Glossary

Afterdamp — the toxic mixture left after an underground explosion or fire, rich in carbon monoxide and carbon dioxide and poor in oxygen.

Asphyxiant — a gas that displaces oxygen (for example nitrogen, argon, carbon dioxide) or interferes with its use by the body (for example carbon monoxide).

Bump test — a short check of a gas detector with a known test gas to confirm that sensors and alarms respond.

Calibration — adjustment of a detector against reference gases so that its readings are accurate.

Closed-circuit breathing apparatus — apparatus that removes carbon dioxide from exhaled breath and adds oxygen, so the same gas is breathed again; used for long operations.

Confined space — a space with limited entry and exit, not designed for continuous occupancy, where a hazardous atmosphere can build up: tanks, vessels, wells, sewers, pits.

Decontamination — removal of hazardous substances from people and equipment after work in a contaminated area.

Entry control — the system for recording who enters a hazardous area, with what air supply and for how long.

Escape set — a short-duration breathing device intended only for leaving a hazardous area.

Firedamp — a flammable mixture of methane and air found in coal mines.

Gas-hazardous work — work in places where a hazardous atmosphere is present or may arise, carried out under a permit and special precautions.

Hydrogen sulfide (H₂S) — a toxic, flammable gas heavier than air, typical for oil and gas production and sewers; at high concentrations it deadens the sense of smell.

Isolation — separating equipment from all sources of hazardous substances and energy, for example by blind flanges and locked valves.

LEL (lower explosive limit) — the lowest concentration of a flammable gas in air at which it can ignite. Detectors usually show flammable gas as a percentage of LEL.

Lung-demand valve — the part of a breathing apparatus that supplies air to the mask as the wearer inhales.

Oxygen deficiency — an atmosphere with less oxygen than normal air (about 20.9 %), which impairs judgement and can cause collapse without warning.

Oxygen enrichment — an atmosphere with more oxygen than normal; materials ignite more easily and burn more intensely.

Photoionisation detector (PID) — an instrument that measures many organic vapours at low concentrations.

Positive pressure — operation of a breathing apparatus in which the pressure inside the mask stays slightly above ambient, so leaks are outward.

Reserve — the part of the air or oxygen supply kept for the return path and emergencies; usually signalled by a warning device.

Self-contained breathing apparatus (SCBA) — breathing apparatus that carries its own air supply, independent of the surrounding atmosphere.

Standby team — a fully equipped team ready to enter at once to assist or rescue the team working inside.

TWA / STEL — time-weighted average and short-term exposure limits for toxic substances at work.

Upper explosive limit (UEL) — the highest concentration of a flammable gas in air at which it can still ignite; above it the mixture is too rich, but dilution with air can bring it back into the flammable range.

Ventilation — supplying fresh air and removing contaminated air, naturally or with fans, to bring the atmosphere back to safe levels.