The Intoxilyser EC/IR is the most commonly used evidential breath testing device in police stations across England and Wales. It replaced the older Intoximeter and Lion Intoxilyser models. While it is an approved device under the Breath Analysis Devices (Approval) Order, its readings are not immune from challenge.
A defence solicitor can obtain disclosure of the device's printout, calibration records, error logs and maintenance history. Any anomalies in the device's operation, such as aborted test cycles, inconsistent readings between the two samples, or recorded fault codes, may indicate a malfunction that undermines the prosecution evidence.
Expert forensic evidence may be instructed to examine the device records in detail. If a significant discrepancy exists between the two breath readings, or the device recorded any irregularities during the test, this technical evidence can be presented to the court to challenge the reliability of the result.
The Statutory Option for Blood or Urine
Under section 8(2) of the Road Traffic Act 1988, if the lower of the two breath readings is no more than 50 micrograms of alcohol per 100 millilitres of breath, the suspect must be offered the statutory option. This means the suspect has the right to elect that the breath specimen be replaced by a specimen of blood or urine for analysis. The choice between blood and urine is made by the police officer, not the suspect.
If the lower reading falls between 40 and 50 micrograms, exercising the statutory option can be tactically significant. Blood and urine tests provide a different measurement that may produce a result below the prescribed limit. The blood alcohol limit is 80 milligrams per 100 millilitres and the urine limit is 107 milligrams per 100 millilitres.
Failure by the police to offer the statutory option when required is a serious procedural error. If your lower breath reading was 50 micrograms or below and the option was not offered, the breath evidence may be inadmissible and the case could be dismissed entirely.
When to Challenge Breathalyser Evidence
Breathalyser evidence should be scrutinised in every drink driving case. Challenges are particularly strong where the reading is close to the legal limit, where there is a significant gap between the two breath samples, or where the suspect has a medical condition that could affect the result. Even high readings can be challenged if device malfunction or procedural errors are identified.
Time is critical in building a breathalyser defence. Device records, calibration logs and custody suite CCTV may only be available for a limited period. Instructing a specialist drink driving solicitor promptly ensures that all relevant evidence is preserved and that the strongest possible challenge is prepared.