25% LEL is a management threshold, not the point of explosion

A gas detector reading of 25% LEL does not mean that the air contains 25% combustible gas. LEL means Lower Explosive Limit. The lowest concentration at which a flame can propagate when a gas or vapor is mixed with air and an ignition source is present is defined as 100% LEL, and the detector displays a percentage of that value. If methane’s LEL is taken as approximately 5% by volume in air, 25% LEL corresponds to about 1.25% methane. This is still below the theoretical lower explosive limit, but it already represents a leak condition entirely unlike normal air.

That is why the word “stop” in the title matters. It does not mean that an explosion occurs automatically at 25% LEL. It means securing time to separate people and ignition sources and control the leak before the risk grows further. Waiting until the concentration reaches 100% LEL leaves no margin for measurement delay, spatial variations in concentration, or an increasing leak. However, 25% is not a universal permissible threshold for every task. Confined-space entry, hot work, shipyards, residential fuel-gas alarms, and process equipment are governed by different rules and permit conditions, and more conservative limits such as 10% LEL or lower are common.

Why stop well before 100%?

First, a detector shows the current value at one point near the worker; it does not guarantee the maximum concentration throughout the space. Gas may be released locally from pipe flanges, tank manways, drains, pits, and gaps in insulation. While the detector reads 25% LEL, a small area near the leak source may already be at a higher concentration. Lighter gases tend to rise and heavier vapors tend to settle, but temperature, pressure, ventilation, obstacles, and worker movement alter the actual distribution. A low reading at one point must not be turned into proof that the entire space is safe.

Second, concentration is not a static number. It can rise sharply within seconds or minutes when a valve opens, residual liquid is agitated, a tank is heated, solvent cleaning takes place, a pump starts, or ventilation stops. In fatal incidents investigated by NIOSH involving manual gauging of oil and gas production tanks, opening a hatch could rapidly release a pressurized cloud of hydrocarbon vapor. Records from multigas detectors worn by the deceased showed both readings above 100% LEL and severe oxygen deficiency. At this rate of change, the interval between 25% and 100% is not a generous working range.

Third, measurement involves response time and uncertainty. Pumped measurements add the sensor response time to the transport time through the hose. A catalytic combustion sensor may respond low when oxygen is deficient or when the catalyst has been contaminated by silicone, sulfur compounds, or other substances. Calibration gas and the actual gas mixture can also produce different responses. One must even account for cases in which some instruments appear to show a low value after a high concentration has exceeded their measuring range. The alarm setpoint is therefore a defensive line that provides a safety margin between the theoretical ignition concentration and field conditions.

Fourth, LEL indicates only the potential for fire and explosion; it does not determine inhalation toxicity. Some hydrocarbons or solvents can exceed occupational exposure limits at concentrations that are only a fraction of their lower explosive limit. Conversely, oxygen deficiency can simultaneously increase the risk of a person collapsing and the likelihood of sensor error. When several vapors are mixed in the workplace, their relative response can differ from that of the calibration gas used by the instrument, so the same displayed value may represent a different level of risk. A reading below 25% LEL therefore cannot eliminate the need for respiratory protection, toxic-gas measurement, oxygen measurement, or ventilation.

Raising the setpoint because alarms occur frequently is not a solution either. Repeated alarms may indicate sensor failure, but they may also signal an actual small leak, inadequate extraction, a change in work practices, or an unsuitable measuring location. First verify the bump test and calibration with certified standard gas, and review the alarm time together with process conditions, valve operations, wind direction, and ventilation operating records. Changing 25% to 40% without removing the cause merely gives workers less time to respond.

The 25% value in KOSHA materials must be read in context

The Korea Occupational Safety and Health Agency’s practical safety and health information for the manufacture of metal products for construction instructs workers performing electric welding where flammable liquid vapors or gases may accumulate to measure the gas concentration, work only at or below 25% LEL, and continue displacement and ventilation. Its material on a 2016 explosion in an adhesive manufacturing process also recommends maintaining the concentration of flammable substances inside enclosed mixers at or below 25% LEL through the injection and displacement of inert gas. The investigation identified the formation of flammable vapor while inerting was not maintained and an electrostatic discharge as causes of the accident.

If 25% is lifted out of this context and read as “work is always permitted below this level,” the source controls disappear. The first document also requires isolation of the welding location, fire-extinguishing equipment, continuous ventilation, a ban on simultaneous painting, and assignment of a fire watch. The second groups together enclosed equipment, inerting, vapor recovery, bonding and grounding, flame arresters, and local exhaust ventilation. Moreover, KOSHA technical materials are not identical to legislation itself, and a workplace permit or risk assessment may establish a lower stop-work value. The 25% figure is one management value among multiple safeguards, not a stand-alone license to work.

In confined spaces and hot work, 10% may apply first

The U.S. OSHA general-industry confined-space standard, 29 CFR 1910.146, defines an atmosphere as hazardous when flammable gas, vapor, or mist exceeds 10% of its LFL. Shipyard guidance treats 10% LEL or more in a confined space as hazardous and states that a value below 10% is not necessarily safe. The mere presence of measurable flammable vapor indicates a leak or ingress, and the concentration may rise above 10% over time as temperature increases or work disturbs the space. NIOSH’s principles for determining IDLH values also explain that routine entry into a potentially explosive atmosphere is limited to 10% LEL or less.

