Atmospheric hazards: oxygen deficiency and enrichment

Most confined space deaths are caused by the air. Of the atmospheric hazards, the most basic is the wrong amount of oxygen. Too little oxygen can cause collapse in seconds and death in minutes, often with no warning at all. Too much oxygen turns ordinary materials into fuel and can make a small spark into a flash fire. This lesson explains how oxygen levels change in confined spaces, what happens to the body as oxygen falls, why enrichment is dangerous, and how to interpret an oxygen reading, including at altitude.

Normal air

Normal air at sea level contains about 20.9 per cent oxygen by volume, about 78 per cent nitrogen, about 0.9 per cent argon and small amounts of carbon dioxide and other gases. The body does not respond to the percentage of oxygen directly. What matters is the partial pressure of oxygen: the share of the total air pressure contributed by oxygen. At sea level, where atmospheric pressure is about 101.3 kPa, the partial pressure of oxygen is about 0.209 × 101.3 ≈ 21.2 kPa.

Here \( x_{O_2} \) is the oxygen fraction by volume and \( P_{\text{atm}} \) is the total atmospheric pressure. Most gas detectors report the percentage by volume, so it is important to understand how that relates to what your body experiences. We come back to this when we look at altitude.

How oxygen is lost in a confined space

Oxygen levels in a confined space fall in two broad ways: oxygen is used up, or it is pushed out by another gas.

Oxygen is consumed by:

• rusting of steel, which can slowly use up the oxygen in a closed steel tank, vessel or ship's hold over days or weeks;
• bacterial action in sewage, manure, sludge, grain, wood chips and other organic materials, which also produces carbon dioxide, methane and hydrogen sulfide;
• fermentation, which consumes oxygen and gives off large amounts of carbon dioxide, for example in wine vats, silos and brewing tanks;
• combustion, from welding, cutting, heating with open flames or running engines;
• some coatings and chemicals that absorb oxygen as they cure or react;
• the people working in the space, slowly, in a very small and unventilated space.

Oxygen is displaced by:

• inert gases deliberately used to purge or protect equipment, especially nitrogen and argon, but also carbon dioxide and helium;
• gases that leak from process lines, cylinders, refrigeration systems or the ground, such as carbon dioxide, methane and refrigerants;
• fire suppression systems that discharge inert gases or carbon dioxide into a room or enclosure.

Displacement can be sudden. Nitrogen purging of vessels is a well-known cause of fatal incidents, because nitrogen is colourless, odourless, non-irritating and almost the same density as air, so it gives no warning and mixes readily. A single breath of nearly pure nitrogen can cause loss of consciousness within seconds, because it actually draws oxygen out of the blood in the lungs.

What low oxygen does to the body

The effects of oxygen deficiency depend on the concentration, the time, the person's health and the work they are doing. The table gives a typical picture at sea level. The exact numbers vary between sources, and individuals vary widely, but the pattern is consistent: judgment and coordination fail before the person realizes anything is wrong.

There are two crucial lessons in this table. First, a person in an oxygen-deficient atmosphere usually cannot tell. Low oxygen does not cause a feeling of suffocation; that feeling comes from a build-up of carbon dioxide, not from a lack of oxygen. People in low oxygen often feel fine, even euphoric, until they collapse. Second, at very low concentrations there is no time to react. A rescuer who enters to help and takes a few breaths can collapse beside the first victim.

What a small drop in oxygen tells you

An oxygen reading of 19.5 per cent may look close to normal. It is not. Because air is about 20.9 per cent oxygen, any gas that displaces air also removes oxygen in proportion. If oxygen falls from 20.9 per cent to 19.5 per cent purely by displacement, the share of air that has been replaced by something else is:

Almost 7 per cent of the atmosphere, or about 67,000 parts per million (ppm), is now some other gas. If that gas is nitrogen, the main danger is the oxygen loss. But if it is a toxic gas such as carbon monoxide or hydrogen sulfide, 67,000 ppm is many times the concentration that kills in minutes. If it is methane, 6.7 per cent is above methane's lower explosive limit of about 5 per cent. This is why an oxygen reading must never be read alone, and why gas detectors measure oxygen, flammable gases and toxic gases together. Lesson 6 explains the testing sequence.

Oxygen enrichment

Oxygen enrichment means more than the normal amount of oxygen. Most Canadian rules set an upper limit of about 23 per cent; Ontario and Alberta both use 23 per cent.

Enrichment usually comes from leaking oxygen cylinders, hoses or torches, oxygen used to 'sweeten' the air in a space (a dangerous practice that should never be done), medical or process oxygen lines, or oxygen released by some chemical reactions. Even a small increase makes a big difference to fire behaviour: materials ignite more easily, burn faster and hotter, and some materials that do not normally burn in air, such as some fabrics, will burn vigorously. Clothing saturated with oxygen can catch fire from a spark and burn intensely, and oxygen can remain trapped in clothing for some time after the person leaves the enriched area.

Never use pure oxygen to ventilate a confined space. Ventilation must use clean air from a safe source. Lesson 8 explains ventilation.

Oxygen and altitude

Because the body responds to the partial pressure of oxygen, a normal 20.9 per cent reading at high altitude provides less oxygen than at sea level. Gas detectors that measure percentage by volume read about 20.9 per cent in clean air at any altitude, but the air is thinner.

People who live at altitude acclimatize, and regulations generally use the percentage by volume rather than partial pressure. But the example shows why a falling oxygen reading at altitude deserves even more caution, and why some employers set stricter limits for work at high elevations.

Carbon dioxide: a displacer that is also toxic

Carbon dioxide deserves special mention. It is produced by fermentation, decomposition, combustion and dry ice, and it is used in fire suppression and beverage systems. It is heavier than air (about 1.5 times as dense), so it pools in pits, cellars, tanks and the bottoms of silos. Unlike nitrogen, carbon dioxide is toxic in its own right at high concentrations. Exposure limits for an eight-hour day are typically 5,000 ppm (0.5 per cent), and concentrations of several per cent cause headaches, rapid breathing and confusion, while concentrations around 10 per cent or more can cause unconsciousness quickly. A space can be deadly from carbon dioxide while the oxygen reading still looks only slightly low. Many standard four-gas detectors do not measure carbon dioxide, so spaces where it is expected need a detector with a carbon dioxide sensor.

Controls for oxygen hazards

The main controls for oxygen deficiency and enrichment are:

• identify the sources of oxygen loss or gain in the hazard assessment, including inert gas lines and fire suppression systems;
• isolate inert gas and oxygen lines by blanking or disconnecting them, not by closing a single valve (lesson 7);
• ventilate with clean air until oxygen is in the acceptable range, and keep ventilating during the entry (lesson 8);
• test before entry and monitor continuously, with the oxygen sensor close to the breathing zone and in the areas where gas could collect (lesson 6);
• never use oxygen to ventilate, and keep oxygen cylinders and hoses outside the space when not in use;
• where oxygen cannot be kept in the acceptable range, use supplied-air respiratory protection, such as an airline respirator with an escape bottle or self-contained breathing apparatus, with the additional precautions required for entering an atmosphere that is immediately dangerous to life or health.

Air-purifying respirators, such as cartridge or dust masks, do not supply oxygen and must never be used in an oxygen-deficient atmosphere.