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Toolbox talk · 10 min · Electrical

Portable Generators and Carbon Monoxide

Hazards: Carbon monoxide poisoning from engine exhaust · Exhaust drawn back inside through doors, windows and vents · Backfeed into building wiring and utility lines · Fire and burns from refueling a hot generator

Why it matters

Carbon monoxide is colorless, odorless and tasteless, and it kills at levels you will never smell. The CPSC's February 2025 report attributes an estimated 858 non-fire CO deaths to portable generators since 2011 — 42% of all consumer-product CO deaths. Those are consumer numbers, not a workplace count, but the mechanism on a jobsite is identical: an engine running where the exhaust cannot leave.

On November 3, 2023, a door installer was fitting the second of four garage doors at a house (OSHA Accident Search, Summary Nr 161385.015). A gasoline generator was running inside the closed garage — not his generator; another subcontractor was using it to power dehumidifiers and fans. The only way in or out of that garage was the door he was installing. He was overexposed to CO and died.

Key points

  • Never indoors, never enclosed. OSHA FS-3286: not in garages, crawl spaces or basements. A partly open space is an enclosed space.
  • Open doors are not ventilation. OSHA states that open windows and doors may NOT prevent CO from building up when a generator sits in an enclosed space.
  • 3 to 4 feet clear, on every side and above. And never placed where doors, windows or vents can pull the exhaust back inside.
  • It is not always your generator. The 2023 case was another trade's machine. Ask what is running before you work in a closed room.
  • Headache, dizziness, nausea — leave. Get to fresh air and get medical attention. Do not go back in until someone trained and equipped says it is safe.
  • No backfeed. Never plug a generator into a building outlet. Only a qualified electrician with a transfer switch ties one to building wiring.

Do / Don't

Do

  • Put the generator outside, away from every opening, before you start it.
  • Run a CO monitor in any space where an engine is working.
  • Shut it down and let it cool before refueling.
  • Tell the other trades where your exhaust is going.

Don't

  • Roll a generator into a garage or basement "just out of the rain".
  • Set it against the wall next to the door you are working through.
  • Walk off a headache in the same room you got it in.
  • Enter to rescue someone without air monitoring and respiratory protection.

Quick site check

  • Walk to every running engine on this site: where is the exhaust going, and who is downwind of it?
  • Any gas-powered saw, compressor, pump or heater working inside a structure today?
  • Do the cords off the generator have GFCI protection, and is the fuel stored away from the hot engine?
Discussion — ask the crew
  1. Where is the generator running on this site right now, and what is downwind or downhill of it?
  2. Is anyone working in a garage, a basement, a crawl space or a partly enclosed room today that another trade is powering?
  3. Have you ever had a headache or felt lightheaded on a job and put it down to the heat or a long night?
  4. Do we have a CO monitor on this site, who carries it, and when was it last bump tested?
  5. Who is allowed to connect a generator to a building's wiring here, and what happens if nobody is?
Refs: OSHA Fact Sheet — Using Portable Generators Safely (FS-3286): shock, CO, fire, noise; the 3 to 4 foot clearance · OSHA Alert 4105 — Preventing Carbon Monoxide Poisoning While Working with Portable Generators · OSHA Fact Sheet — Carbon Monoxide Poisoning (FS-3522): symptoms, the 50 ppm PEL, controls · OSHA Accident Search — Summary Nr 161385.015, event date 11/03/2023 · CPSC — Non-Fire Carbon Monoxide Deaths Associated with the Use of Consumer Products, 2021 Annual Estimates (February 2025)Sign-in sheet included in the printed version

Frequently asked questions

OSHA's fact sheet on portable generators says a generator needs 3 to 4 feet of clear space on all sides and above it for ventilation, and must not be placed near doors, windows or vents where CO could enter and build up in occupied spaces. It is never used indoors or in enclosed spaces.

No. OSHA states directly that open windows and doors may NOT prevent CO from building up when a generator is located in an enclosed space. Enclosed and semi-enclosed means the generator goes outside, not near an opening.

OSHA's permissible exposure limit for CO is 50 parts per million, averaged over an 8-hour period. CO has no color, odor or taste, so the only way to know a level is to measure it.

Headache, fatigue, dizziness, drowsiness or nausea, often with tightness across the chest. With prolonged or high exposure, vomiting, confusion, collapse, loss of consciousness and muscle weakness follow. Get to fresh air and get medical attention.

Not without a transfer switch installed by a qualified electrician. OSHA warns that attaching a generator directly to a building electrical system without a properly installed transfer switch can energize wiring for great distances and create an electrocution risk for utility workers and others.

Sources & references

Content checked against these sources — last reviewed August 28, 2026.

Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.

By — Editorial team of SteelToeTools.com (published by LSEA SAS) Updated