Carbon Monoxide at Work: A Winter Checklist
When the bay doors close, the heaters fire up and the propane forklift keeps running, carbon monoxide builds in places that were fine all summer. You can't see or smell it, and it can drop a crew fast. Here's how to get ahead of it.
I've worked enough northern winters to know the pattern. In October somebody wheels a salamander heater into the shop, a contractor fires up a gas-powered saw inside a half-closed building, and the crew starts "warming up" trucks in the bay. None of it feels dangerous. That's the problem.
A reminder from October
On October 1, 2026, three concrete workers in Monona, Wisconsin, were taken to the hospital after crews used gasoline-powered equipment inside a commercial space under renovation and left it running while they worked, WMTV15 News reported. Firefighters measured more than 270 ppm just inside the back door. The fire chief said the levels "would have been deadly" if the workers had stayed much longer, and that his department gets more carbon monoxide calls in winter. All three were expected to recover. Plenty of CO cases don't end that well.
Know the numbers
OSHA's general industry permissible exposure limit for carbon monoxide is 50 ppm as an 8-hour time-weighted average (29 CFR 1910.1000, Table Z-1). Construction uses the same 50 ppm figure (29 CFR 1926.55, Appendix A). NIOSH recommends a lower limit of 35 ppm averaged over a shift with a 200 ppm ceiling, and lists 1,200 ppm as immediately dangerous to life or health (NIOSH Pocket Guide).
Those are exposure limits, not comfort levels. The Monona reading was more than five times OSHA's average limit, at the door.
Where CO comes from at work
OSHA's carbon monoxide fact sheet names the internal combustion engine as one of the most common sources of workplace exposure. It also lists warehouses, boiler rooms, docks and refineries, and jobs like forklift operator, garage mechanic, welder and diesel engine operator. In winter, the usual suspects are:
Fuel-fired temporary heaters in shops, tents and buildings under construction.
Propane and gasoline forklifts working in closed warehouses and freezers.
Gas-powered saws, pumps, power trowels and pressure washers used indoors.
Vehicles idling in bays to warm up or to run hydraulics.
Boilers and space heaters that haven't been serviced since last spring.
A winter CO checklist
1. Find every combustion source before the cold sets in
Walk the building with the maintenance lead and list every fuel-burning heater, engine and appliance. Note where exhaust goes and what ventilation runs when the doors are closed. If you can't answer that for a piece of equipment, it doesn't run indoors until you can.
2. Swap or vent the worst offenders
OSHA's fact sheet recommends switching from gasoline-powered equipment to electric, battery or compressed-air tools where that can be done safely, and prohibiting gasoline engines in poorly ventilated areas. For tools that must stay fuel-fired, put the engine outside, duct the exhaust, or add mechanical ventilation, and check it works with the doors shut.
3. Put monitors on people, not just walls
A fixed detector by the office door won't help the operator at the back of the freezer. The fact sheet recommends personal CO monitors with audible alarms where exposure is possible, along with regular air testing. Bump-test them, and make the alarm response simple: stop the equipment, get out, report it.
4. Treat confined spaces as their own hazard
CO collects in pits, vaults, tanks and crawlspaces. Under 29 CFR 1910.146, permit spaces need atmospheric testing before entry. Mark them clearly. Our Danger Confined Space labels are OSHA-style 5" x 7" labels for hatches, doors and access points.
5. Train people on symptoms and on rescue
Headache, dizziness, drowsiness and nausea are the early signs OSHA lists. They're easy to blame on a long shift. Teach crews to treat them as a CO warning, and to never rush in after a collapsed coworker without air monitoring and proper respiratory protection. OSHA warns that would-be rescuers can be exposed to fatal levels too.
Signs that do real work
Signs won't ventilate a building, but they put the warning where the decision gets made: at the door of a boiler room, on the cage of a fuel-fired heater, or on the wall of a battery and fuel area. OSHA's sign rule, 29 CFR 1910.145, reserves DANGER signs for immediate hazards, and a CO-rich room qualifies.
Post our DANGER Carbon Monoxide labels at boiler rooms, mechanical rooms, loading docks and anywhere engines run indoors.
They're part of our line of OSHA safety labels, and they're OSHA-style labels sized at 5" x 7".
Where propane cylinders are stored or exchanged for forklifts and heaters, the DANGER Propane No Smoking labels are OSHA-style and belong on the cage.
Running a distribution center with closed docks all winter? Browse our warehouse safety signs for the rest of the building.
Contractors heating an enclosed building for concrete or drywall work should check our construction safety labels too.
For other gas and vapor hazards, including nitrogen, H2S and benzene, see our chemical hazard labels.
The bottom line
Carbon monoxide doesn't give you a second chance to notice it. Inventory your combustion sources, move or vent the engines, monitor the people closest to the exhaust, and mark the rooms where CO can build up. Do it before the first cold snap closes the doors for the season.
VLG labels are printed in house in Kenai, Alaska, a place that knows a thing or two about closed-up winter shops. They're made on premium low-shrinkage calendered vinyl with an industrial strength adhesive and Printed in the USA. Bulk and custom orders are available; minimums apply.
Mark your high-hazard rooms before winter: shop our danger labels.
This article is general safety guidance, not legal advice. Exposure limits, monitoring and ventilation requirements depend on your operation, industry and location, and State Plan states may differ. Check the current standards linked above and consult a qualified safety professional or industrial hygienist about your specific site.
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