What are LEL and UEL?

The lower explosive limit is the minimum concentration of a substance in air needed for an ignition source to cause a flame or explosion. The upper explosive limit is the other end of that band: above it the mixture is too rich to ignite. Between the two limits the substance forms an explosive mixture with air.

The definitions

United States workplace safety regulation (29 CFR 1926.1202) defines the lower flammable limit, also called the lower explosive limit, as the minimum concentration of a substance in air needed for an ignition source to cause a flame or explosion. The upper explosive limit is the other end of the same band: above it the mixture is too rich, there is not enough oxygen left for combustion, and it will not ignite. The band between the two limits is the range in which the substance forms an explosive mixture with air. European classification law works from the same idea when it defines a flammable gas as a gas having a flammable range with air at 20 °C and a standard pressure of 101.3 kPa.

A worked example

For 1,3-butadiene the United States regulation states the figures in its own appendix (29 CFR 1910.1051, Appendix A): a lower explosive limit of 2 per cent and an upper explosive limit of 11.5 per cent by volume, with an autoignition temperature of 420 °C. Below 2 per cent the mixture will not ignite; above 11.5 per cent it will not ignite either; everything in between is explosive.

Why thresholds are a fraction of the limit

In the field the action threshold is not the limit itself but a small percentage of it. For confined space entry a common trigger is ten per cent of the lower flammable limit, and underground construction rules define action and evacuation steps at thresholds such as ten and twenty per cent for methane. In the NIOSH immediately dangerous to life or health values, some substances are set at ten per cent of the lower explosive limit for safety reasons rather than on toxicological grounds. The reason is simple: a gas detector measures one point, while the atmosphere mixes.

The duty side

The Turkish regulation on the protection of workers from explosive atmospheres defines an explosive atmosphere as a mixture of flammable substances in the form of gas, vapour, mist or dust with air under atmospheric conditions, which after ignition spreads combustion throughout the whole mixture. It requires the employer to prepare an explosion protection document, to prevent explosive atmospheres from forming, to prevent ignition where they cannot be prevented, and to mitigate the effects of an explosion.

Where this sits in Optifora

The explosion protection document module is one of the three that are ready today: zones, the zone file, the fourteen-section document and equipment conformity. Its scope is in the module catalogue and the terms are in the OHS glossary.

Obligation	Legal basis	How the page covers it
The definition of an explosive atmosphere is written in the regulation	Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 4(1)(c) — explosive atmosphere: a mixture of flammable substances in the form of gases, vapours, mists or dusts with air under atmospheric conditions, which after ignition burns through the entire unburned mixture.	The definition in the "Its counterpart in Türkiye" section.
The order of measures is written down: prevent the atmosphere, then prevent ignition, then reduce the effects	Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 5(1) — for prevention of and protection against explosions the employer shall act in the order of (a) preventing the formation of explosive atmospheres, (b) where that is not possible, preventing their ignition, and (c) taking measures to mitigate the harmful effects of an explosion; art. 5(2) — the measures taken are reviewed at regular intervals and after significant changes.	The employer duties in the "Its counterpart in Türkiye" section.
The explosion protection document is drawn up before the work starts	Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 10(1) — while fulfilling the duty in art. 6 the employer draws up the Explosion Protection Document; art. 10(2) — the document states in writing that the explosion risk has been determined and assessed, the measures to be taken, the places classified under Annex 1, the places where the minimum requirements in Annexes 2 and 3 will apply, and that the design, operation, control and maintenance of the equipment comply with safety rules; art. 10(3) — the document is drawn up before the work begins and updated on significant changes.	The explosion protection document duty in the "Its counterpart in Türkiye" section.

Obligation

Legal basis

How the page covers it

The definition of an explosive atmosphere is written in the regulation

Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 4(1)(c) — explosive atmosphere: a mixture of flammable substances in the form of gases, vapours, mists or dusts with air under atmospheric conditions, which after ignition burns through the entire unburned mixture.

The definition in the "Its counterpart in Türkiye" section.

The order of measures is written down: prevent the atmosphere, then prevent ignition, then reduce the effects

Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 5(1) — for prevention of and protection against explosions the employer shall act in the order of (a) preventing the formation of explosive atmospheres, (b) where that is not possible, preventing their ignition, and (c) taking measures to mitigate the harmful effects of an explosion; art. 5(2) — the measures taken are reviewed at regular intervals and after significant changes.

