H&S – COMPRESSED AIR: 30TH ANNIVERSARY WCA REGS | INSIGHT


force, considerable concern had been raised over the effectiveness of the statutory decompression tables, with sites, including the Clyde Tunnels, making ad hoc extensions to decompression times. Consequently, the Medical Research Council’s Decompression Sickness Panel, which was another forerunner of the CAWG, commissioned new decompression tables. These had a threshold for stage decompression of 14 psi (1.0 bar(g)) and were trialled on a contract in Blackpool in 1966. They were later published by CIRIA in Report 44, ‘Medical Code of Practice for Work in Compressed Air’, which ultimately ran to three editions. Initially, contractors could seek approval from the HM Factory Inspectorate to use the ‘Blackpool’ Tables; however, by the late 1980s, the Health and Safety Executive (HSE), as it had become, required all contractors to use them. In the period from 1936 to 1966, the annual reports


of the HM Chief Inspector of Factories showed that an average of one fatality occurred approximately every three years due to decompression sickness; in contrast, no fatality has been recorded from the use of the Blackpool Tables. This was a source of considerable pride to Dr Val Hempleman, the author of the Blackpool Tables. In the period leading up to September 1996,


compressed air work was being used extensively on contracts such as: Cromer (6,871 exposures and 49 DCS events at pressures between 1.0 bar(g) and 2.35 bar(g)); Jubilee Line Extension (JLE) Contract 105 – (19,681 exposures and 60 DCS events at pressures between 0.85 bar(g) and 1.35 bar(g)); and, JLE 110 – (2,499 exposures and 28 DCS events at pressures between 1.9 bar(g) and 2.35 bar(g)). Concern was growing about the level of DCS being experienced predominantly by those undertaking physical work on shifts in excess of four hours duration. In September 1996, the Work in Compressed


Air Regulations (S.I. 1996/1656) replaced the 1958 Regulations. The former are goal-setting in nature and supported by the HSE Guidance booklet L96. Amongst the changes was a statutory requirement for a Compressed Air Contractor and a Contract Medical Adviser to be appointed. L96 set out the role of a Hyperbaric Supervisor – someone in day-to-day charge of the compressed air work. Decanting was disallowed, as was the use of vertical locks for stage decompression. The Blackpool Tables became the approved decompression tables for use in the UK. Within the drafting group behind the regulations


there was a hidden agenda – to introduce oxygen- assisted decompression to reduce the incidence of DCS and to set up a regulatory framework, which would allow high-pressure compressed air (HPCA) work at pressures above 3.45 bar(g), along with the use of mixed gas breathing and saturation techniques without the need for further regulatory change. Ultimately, there was too much opposition to the use of oxygen from within HSE and industry to go ahead with that proposal; however, the regulations did not prohibit the use of non-air breathing mixtures or specify exposure technique, and they contained the power of exemption


– including exemption from the 3.45 bar(g) limit, which would allow for HPCA work. Following extensive research in


September 2001, HSE eventually approved oxygen decompression based on the Blackpool Tables with oxygen breathing from the 0.6 bar(g) stage downwards. This was accompanied by rudimentary guidance from HSE on the use of oxygen and non-air breathing mixtures. Shortly afterwards, tables of


German and French origins, making more aggressive use of oxygen at higher stage pressures but with similar outcomes, were allowed by HSE. However, by then the number of exposures on a contract had declined substantially, mainly because of the dominance of mechanised tunnelling. Nevertheless, the principle of oxygen decompression had been established and its benefits are now widely recognised. Looking to the future, BTS CAWG has led the way in


conjunction with the ITA in publishing state of the art guidance on mixed gas breathing and saturation exposure techniques for use at pressures of 3.5 bar(g) up to 20 bar(g) – ITA/BTS CAWG Report 10 ‘Guidelines For Good Working Practice In High Pressure Compressed Air’. These techniques have already been used on major projects, such as the Tuen Mun tunnels in Hong Kong and there is every likelihood the techniques will be used in the UK in the next five years. Decompression from saturation exposures


is managed and undertaken by a long slow linear reduction in pressure at a rate of around 0.2–0.1 bar per hour, the rate decreasing as pressure is reduced, with a moderately high partial pressure of oxygen. The guidance is complemented by the


soon-to-be-published revised harmonised CEN standard for airlock safety (EN 12110 parts 1 and 2), in the drafting of which the BTS is a leading player on behalf of the British Standards Institution (BSI) and the UK. The introduction of 100% oxygen breathing


improved the effectiveness of decompression procedures, however oxygen toxicity has to be considered. There is currently some debate over the use of 50/50 nitrox or 50/50 heliox as a transition gas between working mixture and 100% oxygen breathing as a means of further improving decompression effectiveness whilst still abiding within oxygen exposure limits.


REFERENCES ● 1


Above: Compressed air workers using full face mask breathing systems to deliver 100% oxygen during decompression.


Below: Interior of living compartment of saturation habitat.


Historical information taken from ‘Decompression illness and its regulation in contemporary UK tunnelling – an engineering perspective’. PhD thesis, Donald R Lamont, Aston University, 2006


September 2026


| 35


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45