Carbon Monoxide From Your Own Tools

Why this matters

Techs who test appliances for carbon monoxide every week will run a gas cut-off saw in an attached garage with the overhead door half up, or set a generator beside the basement window, and never think of their own tool as a combustion appliance. It is one, and an unusually dirty one: a small gasoline engine makes far more carbon monoxide (CO) than a tuned furnace, it has no flue, and it runs right where the operator is breathing. NIOSH has documented workers killed by small gasoline engines in spaces with doors and windows open, because "ventilated" by feel was nowhere near enough.

This card is about the CO your own equipment makes. Testing a customer's appliances and responding to a CO alarm in the home is owned elsewhere: see related: Carbon Monoxide Testing + Response Reference. Diesel exhaust from an idling truck has its own card: see related: Diesel Exhaust and the Idling Truck.

What CO does, briefly

CO is colorless and odorless. It binds to hemoglobin far more tightly than oxygen does, so the blood carries less oxygen to the heart and brain while breathing feels normal. The early signs a coworker can observe are the vague ones: a headache, dizziness, nausea, unusual tiredness, a tech who is slow to answer or making simple mistakes. At higher levels come confusion, collapse and death. Two things make the trades worse off than a person resting at home: exertion raises the breathing rate, so a tech wrestling a saw takes in more CO per minute than a person sitting on a couch, and the tech is at the source, where the concentration is highest. The vocabulary of dose and route is owned by the exposure fundamentals card; see related: Dose, Route, and the Exposure You Cannot See.

The numbers that apply to a worker

A TWA can hide a dangerous peak. A worker who spends 45 minutes at 180 ppm and the rest of the day near background has a respectable-looking average. The NIOSH ceiling exists because the peak is what drops someone. Manage to the ceiling during the task, not to the average at the end of the day.

Why the alarm on the wall sounds after you are hurt

A residential CO alarm is built to UL 2034, which is written for occupants, not for workers. It is designed not to sound at low levels, so a house is not evacuated every time someone lights a gas range. The response windows are roughly these: at 70 ppm, alarm between 60 and 240 minutes; at 150 ppm, between 10 and 50 minutes; at 400 ppm, between 4 and 15 minutes. An alarm that is working exactly as listed can sit silent for an hour at 70 ppm, twice the NIOSH limit and over the OSHA one, and for many minutes at levels near the NIOSH ceiling.

Then add placement. The alarm is in the hallway or a bedroom; the tech is beside the engine in the garage or basement, at several times the concentration that reaches the alarm. By the time the hallway unit sounds, the person at the source has been above the worker limits for a long time. The customer's alarm is not your monitor.

A personal monitor is the control that sees it

A personal CO monitor, a small single-gas instrument with an electrochemical sensor clipped near the breathing zone, reads the air you are actually in, in parts per million, and alarms on settings you choose. A common setup is a low alarm at 35 ppm (the NIOSH REL) and a high alarm at 200 ppm (the NIOSH ceiling), with TWA and short-term readings logged. Bump test it before use, per its manufacturer, by exposing it briefly to a known gas and confirming it alarms; a monitor with a dead sensor reads a believable zero.

When it alarms, the rule is the same every time: shut off the engine if your hand is on it, everyone walks out to fresh air, and nobody goes back in until the monitor reads below 35 ppm at the doorway with the space ventilated. A high alarm is not a cue to finish the cut.

Where your own tools make it

• Gas cut-off saws and concrete saws in a garage, basement or enclosed patio. Also a silica task; see related: Respirable Crystalline Silica and the Construction Standard. The fix that solves both is an electric or battery saw with water feed.
• Portable generators in a garage, under a window, beside a door, near a furnace or water heater's combustion air intake, or near an HRV or ERV fresh-air intake that pulls outdoor air into the house. CDC advises keeping generators at least 20 feet from windows, doors and vents.
• Gas pressure washers in a garage, a basement, a parking structure or a carport against a wall. The engine runs for long stretches and the operator stands within a few feet of the exhaust.
• Gasoline water pumps pulled into a flooded basement during water restoration, where the basement is also the place nobody can leave quickly.
• Truck-mounted carpet extraction units whose exhaust sits at the curb a few feet from an open front door, with the hoses holding the door open for an hour. The exhaust drifts in; the house's own exhaust fans can pull it in.
• Propane floor burnishers and buffers in a closed store or a large room overnight. Propane burns cleaner than gasoline but a poorly tuned or long-running unit still makes CO.
• Unvented propane or kerosene construction heaters in a closed-in addition or a basement being finished. Under 29 CFR 1926.154, fresh air must be supplied in sufficient quantity to maintain the health and safety of workers, and special care is required for ventilation when heaters are used in confined spaces.
• Plate compactors and gas trenchers in a trench or an excavation, where CO builds up in the still air because nothing moves it out.

