LOC (Limiting Oxygen Concentration) – when does oxygen become enemy number one?

In everyday life, we think of oxygen as something we can’t live without. In the process industry, however, it’s also a key “ingredient for explosion.” All it takes is a flammable gas, liquid vapour, or dust – and the oxygen content in the mixture determines whether it can be ignited at all. In this post, you’ll get a straightforward, technically accurate explanation: what LOC is, when oxygen turns from ally to problem, and how to use this parameter in explosion protection so that safety isn’t based on “luck.”

Why do we talk about oxygen in the context of ATEX at all?

Because without oxygen (an oxidiser), there’s no combustion – and without combustion, there’s no explosion. That’s the simplest reason why oxygen should be on the watch list just as much as fuel emissions and ignition sources.

Air contains approximately 20.9% vol. oxygen – which is “just right” for most fire and explosion scenarios. And here’s the trap: in many facilities, all the energy goes in two directions – limiting fuel (leaks, dust, vapours) and controlling ignition sources (electrics, ESD, friction, hot surfaces). That’s important. But there’s a third dial, often overlooked: controlling oxygen concentration in the mixture.

The key point for today is simple: understanding the threshold at which a mixture stops being flammable allows you to consciously use inerting as a prevention tool. Not as a “nice addition to the project,” but as a real mechanism that removes one of the conditions needed for an explosion to start.

If you want to check whether your facility is genuinely “closing the loop” on ignition sources, see our post on ignition sources in explosion hazard zones.

What is the limiting oxygen concentration and what does it depend on?

It’s the limiting oxygen concentration below which ignition/combustion is not possible – even if the amount of fuel is “perfectly right.” In practice, this means one thing: if you maintain oxygen below this threshold in a piece of equipment (tank, filter, dryer, cyclone), the explosion scenario can no longer come together.

Two things are worth remembering, as they make the biggest difference in design:

• it is not one universal number for everything,
  
• the result depends not only on the substance, but also on process conditions.
  

The value is influenced by: type of fuel (gas, vapour, dust and the specific material), temperature, pressure, and type of inert gas. An example that usefully “frames” the thinking: CO₂ is sometimes more effective than nitrogen, because it absorbs flame heat better – so the same material may have a different threshold depending on how you “de-oxygenate” it.

For reference (atmospheric conditions, indicative values): methane in a mixture with nitrogen – approx. 12% vol. oxygen, propane – approx. 11.5–12%, and for many solid and cellulosic materials the threshold often falls in the range of approx. 14–17%.

The practical conclusion: sometimes a small reduction from 20.9% is enough to make a mixture lose its ability to explode.

And that’s precisely why this parameter is so “inconvenient” – it shows that oxygen is sometimes the cheapest route to risk reduction, but only when approached with engineering rigour, not guesswork.

How is LOC determined and why can a “table from the internet” lead you astray?

It’s an empirical quantity – determined by testing, not intuition. And that’s the key, if you want to make investment decisions that will hold up technically and formally.

For dusts, PN-EN 14034-4:2004 is used (testing in a 20-litre sphere). The scheme is straightforward: a defined quantity of dust is introduced into the chamber, the atmosphere is a mixture of oxygen and inert gas (often nitrogen), ignition is triggered, and the laboratory observes whether explosion overpressure develops. The oxygen level is then gradually reduced until – after many repetitions – ignition no longer occurs.

For gases and vapours, standards such as EN 15967 or EN 14756 are referenced – the logic is similar: a series of trials, controlled conditions, determination of the threshold.

The most common mistake in plants? Adopting values “from the literature” without checking whether they apply to:

• the same inert gas (nitrogen vs CO₂ makes a difference),
  
• the same temperature and pressure,
  
• the same test method.
  

If you’re planning inerting, you also need one element that’s usually absent from tables: a safety margin. In design practice, this is what “buys” resilience against process fluctuations, oxygen measurement accuracy, automation delays, and typical operational issues (e.g. a temporary drop in inert gas supply).

