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Toolbox Talk

Atmospheric Testing and Air Monitoring

The air is what kills most people in confined spaces, and you can't judge it by looking. Testing turns an unknown atmosphere into a known one. It has to happen before entry, from outside the space, and it has to keep happening the whole time anyone is inside.

Use a calibrated, direct-reading meter, and follow the same order every time — the sequence matters because one reading affects how you read the next.

Test in the right order

Always test the atmosphere in this order:

  • Oxygen first — a meter can't read other gases correctly if oxygen is off.
  • Flammable gases and vapors second.
  • Toxic contaminants third.

Test top to bottom, too. Gases layer — some are heavier than air and pool at the bottom, some are lighter and collect at the top. Lower the probe through the full depth of the space, not just the opening.

Know the numbers

Acceptable entry conditions:

  • Oxygen — 19.5% to 23.5%. Below is deficient; above is enriched and a fire risk.
  • Flammable gas or vapor — below 10% of the LEL.
  • Toxic gases — below their permissible exposure limits, such as hydrogen sulfide and carbon monoxide.

If any reading is out of range, no one enters until ventilation brings it back and a retest confirms it.

Use the meter correctly

  • Calibrate and bump-test the meter per the manufacturer — a meter that isn't calibrated is worse than none, because it lies with confidence.
  • Let the readings stabilize before you trust them.
  • Test from outside the space; never lean in to place the probe.
  • Know what your meter does and doesn't detect — a standard 4-gas meter won't catch every toxic gas.

Monitor continuously — one test isn't enough

Conditions change. Work adds fumes and burns oxygen, temperature shifts move gases, and hazards can seep in from connected pipes or nearby operations. Keep monitoring throughout the entry, and pull everyone out immediately if an alarm sounds. A space that passed at 8 a.m. is not guaranteed safe at noon.

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