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Pressurised atmosphere work

Hazard Knowledge

Work in compressed air means work within any working chamber, airlock or decompression chamber that (in each case) is used for the compression or decompression of persons, including a medical lock used solely for treatment purposes, the pressure of which exceeds 0.15 bar. Access to a pressurised working will involve an air lock or a man lock.

Atmospheric pressure at sea level is fractionally above 1 bar pressure, pressure gauges are scaled to read zero at this at normal atmospheric pressure. In a pressurised atmosphere a reading of 1 bar refers to double that of atmospheric pressure. At 1 bar the equivalent volume of air at atmospheric pressure is halved, therefore inhaling the same volume of gas contains double the concentration of air.

Fires involving compressed air workings will involve an accelerated combustion process due to the richness of the oxygen in the pressurised atmosphere.

There are broadly two types of pressurised atmospheres workings, referred to here as 'elevated pressures' and 'commercial workings':

Elevated pressures

These are premises with a slightly elevated ambient pressure. They will include engineered features within structures to protect escape routes and/or to assist the emergency services in rendering assistance in terms of rescues or firefighting. Examples of these are where elevated pressures are created on protected staircases, crossover tunnels and commercial or medical 'clean areas'. As such engineered features are generally only a few millibars above ambient pressure, they do not present any significant physiological effect on personnel entering the risk area or the duration of a self-contained breathing apparatus set.

Commercial workings

During construction, tunnels may be pressurised to prevent water ingress, particularly where they are bored under a river or in very wet strata. Regulations under the Health and Safety at Work etc. Act 1974 are in place in respect of persons employed in pressurised workings.

The Work in Compressed Air regulations 1996 apply to all people employed in tunnelling, pipe jacking and shaft and caisson sinking operations carried out in compressed air, including the use of tunnel boring or shaft excavating machinery and similar operations, as part of construction work.

The task of firefighting and rescuing persons employed in pressurised workings is principally the responsibility of the contractor on site. The fire and rescue service might respond to a call to pressurised workings and stand by to give advice and provide back-up facilities as necessary. However, subject to any prior arrangement between the contractor and the fire and rescue authority, the contractor's responsibilities under the Health and Safety at Work etc. Act 1974 should make it unnecessary for the fire and rescue service to deal with an incident inside a pressurised commercial working.

Subject to prior agreement and arrangement, it might be reasonably foreseeable that fire and rescue authorities could be requested to provide some element of a contractor's emergency arrangements. Breathing apparatus (BA) command and control procedures appropriate to the risk should be established along with any minimum provisions for a safe system of work.

Health risks

When working in compressed air, the body's internal pressures balance to match the external pressure; therefore the amount of air inhaled at 1 bar pressure will be double that at atmospheric pressure.

Three types of health problem can be brought about by working in compressed air:

  • Barotrauma: where a change in surrounding pressure causes direct damage to air-containing cavities in the body directly connected with the surrounding atmosphere, principally ears, sinuses and lungs
  • Decompression illness: which predominantly occurs as a condition involving pain around the joints, or, more rarely, as a serious, potentially life-threatening condition that may affect the central nervous system, the heart or the lungs
  • Dysbaric osteonecrosis: which is a long-term, chronic condition damaging the long bones, hip or shoulder joints.

Breathing apparatus

The working duration of self-contained breathing apparatus (SCBA) is significantly reduced by increased pressure. The balance of the exhale valve and diaphragm controlling the demand valve are likely to be affected by the increase in the external pressure. In addition, the stress and exertion of working in these environments is likely to increase breathing rates.

See also: The Work in Compressed Air Regulations 1996 and Work in Compressed Air Regulations (Northern Ireland) 2004