Compressed Air Working (Tunnels)

Compressed air working raises air pressure inside a tunnel to prevent groundwater ingress during construction.
Workers enter and exit through airlocks that gradually change pressure to prevent decompression sickness.
Decompression sickness (the bends) occurs when dissolved gases form bubbles in the blood and tissues.
Symptoms include joint pain, skin rashes, dizziness, paralysis, and in severe cases, death.
Working under compressed air is governed by the Work in Compressed Air Regulations 1996.
A contract medical adviser (CMA) must be appointed to oversee worker health and fitness for compressed air.
Exposure times are strictly limited based on the working pressure and decompression tables.
Medical locks (treatment chambers) must be available on site for emergency recompression treatment.
Workers must pass a medical examination specific to compressed air fitness before starting work.
Fire risk is increased under elevated air pressure; strict controls on ignition sources are essential.

Decompression sickness	Incorrect decompression causes nitrogen bubbles in blood and tissue, causing paralysis, organ damage, and death.
Strict time limits	Exceeding exposure time at pressure without correct decompression causes severe illness requiring emergency treatment.
Legal requirement	The Work in Compressed Air Regulations 1996 mandate medical supervision, time limits, and emergency treatment capability.

Do

Don't

Pass the compressed air medical examination before starting any work under pressure.
Follow the decompression schedule exactly as specified by the CMA.
Log entry and exit times through the airlock accurately for every shift.
Report any symptoms of decompression sickness immediately, however minor.
Ensure the medical lock is staffed, tested, and ready for emergency treatment.
Comply with the maximum exposure time for the working pressure being used.
Avoid flying or travelling to altitude for the period specified after decompression.
Follow the strict fire prevention controls required under elevated air pressure.
Attend the pre-shift medical briefing and fitness check before entering the airlock.
Carry your personal compressed air record card at all times during the works.

DON'T enter compressed air without a valid medical certificate of fitness.
DON'T exceed the maximum permitted exposure time at the working pressure.
DON'T rush through decompression; follow the schedule exactly as prescribed.
DON'T ignore joint pain, skin rash, or dizziness after decompression.
DON'T fly or travel to altitude within the restricted period after working under pressure.
DON'T use ignition sources under compressed air without specific fire controls.
DON'T bypass the airlock or attempt to equalise pressure by other means.
DON'T consume alcohol before or immediately after compressed air working shifts.
DON'T allow the medical lock to be used for storage or any non-emergency purpose.
DON'T work under compressed air if you feel unwell or have a respiratory infection.

See also: Tunnelling Safety Awareness | Tunnel Ventilation Requirements

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