Toolbox Talk: Propane Heaters and Carbon Monoxide on Winter Pours

Short answer: Propane heaters produce carbon monoxide, and in a tented or enclosed pour it builds up fast. OSHA's 29 CFR 1926.154(a) requires enough fresh air to keep workers safe, with mechanical ventilation if natural air isn't enough. Exposure must stay under OSHA's 50 ppm 8-hour limit (29 CFR 1926.55). Heaters stay 10 feet from tarps.

Below is a ready-to-deliver talk for concrete crews, framers and anyone working inside heated enclosures this winter. It runs about ten minutes. Read it through once, fill in the bracketed site details, and keep the sign-off sheet at the bottom.

What do the OSHA rules say about temporary heaters?

The rules are spread across four sections of 29 CFR Part 1926. None of them gives a ventilation rate in cubic feet per minute. They set a performance standard (enough fresh air to keep people healthy) plus hard distances.

Your state may run its own OSHA-approved plan with stricter rules, and the heater manufacturer's instructions apply on top of all of this.

The toolbox talk (about 10 minutes)

1. Why we're talking about this (1 minute)

"We're tenting and heating [the footings / the slab / the second floor] this week. The heaters keep the concrete and us warm. They also make carbon monoxide.

CO has no color, no smell and no taste. OSHA says large amounts can knock you out in minutes without warning. Propane is not a 'clean' exception. NIOSH's own toolbox talk says all fuels, even propane, make carbon monoxide."

2. What it does to you (1 minute)

"CO pushes oxygen out of your blood. OSHA lists the early signs as headache, tiredness, dizziness, drowsiness, nausea and tightness across the chest. Later it's vomiting, confusion, collapse and passing out.

The trap is that early CO feels like a normal cold, tired winter day. If two or three people inside the same tent all have a headache, that's not a coincidence. Even people who recover can be left with permanent heart and brain damage."

3. How much is too much (1 minute)

"OSHA's limit is 50 parts per million averaged over an 8-hour shift. NIOSH recommends 35 ppm and says never go above 200 ppm, even briefly. At 1,200 ppm NIOSH calls it immediately dangerous to life.

We can't see any of those numbers, so we measure. [Name] has the CO monitor. It alarms at [setting]. If it alarms, we don't argue with it."

4. Fresh air, every time (2 minutes)

"The rule is simple. If the heater is burning inside an enclosure, fresh air has to get in. If natural air isn't enough, we use mechanical ventilation. On this job that means [the fan at the north end / the duct through the tarp / the open bay].

• Don't close up openings to hold heat in without telling me.
• Don't add a second heater without asking. More heaters means more CO in the same air.
• Gas-powered saws, trowels, generators and pumps add CO too. OSHA says not to run gas engines in enclosed spaces. Generators stay outside, away from openings."

5. Heater setup (2 minutes)

"Before any heater gets lit:

• At least 10 feet from the poly and tarps, and the tarps are tied down so wind can't blow them onto it.
• At least 6 feet from the propane cylinder, and blower or radiant heaters never pointed at a cylinder within 20 feet.
• Set level, on solid ground or proper insulation if it's a wood floor.
• Flame-failure shutoff working. If it's been bypassed, taped, or the heater keeps going out, tag it and tell me.
• Hoses and regulators in good shape. Don't use the regulator or hose to hold the heater up.
• Fire extinguisher within 50 feet. Ours is [location].
• Spare cylinders stay outside. Never store propane inside the building."

6. If someone goes down (2 minutes)

"OSHA's steps:

1. Get them out to fresh air in an open area.
2. Call 911. Our site address is [address].
3. If they're not breathing, start CPR if you're trained.

Don't run back into a tent that's full of CO to drag someone out without thinking. OSHA warns rescuers can be overcome too. Shut the heater off from outside if you can, open the enclosure, and let the monitor tell you when it's safe.