These criteria cannot simply be applied as law to every workplace in Korea. Conversely, the 25% value in KOSHA material cannot override an OSHA-regulated confined-space requirement or an internal hot-work permit. Even within one workplace, the purposes of a routine patrol alarm, cancellation of confined-space entry, suspension of welding, and entry by an emergency-response team differ. The strictest applicable requirement among current legislation, the latest permit, client rules, process safety information, and instrument manufacturer instructions must be checked first. Conditions for starting work and conditions requiring immediate suspension during work should also be documented separately.

NFPA’s 25% is not the same as an industrial hot-work permit value

Even when 25% LEL appears in NFPA materials, the scope of application must be checked first. Materials from the development of NFPA 715 address alarm concentrations for fuel-gas warning equipment and detectors in buildings. They explained that alarms at or below 25% LEL were permitted at the time, while equipment set to 10% LEL would alarm sooner and improve safety. Later review materials also discussed the fact that relevant product standards had adopted a 10% alarm. The 2026 edition’s scope review includes a distinction intended to prevent NFPA 715 from being applied unchanged to certain industrial processes or hazardous locations involving electrical equipment.

In other words, a 25% value discussed as a product-performance limit for residential and building fuel-gas alarms cannot be transferred to the welding-permit threshold inside a petrochemical tank. NFPA’s own materials show both the benefit of early detection and the boundaries of application. Even when the number is the same, the required safety margin changes depending on who is warned and when, whether an automatic shutdown follows the alarm, and whether ignition work is under way.

What to do when the reading reaches 25%

If the site procedure establishes 25% LEL as the stop-work threshold, do not approach the leak source to investigate the alarm or switch off the instrument. First suspend the permitted work, isolate ignition sources safely, and evacuate workers and nearby personnel in the designated direction. Report the location, time, highest reading, and upward trend to the central control room and supervisor by radio or emergency communication. However, operating the switch of non-explosion-protected electrical equipment inside the hazardous area can itself provide an ignition source, so follow the location and method specified in the emergency procedure.

The next steps must be carried out by trained and authorized personnel. Isolate the leak source by remote shutdown or process shutdown, operate the designed ventilation or vapor-recovery system, and continuously measure at upper, middle, and lower levels and at suspected leak points using suitable explosion-protected instruments. Toxic gases and oxygen must be checked as well. A low %LEL does not guarantee low exposure to toxic substances such as benzene or hydrogen sulfide, and some LEL sensors may be unreliable in oxygen-deficient conditions. If rescue is required, activate the preplanned rescue system rather than making an unprotected re-entry.

Restarting means restoring conditions, not obtaining one acceptable reading

Resuming work as soon as the alarm falls below 25% defeats the purpose of stopping. First identify the cause of the leak or concentration increase and isolate or repair it. After the prescribed ventilation, remeasure for a sufficient period at multiple locations, and confirm the instrument’s bump-test and calibration status and its suitability for the target gas. If the concentration may change again during work, use personal and area monitors for continuous monitoring.

The person authorizing restart must reconfirm the start conditions in the applicable legislation and permit. If the criterion for confined-space entry or hot work is below 10% LEL, a reading that has fallen to 12% or 20% still does not allow restart. If the internal standard requires 5% LEL or “not detected,” that requirement takes precedence. Oxygen, toxic-substance, ventilation, isolation, grounding, fire-watch, and emergency-rescue conditions must all be restored before a new permit can be issued or an existing permit reauthorized.

Checks to make before setting an alarm

  1. Distinguish whether the task is routine operation, confined-space entry, hot work, commissioning, or leak response.

  2. Compare the current versions of Korean legislation, client and workplace procedures, and—at overseas sites—the regulations of the relevant jurisdiction.

  3. Document the alarm value, immediate stop-work value, evacuation value, automatic-shutdown value, and restart value separately.

  4. Confirm the expected gas and calibration gas, sensor technology, measuring range, response time, cross-sensitivity, and oxygen dependence.

  5. Determine measuring locations and the continuous-monitoring method to reflect the leak source and the geometry of the space.

  6. State in the work permit who stops the work, shuts down the process, remeasures the atmosphere, and authorizes restart after an alarm.

Conclusion: 25% is a signal to stop before it is too late

Work is stopped at 25% LEL not because that value is the point at which an explosion begins, but to secure a response margin before the actual LEL is reached, accounting for local high concentrations, rapid concentration increases, measurement delay, and sensor error. This explanation must not be turned into “below 25% is safe.” As OSHA and NIOSH materials show, 10% can define a hazardous atmosphere or a routine-entry limit in some situations, and hot-work permits or internal standards may set still lower limits.

The best question in the field comes after “Why 25%?” Ask which rules apply to the task, whether there is a lower stop-work threshold, who stops what when the alarm sounds, and which conditions must be restored before work resumes. The LEL number is an instrument reading that supports judgment. Safety comes from an operating system that connects the number to risk assessment, isolation, ventilation, ignition-source control, continuous monitoring, and work permits.

Sources

Cover photograph

Original photograph from Pexels