What is the normal oxygen concentration in the atmosphere?

Normal atmospheric oxygen concentration is approximately 20.9% by volume. In confined spaces, this can be reduced (oxygen depletion) by chemical reactions, biological processes, or displacement by other gases. Entry should not take place if oxygen levels fall below 19.5% without appropriate respiratory protective equipment.

Below what oxygen concentration is a confined space atmosphere considered oxygen-deficient and dangerous?

An atmosphere is considered oxygen-deficient when the oxygen concentration falls below 19.5%. At this level, breathing becomes impaired and cognitive function is affected. Below 16%, there is a risk of loss of consciousness. Below 6%, death can occur within minutes. Continuous monitoring is essential in confined spaces to detect oxygen depletion.

What does 'LEL' stand for in atmospheric monitoring?

LEL stands for Lower Explosive Limit (also called the Lower Flammable Limit, LFL). It is the minimum concentration of a flammable gas or vapour in air that can ignite and sustain combustion. Below the LEL, the mixture is too lean to burn. In confined spaces, gas concentrations should be maintained well below 10% of the LEL for safe working.

Free mock exam

Hazards & Atmospheric Monitoring — Confined Spaces Practice

Free topic-focused mock exam: 15 random questions from a pool of 40 on Hazards & Atmospheric Monitoring, drawn from the wider Confined Spaces bank. 20-minute timer, 70% to pass, worked explanation on every question. Use this when you've taken the full mock once and want to drill the topics you got wrong.

Questions: 15
Time limit: 20
Pass mark: 70%
Bank size: 40

Free · no sign-up · timer can be ignored · retake as many times as you want

Sample questions

01What is the normal oxygen concentration in the atmosphere?
- A.20.9%
- B.16%
- C.19.5%
- D.23.5%
Why: Normal atmospheric oxygen concentration is approximately 20.9% by volume. In confined spaces, this can be reduced (oxygen depletion) by chemical reactions, biological processes, or displacement by other gases. Entry should not take place if oxygen levels fall below 19.5% without appropriate respiratory protective equipment.
02Below what oxygen concentration is a confined space atmosphere considered oxygen-deficient and dangerous?
- A.Below 23%
- B.Below 20.9%
- C.Below 19.5%
- D.Below 16%
Why: An atmosphere is considered oxygen-deficient when the oxygen concentration falls below 19.5%. At this level, breathing becomes impaired and cognitive function is affected. Below 16%, there is a risk of loss of consciousness. Below 6%, death can occur within minutes. Continuous monitoring is essential in confined spaces to detect oxygen depletion.
03What does 'LEL' stand for in atmospheric monitoring?
- A.Lower Explosive Limit
- B.Least Effective Limit
- C.Lower Emission Limit
- D.Low Energy Level
Why: LEL stands for Lower Explosive Limit (also called the Lower Flammable Limit, LFL). It is the minimum concentration of a flammable gas or vapour in air that can ignite and sustain combustion. Below the LEL, the mixture is too lean to burn. In confined spaces, gas concentrations should be maintained well below 10% of the LEL for safe working.

FAQ

Is the Hazards & Atmospheric Monitoring mock exam free?

Yes — 100% free, no sign-up needed. Questions are pulled at random from a substantial bank, with a worked explanation on every question after you submit.

Can I retake the mock exam?

Yes. Every attempt picks a different random selection and reshuffles the options, so each retake gives you genuinely new questions. No limit on retakes.

Does this count towards the real qualification?

No — this is independent practice material. To gain the actual qualification you need to sit the official assessment through an approved provider.