Monitoring the Atmosphere in a Confined Space

General information on how to monitor for a hazardous atmosphere in a confined space.


Monitoring atmospheric hazards using direct-reading instruments is the first and most critical step for confined space safety.  Oxygen deficiency or enrichment should be monitored first.  Next, explosive gases, vapors, and dust need to be monitored.  Last, toxic gases and vapors are monitored.  This order is critical because explosive gas monitors may not provide accurate readings if the oxygen level is too low. Before entering any confined spaces, it is important to monitor the air, ventilate if necessary, and continue to monitor as long as an entrant is working inside.

The accepted levels for the three main atmospheric hazards (that should be monitored in the following order) are:

  • Oxygen: must be between 19.5% and 23.5%
  • Explosive gases or vapors: must be less than 10% of the Lower Explosive Limit (LEL); airborne combustible dust must also be below the LEL
  • Toxic gases or vapors: must be below OSHA's Permissible Exposure Limit (PEL) for that chemical

Oxygen must always be sampled first.  If there is not enough oxygen, the reading for explosive gases and vapors may be inaccurate. When the air contains less than 15% oxygen, the reading for explosive gases and vapors may be falsely low.  Thus, oxygen must be tested first in order to render accurate LEL readings. If the sensor doesn’t register, do not enter the space as oxygen content may be very low.

This isn’t just good practice. OSHA requires it in its recent standard on confined entry in construction:

“Before an employee enters the space, the internal atmosphere must be tested, with a calibrated direct-reading instrument, for oxygen content, for flammable gases and vapors, and for potential toxic air contaminants, in that order. Any employee who enters the space, or that employee's authorized representative, must be provided an opportunity to observe the pre-entry testing required by this paragraph.” 1926.1203(e)(2)(iii)

Direct-reading Instruments

The air monitors used to test the air in confined spaces are called "direct-reading instruments" because they “read” an air sample. Air monitors test for oxygen. They also test for explosive and toxic gases and vapors.  Only the following gases and vapors can be specifically detected by chemical-specific sensors:

  • oxygen
  • carbon monoxide
  • chlorine
  • hydrogen
  • ammonia
  • hydrogen sulfide
  • hydrogen cyanide
  • nitrogen oxide
  • sulfur dioxide
  • ozone

If a confined space contains one of the chemicals listed above, a sensor can be used to detect that chemical. A hydrogen sulfide sensor, however, will not detect solvent vapors from thinners or glues. When dealing with unknown chemicals, it is better to use a non-specific sensor which is sensitive to small amounts of toxic gases and vapors. 

It is important to note that not all direct-reading instruments are universally appropriate for any confined spaces.  Please refer to the sample of manufacturer links under 'Availability' to view the various specifications for the appropriate confined space.

How to Monitor in a Confined Space

Always monitor the inside of a confined space from outside of the space by collecting the air sample through a sampling probe inserted into the space.  A probe is a long, hollow, narrow tube connected to the inlet of the monitor (Figure 1).  To obtain a sample, keep the cover to the space enclosed so that the hazardous air inside the space does not mix with the outside air and to avoid a potential ignition of flammable gases.

Figure 1.  A sampling probe connected to an air monitor. (Source:

Some air monitors draw a sample with a motorized or hand-activated pump.  Others allow the air to passively move across the sensor, also known as "passive diffusion."

If the initial test readings indicate the air is within safe limits, remove the cover to the space and begin sampling vertically every four feet (Figure 2).  Some monitors require some time to fully respond.  To be safe, allow at least two to three minutes for a full response.  The vapor density of a chemical generally determines where it can be found in a confined space.  Sampling every four feet ensures that you will detect all the gases and vapors which accumulate at different levels within the space.

Figure 2.  Always monitor the air at different levels or layers within the confined space. (Source: CPWR Confined Space Instructor Manual 2006)

To help direct the sampling probe, attach it to a long, sturdy pole. Sample each vertical layer horizontally within a radius of six feet (space permitting) slowly and carefully as the air within a confined space may be unevenly mixed. Hazardous gases may exist within several feet of an area that reads safe.  Since the air within a confined space can change quickly, it is recommended that you monitor continuously while you are in the space because sensors in direct-reading monitors take time to respond fully. When sampling air requiring more than 10 feet deep of the sample line, the instrument needs more time to respond.

Monitors (Figure 3) can be worn on a belt or in an outside shirt or pocket. If possible, tubing should be near the entrant’s breathing zone to measure the content of the air the entrant is actually breathing.

Figure 3. A wearable single gas detector for hydrogen sulfide by BW Technologies (Source:

Types of Direct-reading Air Sensors

Direct-reading instruments like air monitors contain several types of sensors to generally measure the air (Figure 4).  Three types of sensors used in air monitors are:

Electrochemical Cells
Electrochemical cells detect oxygen and a small number of other specific chemicals, including carbon monoxide and hydrogen sulfide.  They may respond to gases for which they were not intended, resulting in false readings.

