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Atmospheric Testing

Atmospheric testing may be performed only by persons qualified and trained to operate the testing instrument. Initial air sampling will be conducted from outside the space, and will be performed when possible at various levels within the confined space (e.g. at least top, middle and bottom), and around all conduits, pipes, or cables. Testing must be performed within 15 minutes of entry.

Atmospheric conditions will be considered unacceptable if:

  • Oxygen levels are less than 19.5 percent or greater than 23.5 percent by volume,
  • A combustible gas is present at greater than 10 percent of its lower explosive limit (LEL),
  • Airborne combustible dust obscures vision to five feet or less,
  • A toxic substance exceeds an OSHA or American Conference of Governmental Industrial Hygienists (ACGIH) limit where exposure could result in death, acute illness, or impairment of ability to self-rescue. Previous contents of a container may pose a specific gas/fume/vapor. Air monitoring equipment may need to be specialized to the specific gas, such as nitrogen dioxide (i.e. silo gas found on farms). Common multi-gas air monitors are configurated to test for hydrogen sulfide and carbon monoxide.
  • Any atmospheric condition recognized as immediately dangerous to life or health (IDLH) is present.

Precautions must be taken to prevent personnel exposure when the limits expressed above are not met. These precautions include providing forced air ventilation (preferred) or appropriate respiratory protection in accordance with Virginia Tech's Respiratory Protection Program.


Air monitors
  • Air monitors shall be direct-reading instruments and calibrated in accordance with the manufacturer's recommendations. 
  • Personnel performing air monitoring must be familiar with the instrument being used. 
  • Atmospheric testing must include oxygen concentration, combustible gases, and any known or suspected toxic substances (commonly hydrogen sulfide and carbon monoxide). 
  • A properly calibrated direct reading gas monitor must be used. 
  • Direct reading gas detector tubes or other acceptable means may also be used to test potentially toxic atmospheres.
  • Intrinsically safe equipment will be used if a flammable atmosphere is present, or is suspected of being present.
  • Unacceptable levels may be indicated on a scale or by a visual alarm and must be indicated by an audible alarm. 


The purpose of regular instrument calibration is to provide accurate gas-concentration readings that could prevent worker illness, injury, or death. Proper warnings depend on the monitor's detection capabilities, and its ability to translate its findings into an accurate reading. Calibration adjusts the monitor's accuracy when reference points have shifted, making the readings unreliable. "Calibration drift" happens to all sensors over time, which reinforces the need for regular maintenance. Some common causes are:

  • Degradation caused by exposure to phosphates, phosphorus-containing components, or lead-containing components;
  • Gradual chemical degradation of sensors;
  • Use in extreme environmental conditions, such as high/low temperatures and humidity, and high levels of airborne particulates;
  • Exposure to high concentrations of the target gases and vapors;
  • Exposure of catalytic hot-bead LEL sensors in the instruments to volatile silicones, hydride gases, halogenated hydrocarbons, and sulfide gases;
  • Exposure of electrochemical toxic gas sensors to solvent vapors and highly corrosive gases; and/or
  • Handling/jostling the equipment causing enough vibration or shock over time to affect electronic components and circuitry

Each atmospheric testing instrument shall be calibrated on a schedule and in the manner recommended by the manufacturer. Calibration records must be maintained by the department if the instrument does not track it automatically.

Follow manufacturer's guidelines and instructions for proper calibration.

Contact Information

Robin McCall-Miller, Occupational Safety Program Manager

Phone: 540-231-2341