At Atex Doradztwo, we link these decisions to the explosion hazard assessment – because only then do you know where the oxygen threshold actually matters, and where the risk stems from a different element of the scenario.

When does oxygen become enemy number one?

In two situations: when there is “too much” oxygen and when you deliberately make it “too little,” but in the wrong place. It sounds like a contradiction, yet it’s one of the most important paradoxes in process safety.

When there’s too much oxygen – oxygen enrichment

Even at concentrations above approximately 23–24% oxygen in air, fire and explosion hazards increase markedly. The mixture becomes more easily ignitable, the required ignition energy drops, and materials that would normally only smoulder can burn vigorously or even undergo spontaneous ignition.

The worst part is that the senses won’t detect it. A person can function “normally” while the environment is already “primed” for rapid fire.

When there’s too little oxygen – the conflict between installation safety and human safety

Inerting makes sense, but only where there is space and control for it. A person begins to feel the effects of oxygen deficiency at around 16% (fatigue, headache, reduced concentration). At around 10%, the risk of loss of consciousness rises, and below that it becomes lethally dangerous.

This leads to a practical rule that should be applied without exception: full inerting – in closed equipment (silos, filters, reactors), not in spaces where people work permanently. Where operation, servicing, or entry into apparatus is involved, we need organisational and technical solutions that “join” process safety and occupational safety rather than pit them against each other.

Where does LOC make the biggest difference in installations?

Where we have flammable mixtures and an enclosed space in which we can stably control the atmosphere. Three groups of applications appear most frequently here.

Tanks and solvent storage

The typical scenario is nitrogen blanketing of the vapour space above the liquid, maintaining slight overpressure of inert gas, and continuous oxygen measurement with alarms and valve automation. In practice, this is not a “gadget” – it’s a way to handle fluctuations in emissions and evaporation without relying solely on ventilation.

Silos and dust collection systems

With dusts, the problem is often not only ignition but also secondary explosion (dislodgement of settled dust, rapidly forming a cloud at the wrong moment). For this reason, some facilities combine approaches: explosion relief, isolation, and in critical equipment also inerting.

If you want to approach this topic from a process perspective (rather than a purely equipment-based one), you’ll find our post on how to design a safe dust collection system in Ex zones and avoid secondary explosions useful.

Dryers and reactors

Here the biggest challenge is often variability. Process conditions can “pass through” ranges in which the mixture is flammable – and in those cases, controlling oxygen (below the threshold) acts as a process safety switch. The catch is that the threshold can shift with temperature and pressure, so the solution must be tailored to the specific technology, not to a generic scheme.

How to translate LOC into decisions that stand up in audits and in the budget?

It’s a number that helps you decide: whether and where to invest in inerting, and how to design it so it doesn’t become a “dead safeguard.” Management and maintenance teams don’t need a chemistry lecture – they need answers about what this changes in terms of risk, costs, and downtime.

A properly calculated and applied oxygen threshold can:

• reduce the risk of a catastrophic explosion to near zero in a specific part of the installation,
• limit “over-engineering,” i.e. spending on solutions that look good on paper but don’t actually change the ignition scenario,
• bring order to automation: what you measure, what thresholds you set, what should happen on alarm.

But there’s also the other side: ignoring the oxygen threshold creates a false sense of security (“we have a filter, we have ventilation, we have Ex-rated equipment”). Near-miss events often occur precisely when the system is operating at the edge of its effectiveness – and only “one element is missing” for things to go further.

If the topic needs to be closed not only technically but also formally, the key is linking it to ATEX documentation. In practice, we always come back to the DZPW – because that’s where it should be clearly described how we control risk, what the thresholds, procedures, and responsibilities are. For that reason, it’s worth linking to the post that addresses the most common problem in facilities: what the DZPW should contain.

If you need end-to-end support, Atex Doradztwo handles it in one continuous process: explosion hazard assessment, selection of the approach (including inerting), documentation update, and preparation of solutions that can be sustained in operation.