Too much oxygen makes materials ignite more easily and burn fiercely; above about 23.5 % is commonly treated as oxygen-enriched. Too little oxygen endangers life; below about 19.5 % is commonly treated as deficient, and nitrogen or argon leaks can reduce oxygen with no warning smell. Oxygen monitoring in plant spaces, confined spaces and around vents protects people.
- State the usual alarm limits for oxygen enrichment and deficiency
- Explain why nitrogen and argon are asphyxiants
- Identify places where each hazard arises
- Separate area monitoring from process analysis
Oxygen enrichment
In an oxygen-enriched atmosphere, normal materials, such as clothing, grease, hair and paper, ignite more easily and burn much harder. Many standards treat an atmosphere above about 23.5 % oxygen as enriched and require action. Hazards arise near oxygen plants, liquid oxygen storage, medical oxygen systems, cutting and welding with oxygen, and from vents that release oxygen-rich gas into a small space.
Venting an oxygen-rich analyser sample into a small room or cabinet. The oxygen builds up and clothes can become a fire hazard.
Oxygen deficiency
People can be harmed when oxygen falls. A common lower limit for entry into confined spaces is 19.5 %. Lower levels reduce judgement and coordination; at very low levels, roughly below 10 %, unconsciousness can follow within minutes, and exposure may be fatal. Nitrogen and argon have no smell or colour and cause no sensation of suffocation before collapse, so a leak into a closed space is dangerous.
Where it happens
- Nitrogen, argon and CO2 systems in enclosed rooms, pits and tanks.
- Entry into vessels after purging with inert gas.
- Analyser shelters containing inert gas cylinders or vents.
- Cold stores and controlled-atmosphere fruit stores that run at low oxygen: Controlled-Atmosphere fruit storage explained.
Monitoring
- Personal and area oxygen monitors are safety devices that alarm at both limits, for example below 19.5 % and above 23.5 %. They are a different product class from process analysers.
- Process oxygen analysers measure process gas for control and quality; they are not a substitute for area monitors.
- Ventilate enclosed spaces, vent analyser samples safely, and follow permit-to-work rules for entry.
Purging a vessel to low oxygen is a deliberate hazard to people who may enter it: Inerting and purging: how they work and how to prove them.
A rule to remember
Treat nitrogen and argon as dangerous in closed spaces, and oxygen as dangerous in any space where it can concentrate. Check the oxygen level before entry, every time.
- Above about 23.5 % oxygen is enriched; below about 19.5 % is deficient.
- Nitrogen and argon give no warning before collapse.
- Vent analyser samples safely and never into small enclosed spaces.
- Area monitors are separate from process analysers.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What oxygen level is commonly used as the lower limit for entry?
About 19.5 %.
Why are nitrogen leaks dangerous?
They displace oxygen without smell or irritation, so people may collapse without warning.
Why avoid venting oxygen-rich sample into a cabinet?
Oxygen can build up and make materials burn fiercely.
Key terms
Frequently asked questions
What oxygen level is safe to breathe?
Normal air is 20.95 %; below about 19.5 % is generally treated as deficient and above about 23.5 % as enriched.
Is nitrogen toxic?
No, but it displaces oxygen and can cause asphyxiation in closed spaces.
Can a process oxygen analyser be used as a personal monitor?
No; use a device built and certified for personal or area monitoring.
How should an oxygen analyser vent?
To a safe place suited to the gas, away from people and ignition sources.
