Carbon Monoxide Headquarters

Considerations in setting CO Threshold:

Low enough to Warn / Turn-off engine before motor vehicle occupant(s) are harmed.

High enough that even extreme driving conditions (eg. traffic congestion, road tunnels, parking garages) or smoking would not produce "false" alarms.

Sydney Tunnel - 150 ppm CO peak value (ref. Routley & Sugo, 1997)

Cahill Expressway - 80 ppm CO peak value (ref. Routley & Sugo, 1997)

Public parking garages - 100-200 ppm (ref. general literature)

Low enough to protect all subgroups of the population, especially those at highest risk (CHF, CAD, fetal, very old, etc.)

EPA (USA), WHO - 9 ppm (TWA - 8 hrs.)

ACGIH (USA)- 25 ppm (TWA - 8 hrs.)

NOHSC (Australia) - 30 ppm (TWA - 8 hrs.)

NIOSH, OSHA (USA) - 35-50 ppm (TWA - 8 hrs.)

NOHSC (Australia) - 100 ppm (STEL)

Low enough to allow for the interactive effects of CO₂, depleted oxygen, heat, etc., all factors that speed CO uptake and increase lethality.

Provide multiple thresholds to provide -

Low alarm warning

High alarm warning

Engine shut-down

10% COHb may be too low for practical reasons, notwithstanding some of the considerations above, while 20% COHb is almost certainly too high.