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Delta P Hazards (ΔP) in Diving Operations

Safe At Work Ontario
  • Issued: August 30, 2012
  • Revised: March 1, 2014
  • Content last reviewed: March 2014
  • See also: Diving

Disclaimer: This resource has been prepared to help the workplace parties understand some of their obligations under the Occupational Health and Safety Act (OHSA) and regulations. It is not legal advice. It is not intended to replace the OHSA or the regulations. For further information please see full disclaimer.

There are many water flow hazards that may affect a diving operation. The focus of this fact sheet is "Delta P" hazards (ΔP). Differential pressure, known as "Delta P," can present unique and deadly hazards to divers. Differential pressures are created when two bodies of water intersect, each with a different water level, such as at a dam. The forces can be considerable when the water flows from a high pressure area to a low pressure area. A diver working on the high pressure side may be drawn into the flow and trapped. Serious injuries or death may occur.

Many occupational diving incidents in Ontario have occurred at water control structures such as dams, intakes and other structures built to control water levels. These incidents might have been prevented if owners, employers and diving supervisors had taken every reasonable precaution in the circumstances to protect workers, including putting in place adequate measures and procedures to identify and control (ΔP) hazards.

Delta P hazards must be identified before the dive begins because once a diver is drawn into the hazard it is usually too late to conduct a rescue.

Some general duties of workplace parties

Employers, Owners & Constructors

The Occupational Health and Safety Act (OHSA) and Regulation for Diving Operations (O. Reg. 629/94) require every employer, owner and constructor of a project with a diving operation to:

  • prepare operational and contingency plans in writing with input from one or more of the appointed diving supervisor(s).

The operational plan shall include:

  • site specific measures and procedures to identify and handle (Delta P) hazards in the diving operation before beginning a dive and
  • state which water control authority would need to know about the diving operation in order to prevent a sudden release of water upstream of the diving operation or change to water flow at an intake

The contingency plan must include the site-specific measures and procedures to manage a diving emergency in the event a diver is trapped by a Delta P hazard.


Diving supervisors' duties under the OHSA and Diving Regulation 629/94 include:

  • ensuring a diver is not exposed to a (Delta P) hazard
  • terminating the dive operation if the continuance is likely to endanger a diver and
  • ensuring the dive site is adequately equipped to deal with emergencies such as the rescue of a diver entrapped by Delta P.


Workers' duties under the OHSA and Diving Regulation include:

  • understanding the diving operations plan and the contingency plan particularly as they relate to Delta P hazards
  • knowing indicators of (Delta P) hazards at the dive site
  • reporting any (Delta P) hazards to their supervisor and
  • refusing unsafe work

Protecting workers

It's the responsibility of employers, owners, constructors and supervisors to ensure all workplace parties comply with the provisions of the OHSA and its regulations. There are many factors to consider for safe diving practices.

Additional KEY Obligations under the OHSA and regulations

  • The employer must ensure all people who participate in a diving operation are competent.
    [Diving Regulation subssection 4.1(2)].
  • The employer, constructor and owner must ensure a water control facility or water intake plant within one kilometre of the dive site is notified of the diving operation generally at least 24 hours before it commences. [Diving Regulation clause 9(1)(c)].
  • The diving supervisor must brief all workers associated with the diving operation on the operational plan, the contingency plan and the procedures to be followed during the diving operation;
    [Diving Regulation clause 12(3)(b)].
  • The employer, the owner and the diving supervisor must ensure a diver is protected from a hazardous water flow caused by (Delta P), [Diving Regulation section 54].

Health and safety considerations and best practices

See guidance on diving (PDF) at locations where (Delta P) hazards may exist.

More information about safety on construction projects

Toll–free Number

Call 1-877-202-0008 any time to report critical injuries, fatalities or work refusals. Call 8:30 a.m. to 5 p.m. Monday to Friday for general inquiries about workplace health and safety. Always call 911 in an emergency.

Disclaimer: This web resource has been prepared to assist the workplace parties in understanding some of their obligations under the Occupational Health and Safety Act (OHSA) and the regulations. It is not intended to replace the OHSA or the regulations and reference should always be made to the official version of the legislation.

It is the responsibility of the workplace parties to ensure compliance with the legislation. This web resource does not constitute legal advice. If you require assistance with respect to the interpretation of the legislation and its potential application in specific circumstances, please contact your legal counsel.

While this web resource will also be available to Ministry of Labour inspectors, they will apply and enforce the OHSA and its regulations based on the facts as they may find them in the workplace. This web resource does not affect their enforcement discretion in any way.