DECOMPRESSION SICKNESS AND TUNNEL WORKERS
Courtesy of Port of Miami
Limitations of the Current OSHA Decompression Tables
Though the OSHA Decompression Tables are the required standard, many factors indicate they need updating.
- DCS has occurred while using these tables.
One example occurred among tunnel workers on a Milwaukee project who followed these tables. A study of these workers found an unacceptable incidence of dysbaric osteonecrosis. The project involved pressures up to 36 psi (pounds per square inch).
- Many modern projects require greater pressures.
The maximum worksite pressure allowed by the OSHA Tables is 50 psi. This is also equivalent to ~3.5 bar gauge (1 bar is about equal to atmospheric pressure at sea level). Today, the modern Tunnel Boring Machine allows projects at greater depths or unstable soil conditions that exceed 50 psi. One example is a project at Lake Mead, Nevada that may use pressure up to 200 psi (14 bar gauge).
- The Tables use continuous rather than staged decompression.
Using continuous decompression, workers continue making nitrogen gas bubbles at initial higher pressures while slowly, continuously decreasing pressure. Workers also spend more time at low pressures that do not fully suppress bubble production.Other industrialized nations use staged decompression. This method stops workers at set depths and times in the final stages of ascent. This allows excess nitrogen to escape from the body without causing harmful effects.
- Using 100% oxygen during decompression is not allowed.
Breathing oxygen during decompression can decrease decompression time and the chance of DCS. This practice is used often in many industrialized nations. Some actually require its use. The OSHA Tables do not allow this, due to fire concerns.
- The decompression schedule is the same for all exposures over eight hours.
The OSHA tables use a single decompression schedule for any exposure that is over eight hours. So, if you work a double shift (16 hours), you would decompress on an “over 8 hour” schedule that is only slightly longer than the decompression time for a regular eight hour shift. This is despite the exposure is a saturation exposure. Saturation exposures occur when a person absorbs the maximum partial pressure of gas at the depth at which he or she is working.
- Kindwall E, Edel P, Melton, H. “Safe Decompression Schedules for Caisson Workers,” (NTIS, PBB5-103612), NIOSH Research contract 5-R01-OH-00947-03, Dec 1983, pp. 126.
Page last reviewed: June 18, 2012
Content source: National Institute for Occupational Safety and Health