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Decompression Sickness

Underwater construction and tunnel operations use compressed air to stabilize soil and keep out water. In these conditions, workers are at risk of decompression sickness (DCS).

DCS, also called the “bends,” occurs when not enough pressure is released from the body following exposure to increased pressure. DCS is more commonly noted among scuba divers, but also occurs among those working in conditions that use compressed air, like tunnel construction.

Moving too quickly from an area of high pressure to low pressure can produce nitrogen gas bubbles in the body. If outside pressure is released too quickly, the gas is released in the body. This can be very painful and sometimes fatal.

Symptoms may include:

  • joint pain
  • areas of bone destruction (termed dysbaric osteonecrosis)
  • skin disorders, such as cutis marmorata, which causes a pink marbling of the skin
  • spinal cord and brain disorders, such as stroke, paralysis, paresthesias, bladder dysfunction, etc.
  • cardiopulmonary disorders, such as shortness of breath
  • arterial gas embolism (gas bubbles in the arteries that block blood flow)

Prevention and treatment

DCS can be treated or prevented using a decompression chamber guided by decompression tables. The tables direct the time and pressure intervals needed to ensure workers are brought back to surface pressure safely.1

A decompression chamber is used to gradually bring workers back to normal surface pressures. Decompression tables detail the time and pressures needed to do this safely. The pressure changes and time intervals defined in the tables are chosen based on the worker’s exposure pressure and work shift length.1

NIOSH-developed Decompression Tables

NIOSH compression tables were developed after a tunnel job from 1971-1973 in Milwaukee used pressures to 36 psi that resulted in a very high level of dysbaric osteonecrosis among workers. This is an illness that causes part of the bone tissue to die.

In the 1980s, NIOSH let a contract to Dr. Eric Kindwall and Mr. Peter Edel to develop stepwise (staged) tables. This effort was summarized in a final report, Criteria for Interim Decompression Tables for Caisson and Tunnel Workers (also known as the Edel-Kindwall Caisson Tables) 2

This report provided four decompression tables developed by NIOSH. These tables are accessible for use by workers, employers, and safety and health professionals:

  • G3A Air Interim Decompression Tables: Utilized for pressures in the range from 14 psi to 50 psi to be used at altitudes no greater than 800 – 1000 feet.
  • G3A1 Air Interim Decompression back-up Tables for G-3A where environmental or individual conditions warrant: Serves as a backup for Table G3A where environmental or individual conditions warrant, based on the most severe conditions which might be anticipated and under which such conditions should produce a decompression sickness incidence that is significantly improved over current practice.
  • G3B1 Oxygen Interim Decompression Tables: Note that these should not be used unless the contractor is willing to obtain special training for the personnel involved and properly maintain the required oxygen equipment.
  • G3C Air Saturation Interim Decompression Table: These should be used when operational needs or emergencies required workers to remain under hyperbaric conditions for more than 8 hours.

Limitations of the NIOSH-developed Decompression Tables:

  • The NIOSH tables do not address pressures greater than 50 psi. Higher pressures are common among underground construction jobs. Some jobs use tables from other countries such as France or Germany that address higher pressures.
  • The NIOSH tables do not address the use of technical gas mixtures. Divers and technical gas mixtures are used for underground construction operations. Commercial divers use Trimix, which is a mixture of oxygen, nitrogen, and helium, or other mixtures. Tables for these gas mixtures exist, but are usually trademarked.

Additional decompression tables exist. Many of the tables were developed for specific purposes, such as the U.S. Navy Tables. Other tables have been developed by countries and used in those countries.


  1. Hamilton RW Bill, Kay E. (2008) Boring deep tunnels. Third conference on U.S. – Japan panel on aerospace-diving physiology & technology and hyperbaric medicine.
  2. Kindwall E, Edel P [1981]. Criteria for interim decompression tables for caisson and tunnel workers. Milwaukee, WI: Sea-Space Research Co., Inc. NIOSH contract no. 210-80-0110 for NIOSH Office of External Coordination and Special Projects.