The occupants are the second exposure. They are not your employees, OSHA does not reach them, and your duty to them is ordinary care: a generator that is fine for the crew outdoors can still be feeding a bedroom window.

A worked afternoon: one saw, one garage

A plumbing crew cuts a trench in an attached garage slab to reroute a drain. The saw is a gas cut-off saw. Overhead door raised about two feet for "ventilation," the door to the kitchen closed. The tech wears a personal monitor with alarms at 35 and 200 ppm. The readings below are illustrative of what such a monitor might log; they are not measurements of any particular saw.

• The cut: 45 minutes of intermittent cutting, with the monitor averaging about 180 ppm over that time and peaking at 260 ppm. The low alarm sounded within the first minutes and was acknowledged; the high alarm sounded at the peak.
• The rest of the day: 7 hours 15 minutes of other work at about 5 ppm.
• The TWA: (0.75 hours times 180 ppm plus 7.25 hours times 5 ppm) divided by 8 hours, which is (135 plus 36.25) divided by 8, or about 21.4 ppm.

On paper the tech is under both the OSHA PEL of 50 and the NIOSH REL of 35 for the day. That is the misleading half. The NIOSH ceiling of 200 ppm was exceeded during the cut, and the high alarm said so in real time; the correct response was to stop cutting and walk out at that alarm, not to finish the trench and read a reassuring TWA afterward.

Inside the house. The kitchen door was closed but not sealed. Say the CO that migrated in held the kitchen near 60 to 80 ppm for an hour or so. A UL 2034 alarm in the hallway might sound at 70 ppm anywhere from 60 to 240 minutes in, so it may never have sounded at all. A family member napping in the next room was exposed at levels above the worker limits with no warning. No regulation required the crew to measure that; the crew's duty of care did.

What should have happened, in hierarchy order: substitute an electric saw with water feed (a battery saw on a short slab cut, a corded wet saw on a long one), which removes the CO at its source and controls the silica. If the gas saw were unavoidable, it belongs outdoors, cutting an exterior slab, or the garage needs mechanical exhaust to outdoors, not a two-foot gap under the door, plus the kitchen door sealed with tape and the occupants out of the adjoining rooms. See related: How to Ventilate a Small Space You Are Working In.

What changes the answer. A fully open overhead door on a breezy day and a short cut may keep the monitor under 35 throughout, which is the reason to wear the monitor rather than to assume the worst or the best. A detached garage removes the occupant exposure. The same saw in a basement makes everything worse, because there is no door to raise.

If someone shows signs

• Get everyone out to fresh air and shut the engine off from where you stand only if you can do so without staying in the space.
• Call 911 for anyone who is confused, faints, has chest pain or trouble breathing, or does not improve quickly in fresh air. Say carbon monoxide is suspected. Poison Control (1-800-222-1222 in the U.S.) can advise for milder cases.
• Do not let the person drive. CO affects judgment, and the symptoms can return.
• Ventilate before anyone goes back in, and read the monitor at the door. Re-entry waits until it reads below 35 ppm.
• Follow mainstream first aid guidance (American Red Cross, American Heart Association) while waiting for responders. Describe what you saw to them; do not try to judge yourself how badly someone was exposed. Hands-on first aid certification is what makes a responder.

References

• 29 CFR 1910.1000, Table Z-1, and 29 CFR 1926.55 with its Appendix A, the carbon monoxide permissible exposure limit in general industry and construction
• 29 CFR 1926.154, Temporary Heating Devices (construction)
• NIOSH Pocket Guide to Chemical Hazards, carbon monoxide (REL, ceiling and IDLH); NIOSH Alert on preventing carbon monoxide poisoning from small gasoline-powered engines and tools
• UL 2034, Standard for Single and Multiple Station Carbon Monoxide Alarms, for the response windows a residential alarm is listed to; CDC guidance on generator placement
• See related: Carbon Monoxide Testing + Response Reference; Diesel Exhaust and the Idling Truck; The Hierarchy of Controls on a Service Call; How to Ventilate a Small Space You Are Working In