Carbon Monoxide and Combustion Gas Safety for the Maker Shop: Forges, Generators, and Engine-Powered Tools Indoors

This site's shop safety content covers combustible dust, chemical and solvent storage, fire detection, and respirators for airborne particulates — all real hazards, and none of them the one that actually kills people most often in small home shops: carbon monoxide. Dust explosions and solvent fires are dramatic and get the attention; a backup generator or a coal forge quietly filling an attached garage with CO while you work is far more common and far more lethal, precisely because it's invisible, odorless, and gives almost no warning before it causes confusion and loss of consciousness.

Where CO Actually Comes From in a Maker Shop

• Portable generators run during a power outage — by far the most common cause of home CO deaths in general, and directly relevant to anyone running this site's backup power setup for a shop. A generator produces far more CO per hour than people expect, and running one in a garage "with the door open" is still frequently fatal; CO from a generator in an attached garage can reach lethal concentrations in the house itself within an hour even with the big door open, because an open garage door is not remotely equivalent to true outdoor ventilation.
• Coal and propane forges used for blacksmithing, bladesmithing, or casting work covered elsewhere on this site — both produce meaningful CO, and a forge running in a garage or shed with the door down is a genuine hazard even though the fire itself looks and smells like it's "venting fine."
• Gas or propane-powered tools run indoors: pressure washers, cut-off saws, and generators are the big ones, but any small gas engine run in an enclosed space applies.
• Space heaters that burn fuel (kerosene, propane) rather than running on electricity, used to keep an uninsulated shop warm in winter — a seasonal risk that spikes exactly when shop doors and windows are sealed up tightest against the cold.
• Vehicles or small engines left idling inside a garage while you work on something unrelated nearby, even briefly.

Why It's So Dangerous

Carbon monoxide binds to hemoglobin roughly 200–250 times more readily than oxygen does, which means even a modest concentration in the air steadily displaces your blood's actual oxygen-carrying capacity over time. Early symptoms — headache, dizziness, fatigue, nausea — are easy to mistake for being tired, dehydrated, or coming down with something, especially in a shop environment where you're already exerting yourself and breathing other fumes. By the time symptoms are severe enough to be unmistakable, judgment and coordination are often already impaired enough to make self-rescue difficult. There is no smell, no color, and no irritation to warn you — unlike smoke or a chemical odor, your body gives you no natural early warning system for CO at a biological level.

Approximate CO LevelEffect 9 ppmASHRAE's recommended maximum for sustained indoor air quality — well below anything symptomatic, but the benchmark "clean air" target 50 ppmOSHA permissible exposure limit for an 8-hour workday 200 ppmMild headache within 2-3 hours 800 ppmDizziness, nausea, convulsions within 45 minutes; possible unconsciousness within 2 hours 1600+ ppmHeadache, dizziness within 20 minutes; death possible within 1-2 hours

These figures are approximate and vary by individual and exposure duration — they illustrate the shape of the danger curve, not a precise medical threshold, and should not be treated as a safe-exposure guide.

What Actually Works

• A UL 2034-listed CO detector in the shop itself, not just the one in your house's hallway. A detector in the living room will not alert you in time if you're the one standing next to a forge or a generator in a detached shop or garage — put a dedicated unit where you actually work. Battery-backed plug-in units are inexpensive and this is one safety item with no good reason to skip.
• Never run a generator, gas engine, or fuel-burning heater in an attached garage, basement, or any enclosed shop, door open or not. "Enclosed" includes a one-car garage with the big door up but the house-side door shut — CO migrates through small gaps and HVAC returns far more readily than people expect. Generators belong outdoors, well away from windows, doors, and vents, running downwind of the structure.
• Forges need real, moving ventilation, not just an open door — a flue, hood, or powered exhaust that actually removes combustion gas from the space, the same category of active ventilation this site's shop articles already recommend for laser and 3D printer fumes, but sized for combustion byproducts rather than VOCs or ultrafine particulate.
• Know the early symptoms and treat them as a reason to get outside immediately, not push through the headache to finish a project. If a headache, dizziness, or nausea shows up only while you're in the shop and clears once you step outside, that pattern itself is diagnostic — don't go back in without first finding and fixing the source.
• A battery or crank-powered standalone detector is worth having specifically for generator use during outages, since that's exactly when you might not have reliable power to a plug-in unit at the location the generator is actually running.

If You Suspect Poisoning

Get into fresh air immediately and call emergency services — CO poisoning is a medical emergency, and symptoms can continue to worsen even after exposure stops. Don't wait to see if it passes on its own, and don't go back into the space to shut off the source yourself until it's been ventilated; let the people responding handle that once you're safely clear.

Dust collection, fire suppression, and chemical storage get a lot of attention in shop safety content because they're visible, tangible hazards you can see and smell the risk of. CO asks for the opposite kind of vigilance: a cheap detector and a hard rule about what never runs in an enclosed space, maintained precisely because the hazard itself gives you no sensory warning to rely on.