In alcohol-related medico-legal cases, the Henry's Law constant is used to calculate blood alcohol concentration (BAC) from expired breath alcohol. The standard blood:breath ratio used in breath analysers is:
- A 100:1
- B 2100:1 ✓
- C 1000:1
- D 5000:1
Correct answer: B. 2100:1
Explanation
Breath analysers convert expired alveolar breath ethanol concentration to estimated BAC using Henry's Law and an assumed blood:breath partition ratio of 2100:1 (i.e., 2100 mL of alveolar breath contains the same amount of ethanol as 1 mL of blood). This ratio is a population average and can vary from 1700:1 to 2400:1 between individuals and with temperature, leading to potential errors. In India, the Motor Vehicles Act prescribes a blood alcohol limit of 30 mg/100 mL; breath equivalent is approximately 14.3 micrograms/100 mL expired breath.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.