Tell your doctor you may have been exposed to CO if you get sick later."

7. It's bad for the concrete too (30 seconds)

"ACI 306R-16 defines carbon monoxide as a gas in heater and engine exhaust that can cause dusting of concrete surfaces less than 24 hours old. A badly vented heater can cost us the slab surface as well as a trip to the hospital."

Discussion questions

Pick two or three and let the crew answer before you do:

1. Three people inside the tent have headaches by mid-morning. What do we do?
2. The wind keeps blowing the poly toward the heater. What's the fix?
3. Why isn't propane a safe fuel just because it's cleaner than gasoline?
4. The CO monitor alarms but everyone feels fine. Do we keep working?
5. Where is the nearest fire extinguisher right now?
6. What's the difference between a cylinder that's connected and in use inside, and one that's being stored?

Sign-off record

Keep a record of who attended. It's evidence the hazard was covered under 29 CFR 1926.21(b)(2), which requires instructing each employee in recognizing and avoiding unsafe conditions.

Questions raised and follow-up actions: ______________________________

How much ventilation does OSHA require for a propane heater?

OSHA doesn't give a number. 29 CFR 1926.154(a)(1) requires fresh air "in sufficient quantities to maintain the health and safety of workmen" and mechanical ventilation where natural air is inadequate. That makes the heater manufacturer's instructions and a CO monitor your practical test. OSHA's fact sheet tells employers to provide personal CO monitors with audible alarms where exposure is possible and to test the air regularly.

Can propane cylinders be used inside a building under construction?

Yes, with conditions, but they can't be stored there. 29 CFR 1926.153(h) allows containers inside when keeping them outside is impracticable. It requires excess flow valves, approved regulators, hose rated to at least 250 psig, and an individual container no larger than 245 lb water capacity (nominal 100 lb LP-gas). Two or more heater-container units on the same unpartitioned floor need their containers at least 20 feet apart (1926.153(h)(14)). Storage of LPG within buildings is prohibited under 1926.153(j).

Are salamanders and oil-fired heaters covered too?

Yes. Solid fuel salamanders are banned in buildings and on scaffolds (1926.154(d)). Oil-fired heaters need a primary safety control that stops fuel flow on flame failure (1926.154(e)(1)). Portable LP heaters, including salamanders, need an approved automatic shutoff on flame failure (1926.153(h)(8)).

Asking on site

A foreman can ask Verika "how far does the heater have to be from the poly?" and get 10 feet, with 29 CFR 1926.154(b)(4) cited. If your company has its own heater or enclosure procedure loaded, Verika answers from that too and shows both sources. If nothing on file covers the question, it says so. The concrete side of heated pours is covered on our Wisconsin cold-weather concreting answer and our guide to how long to keep blankets on new concrete. If you're thawing ground under heaters before a pour, see can footings be placed on frozen ground. Heater safety is also talk #1 in our winter toolbox talk topics.

Sources

1. 29 CFR 1926.154, Temporary heating devices — eCFR, current as of October 2026
2. 29 CFR 1926.153, Liquefied petroleum gas (LP-Gas) — eCFR, paragraphs (h) and (j)
3. 29 CFR 1926.150, Fire protection — eCFR, paragraph (c)(1)(vi)
4. 29 CFR 1926.55, Gases, vapors, fumes, dusts, and mists (Table 1: carbon monoxide) — eCFR
5. 29 CFR 1926.21, Safety training and education — eCFR
6. Carbon Monoxide Poisoning fact sheet (DSG FS-3522) — OSHA, 2012
7. NIOSH Pocket Guide to Chemical Hazards: Carbon monoxide — NIOSH
8. Construction Toolbox Talk: Carbon Monoxide Poisoning, Publication No. 2022-130 — NIOSH and CPWR, August 2022
9. ACI 306R-16, Guide to Cold Weather Concreting (preview, chapter 2 definitions) — American Concrete Institute, 2016