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The Importance of Gas Testing: Hazards, Safety Precautions, and Exposure Limit


Gas testing is an essential activity that is crucial in ensuring the safety of workers in various industries. The process involves testing the air or gas in a specific area to identify any hazardous gases, vapors, or fumes that may pose a threat to human health or cause explosions or fires. In this article, we will discuss the hazards of gases, precautions to take during gas testing, and the various exposure limits to keep in mind.

Abbreviations Used

Before diving into the topic, it is essential to understand the following abbreviations used in gas testing:

  • Gas: Any substance of very low density and viscosity (no shape, fills the free space of the container) Vapour: Mist or fumes suspended in the air
  • Liquid: Any flowing substance with little tendency to disperse (no shape, takes shape of the container) PPM: Parts Per Million
  • TLV-TWA: Threshold Limit Value – Time-Weighted Average
  • STEL: Short Term Exposure Limit
  • IDLH: Immediately Dangerous to Life and Health
  • LEL: Lower Explosive Limit UEL: Upper Explosive Limit

Hazards of Gases

Gas hazards can be categorized into three main categories: oxygen hazards, explosive gases and flammable vapors, and harmful or toxic gas hazards.

  • Oxygen Hazards: Inadequate oxygen supply in the air we breathe can be fatal. Similarly, if there’s too much oxygen, it can be a significant safety concern.
  • Explosive Gases and Flammable Vapors: When explosive gases and flammable vapors mix with air in specific proportions, they can lead to explosions or fires.
  • Harmful or Toxic Gas Hazards: These hazards cover a wide range of gases, from inert gases like nitrogen, which can displace oxygen, to nerve gases like Hydrogen Sulphide, which chemically interferes with the cells in our body.

Safety Precautions

Gas testing must be completed by trained competent personnel. To prevent incidents, three main types of gas testing are undertaken as safety precautions for hazards:

  1. Before entering a confined space or Excavation.
  2. Before undertaking any type of hot-work.
  3. During inerting.
Gas Testing

Gas testing is involved in the following activities:

  1. Hot work of any type where heat is used or generated, e.g. by welding, flame cutting and grinding, etc.
  2. Work which may generate sparks or other sources of ignition.
  3. Work which may cause an uncontrolled release of hydrocarbons, other flammable or toxic materials.
  4. Electrical instrumentation work which may cause sparks.
  5. Entry into confined spaces and gas alarm


  • Gas checks are necessary to ensure safety in confined spaces and Hot work etc.
  • Lack of oxygen can cause suffocation or even death.
  • An excess of oxygen can create a fire or explosion hazard.
  • The presence of flammable gas or flammable vapors can lead to fires or explosions.
  • The presence of Toxic gas can.
  • Gas checks certify that the atmosphere within a confined space is free from flammable, explosive or toxic substances and that the oxygen content is at a safe level.
  • Hot work gas testing is carried out to ensure that there are no leaks or accumulations of flammable vapors or explosive gases at the worksite during hot work.

In other words, to certify that the atmosphere is safe to work in!

 Gas Detector: 

  • Gas detector should be calibrated – Calibration validity is 3 months as per KNPC.
  • Gas testing only to be carried out by Authorised Gas Tester.


    1. O2 (Oxygen)
    2. LEL (Lower Explosive Limit)
    3. CO (Carbon mono oxide)
    4. H2S (Hydrogen sulphide)

Multi gas Monitor

Exposure Limits of Gases:

  • O2 (Oxygen)- required in between 19.5 to 23.5%
  • LEL- shall be zero for hot work and 10% for cold work is allowed.
H2S (Hydrogen sulfide)10 PPM15 PPM33 PPM*
CO (Carbon mono oxide)25 PPM400 PPM1500 PPM
Cl2 (Chlorine)0.5 PPM01 PPM10 PPM
SO2 (Sulfur dioxide)02 PPM05 PPM100 PPM
NH3 (Ammonia)25 PPM35 PPM100 PPM

IDLH of H2S : 100PPM as per NIOSH, 33PPM as per KNPC

TLV-TWA: Threshold Limit Value – Time-Weighted Average:

Limit which is set for exposure up to 8 hours per day which results in no short or long term ill effects

STEL: Short Term Exposure Limit:

Limit which is set for exposure for 15 minutes (limit 4 times per day with 1 hour between exposures)

IDLH: Immediately Dangerous to Life and Health:

Limit which is set for no exposure – Work not allowed


All combustible gases and  vapours are  characterized by explosive limits between  which, the  gas  or  vapour mixed with air is capable of sustaining the spread of flame. These can be referred to as the Explosive or Flammable limits

Explosive ranges of other gases


LEL – Lower Explosive Limit:

LEL of methane is 5% by volume i.e. 5 volumes of methane mixed with 95 volumes of air.  This is the minimum quantity of Methane in a methane/air mixture that will ignite to produce a self-propagating flame if exposed to a hot source or a spark from electrical or mechanical equipment. If there is less than 5% of Methane in the air by volume the mixture is  too  lean to  support combustion.

 UEL – Upper  Explosive Limit:

UEL refers to the  highest concentration of a gas in the atmosphere which results in a combustible mixture. For example, the UEL of Methane in air is 15% by volume, which means that if there is more than 15% of Methane in the air by volume the mixture is too rich to support combustion.


15 to 19%            : Strenuous work is difficult

12 to 14%            :  Respiration and pulse increases, co-ordination and judgment impaired

9 to 11%             : Nausea, fainting, blue lips, unconsciousness

6 to 8%               : Death within 8 minutes, recovery if resuscitated within 4 minutes

Remember! That rusting of steelwork and some chemicals in confined spaces can reduce the oxygen in the atmosphere to dangerous levels very quickly.


Above 23% oxygen serious fire risk causing some materials to spontaneously combust, for example, oily cloth and steel wool >(30%)

Prolonged exposure causes damage to breathe processes and intoxication which seriously disturbs judgment with (industrial 02 gas)

Most gas detection apparatus is uncertified if the oxygen content is increased above atmosphere levels

The minimum content of Oxygen required for a Catalytic Gas detector reading LEL is 13% or more to read accurately.     

4 thoughts on “The Importance of Gas Testing: Hazards, Safety Precautions, and Exposure Limit”

  1. Very interesting topic., it enhanced more of my knowledge with regards to handling of hazardous gases! Thank you very much.., and i want more more exciting topics.

  2. I searched for these details on various sites, but didn’t get a clear picture.
    But you gave me the exact details which I have been searching for.
    Thank you a lot for your efforts.
    Keep up the good work…

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