Combustible Gas Sensors
Combustible gas sensors are non-specific detectors that measure the total amount of explosive gas as a percentage of the lower explosive limit (LEL). Combustible gas sensors are used in “LEL meters.”

Broad-Band Sensors
Broad-band sensors are non-specific detectors which respond to explosive and toxic gases and vapors. These sensors are good screening tools for detecting toxic chemicals in confined spaces.

Figure 4. A multigas air monitor by MSA (Source:

It is important to note that sensors cannot reliably detect the presence of combustible dust, and are not a substitute for a hazardous dust inspection and control program.

Detector Tubes for Toxic Gases/Vapors

Detector tubes, which can measure approximately 200 different substances, can be used to detect the presence of toxic gases or vapors in confined spaces. However, to use a detector tube you must know the specific gas or vapor that you are testing for (figure 3).  If you use detector tubes to sample a confined space, always check to make sure that:

  •  the right tube is used for the specific chemical being sampled
  •  the tube has not expired
  •  both ends of the tube are broken before use
  •  the sample pump passes a leak test
  •  the correct amount of air is drawn for each sample
  •  the air sample is collected from near the entrant's breathing zone

The accuracy of detector tubes, which can be off by as much as 25%, is affected by temperature, humidity, pressure, and the presence of other chemicals.

Risks Addressed:

Working in confined spaces can present some conditions that can be immediately dangerous to life or health (IDLH).  A confined space is any enclosed area not designed for continuous human occupancy that has a limited or restricted means of entry or exit.  In a confined space, existing ventilation is not sufficient to ensure that the space is free of a hazardous atmosphere, oxygen deficiency, or other potential hazards.  Monitoring the atmosphere is a work practice (one part of an established confined space program) that ensures the space is free of a hazardous atmosphere.

How Risks are Reduced:

Although a hazardous atmosphere itself can remain or arise during work in a confined space, monitoring the air to alert entrants of a hazardous atmosphere prior to entry or as it happens by way of alarm signals (audible and visual) from properly calibrated direct-reading instruments can help personnel determine proper safety operational procedures; this in turn can help reduce the risk of exposure.  To ensure effective hazardous atmosphere monitoring, alarm points should be set at 19.5% and 23.5% for oxygen, 10% of the Lower Explosive Limit (LEL) for the gas or vapor, and the amount of the Permissible Exposure Limits (PEL) or Threshold Limit Values (TLV) in parts per million for the specific chemical sensors.

Additional Considerations:

  • It is recommended users follow all manufacturer guidelines before using air monitors to ensure worker safety and compliance with applicable local, state or federal regulations.
  • Air monitoring for hazardous atmospheres is just a part of a confined space safety program.  To ensure safety, please adhere to other guidelines of a confined space program.  This may include donning personal protective equipment, isolating the permit space, ventilating confined spaces, understanding personnel roles and responsibilities through training, and having retrieval & rescue plans in place.  Work in confined spaces always requires a confined space entry permit.
  • Confined spaces should be monitored continuously when an entrant is inside.  If the confined space is unoccupied for longer than 20 minutes, you must perform entry testing again.
  • Air monitors should be equipped with battery indicators.  Always check the battery and find out how long it can hold a charge.
  • Always check the sampling pump on an air monitor before use.  Leaks can affect the accuracy of the reading because the pump will not draw enough air for a sample.  Sampling pumps and tubes should be marked as certified by a third party certifying body like SEI/ASTM.  Self-certification by the manufacturer may be inadequate for many situations.
  • Air monitors should be bench calibrated (done in the factory or by a certified person) at least once every six months or per the manufacturer's guideline on the equipment (i.e. some may require weekly or before every use).  The date of this bench calibration should be marked on the outside of the instrument. 
  • If a confined space contains explosive gases or vapors, any spark can set off an explosion.  Intrinsically safe electrical and/or electronic equipment should be used as it will not provide that spark.  These should be certified for Class I, Division 1 and 2, Group A.
  • A "locked on" alarm is intended to sound continuously, even when an entrant moves out of the hazardous atmosphere into a safe area.  Some monitors have alarms which can only be turned off by shutting the instrument off and then turning it back on, which should always be done in a safe and clean atmosphere, away from the confined space.
  • OSHA requires at least one “attendant” for the duration of all confined space work.  This attendant must maintain continuous contact with the entrant, must not enter the confined space themselves, and should not perform any work unrelated to the safety of the entrant.  The attendant should be familiar with symptoms of exposure and is responsible for implementing the rescue plan in the event of an emergency.


Le, Jean Christophe, MPH - CPWR The Center for Construction Research and Training

This solution page was adapted from the CPWR Confined Space Instructor Manual 2006.

Hazards Addressed:


JJS Technical Services
To obtain information, visit BW Technologies Gas Monitors or contact 1-866-455-7832

Dräger, Inc.
To obtain information, visit Portable Gas Detection and MSA Gas Detectors and Gas Monitors or contact 1-800-437-2437

Industrial Scientific
To obtain information, visit Portable Gas Detectors or contact 1-800-338-3287

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