In simple terms, gases tend to flow like smoke. For gases lighter than air, the “smoke” goes up; for those gases heavier than air, the “smoke” goes down. Other factors that affect how gas flows in an area include airflow and movement within the area.

Gases lighter than air include:

  • Ammonia (NH3)
  • Methane (CH4)
  • Hydrogen (H2)

Gases in the “breathing zone” (4 to 6’ above floor level) include:

  • Oxygen (O2)
  • Nitric Oxide (NO)
  • Carbon Monoxide (CO)
  • Nitrogen Dioxide (NO2)

Gases heavier than air include:

  • Sulfur Dioxide (SO2)
  • Refrigerant Gases (R22, R404a, R134a, etc.)
  • Propane (C3H8)
  • Hydrogen Sulfide (H2S)
  • Carbon Dioxide (CO2)

To learn more, read the related article, "Guidelines for Effective Gas Leak Detection."