The employer duties in the "Its counterpart in Türkiye" section.

The explosion protection document is drawn up before the work starts

Regulation on the Protection of Workers from the Risks of Explosive Atmospheres, art. 10(1) — while fulfilling the duty in art. 6 the employer draws up the Explosion Protection Document; art. 10(2) — the document states in writing that the explosion risk has been determined and assessed, the measures to be taken, the places classified under Annex 1, the places where the minimum requirements in Annexes 2 and 3 will apply, and that the design, operation, control and maintenance of the equipment comply with safety rules; art. 10(3) — the document is drawn up before the work begins and updated on significant changes.

The explosion protection document duty in the "Its counterpart in Türkiye" section.

Obligation	Legal basis	How the page covers it
The Directive's definition of an explosive atmosphere rests on the same test	Directive 1999/92/EC, art. 2 — "explosive atmosphere" means a mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after ignition has occurred, combustion spreads to the entire unburned mixture.	The EU basis for the notion of the explosive range in the "Definitions" section.
The explosion risk is assessed taking specific headings into account	Directive 1999/92/EC, art. 4(1) — the employer shall assess the specific risks arising from explosive atmospheres, taking account at least of the likelihood that explosive atmospheres will occur and their persistence, the likelihood that ignition sources including electrostatic discharges will be present and become effective, the installations, substances used and processes and their possible interactions, and the scale of the anticipated effects; art. 4(2) — places connected via openings to places where explosive atmospheres may occur shall be taken into account.	The EU basis for the "Why a percentage, not 100%" section: the threshold is assessed over the atmosphere itself.
The explosion protection document is mandatory in the Directive too	Directive 1999/92/EC, art. 8 — in carrying out the obligations laid down in art. 4, a document called the "explosion protection document" shall be drawn up and kept up to date; it shall demonstrate that explosion risks have been determined and assessed, that adequate measures will be taken, which places have been classified into zones under Annex I, where the minimum requirements of Annex II apply, that the workplace and work equipment including warning devices are designed, operated and maintained with due regard for safety, and that arrangements have been made for the safe use of work equipment; art. 7(1) — the employer shall classify places where explosive atmospheres may occur into zones in accordance with Annex I.	The EU source of the document mentioned in the "Its counterpart in Türkiye" section.

Obligation

Legal basis

How the page covers it

The Directive's definition of an explosive atmosphere rests on the same test

Directive 1999/92/EC, art. 2 — "explosive atmosphere" means a mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after ignition has occurred, combustion spreads to the entire unburned mixture.

The EU basis for the notion of the explosive range in the "Definitions" section.

The explosion risk is assessed taking specific headings into account

Directive 1999/92/EC, art. 4(1) — the employer shall assess the specific risks arising from explosive atmospheres, taking account at least of the likelihood that explosive atmospheres will occur and their persistence, the likelihood that ignition sources including electrostatic discharges will be present and become effective, the installations, substances used and processes and their possible interactions, and the scale of the anticipated effects; art. 4(2) — places connected via openings to places where explosive atmospheres may occur shall be taken into account.

The EU basis for the "Why a percentage, not 100%" section: the threshold is assessed over the atmosphere itself.

The explosion protection document is mandatory in the Directive too

Directive 1999/92/EC, art. 8 — in carrying out the obligations laid down in art. 4, a document called the "explosion protection document" shall be drawn up and kept up to date; it shall demonstrate that explosion risks have been determined and assessed, that adequate measures will be taken, which places have been classified into zones under Annex I, where the minimum requirements of Annex II apply, that the workplace and work equipment including warning devices are designed, operated and maintained with due regard for safety, and that arrangements have been made for the safe use of work equipment; art. 7(1) — the employer shall classify places where explosive atmospheres may occur into zones in accordance with Annex I.

The EU source of the document mentioned in the "Its counterpart in Türkiye" section.

What are LEL and UEL?

The definitions

A worked example

Why thresholds are a fraction of the limit

The duty side

Where this sits in Optifora

Legal basis

Legal basis — Türkiye

Legal basis — European Union

Legal basis — United States

Manage this in Optifora

What are LEL and UEL?

The definitions

A worked example

Why thresholds are a fraction of the limit

The duty side

Where this sits in Optifora

Related answers

Legal basis

Legal basis — Türkiye

Legal basis — European Union

Legal basis — United States

Manage this in Optifora