Flammable and toxic atmospheres

After oxygen, the two other great atmospheric hazards in confined spaces are flammable atmospheres, which can burn or explode, and toxic atmospheres, which poison. Both are common, both can be invisible, and both can develop during the entry as well as before it. This lesson explains flammable limits and how they are measured, the toxic gases most often found in confined spaces, how gas concentrations are expressed and converted, and how exposure limits compare between Canada and the United States.

How a flammable atmosphere forms

A fire or explosion needs three things: fuel, oxygen and a source of ignition. In a confined space, fuel can come from gases such as methane, propane and hydrogen; vapors from solvents, fuels, coatings and cleaning chemicals; and fine combustible dusts such as grain, flour, sugar, wood, coal and some metal powders. Because the space traps whatever is released, fuel concentrations can build up quickly, and because the space is enclosed, the pressure from ignition has nowhere to go. A flash fire or explosion in a tank can kill everyone inside and people near the opening.

Every flammable gas or vapor burns only within a range of concentrations in air. Below the lower explosive limit (LEL), also called the lower flammable limit (LFL), the mixture is too lean to burn. Above the upper explosive limit (UEL), it is too rich. Between the two, any ignition source can set it off.

A space that is above the UEL is not safe. As it is ventilated, the concentration falls through the flammable range on its way to safety, and any air leaking in can bring it into the range.

Percent of LEL

Gas detectors used for confined space entry measure flammable gases as a percentage of the LEL, not as a percentage of the air. A reading of 100 percent LEL means the concentration has reached the lower explosive limit and the mixture can burn. A reading of 10 percent LEL means the concentration is one tenth of the LEL.

OSHA's permit space standard treats a flammable gas, vapor or mist above 10 percent of its lower flammable limit as a hazardous atmosphere. Most gas detectors used in the United States are set to alarm at 10 percent LEL, and many employers require all entrants to leave immediately if the alarm sounds. For hot work, many programs require a reading of 0 or close to 0 percent LEL.

Why set limits so low? Because a reading at one point is not the whole space. Gas pockets, stratification, a slug of vapor from disturbed sludge, a change in wind or process, or instrument error can all produce dangerous concentrations a short distance from where the reading was taken. The low limits leave room for those uncertainties.

Combustible dust

Combustible dusts can form explosive clouds in silos, bins, hoppers, dust collectors, ducts and mills. Gas detectors do not measure dust. Dust hazards are controlled by cleaning, by keeping dust settled or wet, by preventing ignition, and by using equipment rated for the hazard. A common rule of thumb is that a dust cloud dense enough to obscure vision at about 1.5 m (5 ft) may be at or near its explosive concentration. Dust explosions are covered further in fire safety courses; for confined space entry, the key point is that a clean gas reading does not mean there is no explosion risk.

Toxic gases and vapors

Many substances found in confined spaces are poisonous. Some, such as hydrogen sulfide and carbon monoxide, can kill quickly at high concentrations. Others, such as solvent vapors, cause dizziness, unconsciousness or long-term damage. The most important ones for confined space work are summarized below.

Hydrogen sulfide (H₂S) is produced when organic matter decays without oxygen, so it is common in sewers, manure pits, wastewater plants, pulp and paper mills, tanneries and oil and gas operations. It is heavier than air, colorless and smells of rotten eggs at low concentrations. At higher concentrations it quickly knocks out the sense of smell, so a worker may think the gas has gone when it is actually rising. It irritates the eyes and lungs, and at several hundred ppm it can cause sudden collapse and death after a few breaths. Disturbing sludge can release a burst of hydrogen sulfide even when the initial test was clear.

Carbon monoxide (CO) is produced by incomplete combustion in engines, generators, heaters, welding and fires, and can migrate into spaces from nearby traffic or equipment. It is colorless, odorless and about the same density as air. It binds to hemoglobin in the blood far more strongly than oxygen, so even modest concentrations reduce the blood's ability to carry oxygen. Symptoms include headache, dizziness, nausea and confusion; high concentrations cause collapse and death. Gasoline-powered pumps, generators and compressors must never be run inside or near the opening of a confined space.

Other common toxic hazards include ammonia from refrigeration and fertilizer; chlorine and other disinfectants in water treatment; sulfur dioxide; nitrogen dioxide from welding and engines; solvent vapors from coatings, adhesives and degreasers; welding fume containing metals; and residues of whatever the space last contained. The safety data sheets for the materials involved, and the process history of the space, guide which toxic gases must be tested for.

Expressing concentrations: ppm and mg/m³

Gases and vapors are usually measured in parts per million by volume (ppm), while exposure limits for some substances, especially dusts and fumes, are given in milligrams per cubic meter (mg/m³). For gases and vapors, you can convert between the two at normal room conditions (25 °C and 101.3 kPa) using the molar volume of 24.45 liters:

Here M is the molar mass of the substance in grams per mole.

Exposure limits

Exposure limits set the maximum airborne concentration a worker may be exposed to. Three kinds are common: an eight-hour time-weighted average (TWA), a short-term exposure limit (STEL) averaged over 15 minutes, and a ceiling that must never be exceeded. The American Conference of Governmental Industrial Hygienists (ACGIH) publishes threshold limit values (TLVs) that are widely used as a scientific benchmark. Many Canadian jurisdictions adopt the ACGIH values, sometimes with a delay or with changes. In the United States, OSHA's legally enforceable permissible exposure limits (PELs) are mostly from 1971 and are often much higher than current ACGIH values, while NIOSH publishes recommended exposure limits (RELs).

Because OSHA's PELs are outdated for many substances, OSHA itself encourages employers to consider more protective limits such as the ACGIH TLVs or NIOSH RELs, and several state plans, such as California's, have adopted more protective limits. Your employer's program and the entry permit will state the limits used at your site. For confined space entry, many programs set gas detector alarms at or below the most protective applicable limit, for example 10 ppm for hydrogen sulfide, and require workers to leave when an alarm sounds.

Toxic atmospheres created by the work

Many toxic atmospheres in confined spaces are created by the work itself. Welding and cutting produce metal fume, ozone, nitrogen oxides and carbon monoxide, and consume oxygen. Painting and coating release solvent vapors, often heavier than air. Cleaning with chemicals can release chlorine or other gases, especially if incompatible chemicals mix. Abrasive blasting produces dust. Scraping out sludge can release hydrogen sulfide, methane and carbon dioxide trapped in it. Each of these must be anticipated in the hazard assessment and controlled with ventilation, monitoring and, where necessary, respiratory protection. Hot work in confined spaces is covered further in HSE-216.