That exhaust
smell.
Start with the thing people most often get wrong: carbon monoxide has no smell. It is colourless and odourless. If a garage smells of exhaust, you are smelling the other products of combustion, unburned fuel and hydrocarbons, and in diesel the particulate as well.
That does not make the smell useless. It makes it a warning light. If exhaust fumes are lingering in an enclosed garage, ventilation is not clearing the air, and carbon monoxide is part of what is not being cleared. The smell tells you the air is not turning over. It tells you nothing at all about how much CO is in it.
What to do first
- If anyone has symptoms (headache, dizziness, nausea, confusion, unusual fatigue), get them to fresh air and call emergency services. Do not diagnose this yourself.
- Check whether the exhaust fans are running. Stand in the garage and listen, then confirm at the fan room or the building management system.
- Check the CO panel for an alarm, fault or trouble condition. A panel in fault means the system that is supposed to be running those fans is not monitoring anything.
- Look for a source. Vehicles idling at a loading dock or in a queue, a delivery truck left running, generator testing, or nearby construction.
Why a garage stops clearing exhaust
In an enclosed garage the fans should either run continuously or be brought on automatically by CO detection (IBC §406.4.2 with the mechanical code provisions). When fumes are hanging around, one of those two things has broken:
- The CO sensors have drifted low and are not reaching setpoint, so the fans never get called
- Sensors are faulted, expired or, occasionally, still bagged from a wash-down
- The fans themselves are failed, belt-slipped, or their drives are not responding
- Setpoints or programming were changed at some point and nobody has verified them since
A drifted-low sensor is the quiet failure mode and the one worth worrying about. The panel displays a comfortable number, the fans stay off, and nothing indicates a problem until someone notices the air.
The limit you are actually being measured against
Cal/OSHA sets the permissible exposure limit for carbon monoxide at 25 ppm as an 8-hour time-weighted average, with a 200 ppm ceiling (Title 8 §5155, Table AC-1). Wherever people work, including a parking structure, that limit applies. A monitoring system that is not reading accurately cannot demonstrate you are under it, which matters for staff safety and for any claim that follows an incident.
What a service visit should establish
Not just whether the panel powers up. Apply certified span gas to every sensor, confirm each one reads correctly and drives the ventilation response at setpoint, verify the fans actually start, replace end-of-life sensors, and issue a dated report with each sensor listed. Fire code requires that inspection and test at least annually in any case (California Fire Code §916.11).
ACES Monitoring does gas detection exclusively, in enclosed garages, warehouses and commercial buildings nationwide.
Sources: Cal/OSHA Title 8 §5155, Table AC-1 · IBC §406.4.2 and IMC ventilation provisions · California Fire Code §916.11.
Common questions
Can you smell carbon monoxide?
No. Carbon monoxide is colourless and odourless. What you smell in a garage is the rest of the exhaust: unburned fuel, hydrocarbons and, in diesel, particulate. The smell means exhaust is accumulating rather than being cleared, and CO is part of that exhaust even though it has no odour of its own.
Is an exhaust smell in a parking garage dangerous?
It means ventilation is not keeping up, which is worth acting on immediately. Cal/OSHA sets the CO exposure limit at 25 ppm as an 8-hour time-weighted average with a 200 ppm ceiling (Title 8 §5155). You cannot judge against those numbers by smell, only with a working monitoring system.
What are the symptoms of CO exposure?
Headache, dizziness, nausea, confusion and fatigue, often described as flu-like. Anyone experiencing these in or near a garage should get to fresh air immediately, and emergency services should be called if symptoms are significant.
Who should I call about garage fumes?
If people have symptoms, emergency services first. Then a gas detection specialist to test whether the CO system is reading correctly and driving the fans, because a smell that persists usually means it is not.
Talk to a gas monitoring specialist
Tell us the building and how many sensors you have. We will tell you what service it needs and what it costs, no pressure.