Blood Alcohol Concentration (BAC)
Blood alcohol concentration, or BAC, is the measure of alcohol present in a person's blood, expressed in milligrams of alcohol per 100 millilitres of blood. Canadian criminal law fixes the threshold for impaired-related charges at 80 mg per 100 mL. Driving with a BAC at or above 80 mg within two hours of operating a conveyance is an offence under section 320.14(1)(b) — the over 80 offence.
Mass Tsang's DUI lawyers examine every step of the BAC evidence chain. For more, see our blog posts on the over 80 rule and how to fight an impaired driving charge.
How BAC is measured
Two measurement methods matter at law. Roadside Approved Screening Devices (ASDs) provide a PASS, WARN, or FAIL reading that gives officers grounds to investigate further — they do not produce a court-admissible BAC. Evidentiary breath testing at the police station, on an Approved Instrument operated by a qualified technician, produces the BAC readings that prove the offence. Two samples are taken at least 15 minutes apart; the lower reading is used in court.
Blood samples
Where breath testing isn't possible — typically in serious crashes where the driver is hospitalized — a blood sample can be taken under section 320.28. The sample is analyzed and converted to a BAC reading. Blood-based prosecutions involve additional procedural and expert requirements.
The warn range and administrative penalties
Below the criminal threshold, Ontario imposes administrative penalties on drivers in the "warn range" (50–79 mg). Penalties include immediate licence suspension, vehicle impoundment, fees, and remedial measures for repeat offenders. Warn range is not a criminal charge — but it is not nothing.
Challenging BAC evidence
Defending an over 80 case often involves challenging the BAC evidence. Common attacks include: timing of breath samples ("as soon as practicable"); calibration and maintenance of the Approved Instrument; qualifications of the technician; chain of custody; statutory presumptions; and the lawfulness of the underlying stop and demand. Each angle, properly pursued, can break the Crown's case.
Related glossary terms
