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Workplace health and safety

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Hypoxic blackout at recreational snorkelling workplaces

Definitions
Background
Recommendations

Definitions

Where a person holds their breath on the surface, the irresistible urge to breathe is initiated mainly by a rise in CO2 levels and to a lesser extent by a fall in arterial O2. If a person’s arterial O2 levels drop below a critical level, hypoxia will develop and the person will suddenly lose consciousness and "blackout".

The term "shallow water blackout" is often used to describe this condition but is also used to describe a condition associated with closed circuit rebreathers. Hence the expression "hypoxic blackout" is used here.

In recreational snorkelling, there are two causes for this condition that can affect a "breath hold diver" who leaves the surface with a single breath. They are hyperventilation and ascent. These conditions may occur concurrently or independently.

Hyperventilation occurs when the person takes repeated deep breaths before descending. This affects the relative balance of CO2 and arterial O2 and can reduce the person’s urge to breathe. It is a condition that is independent of depth and has been recorded in swimming pools.

Hypoxia of ascent occurs when the person descends to depth. There the elevated partial pressure of O2 is sufficient for the person whilst they remain at depth, but falls rapidly to critical levels as the person ascends. This condition is more prevalent in persons who train to extend their breath hold and who dive deep, such as competitive breath hold divers, spear fishermen and photographers. Deaths have been recorded in depths from 3m to 30m.

Background

In December 2002 and January 2003 two breath hold divers died in separate incidents at recreational workplaces off the Queensland coast. Although post mortem examination reports have not been completed there is some evidence to suggest that both incidents were caused by hypoxic blackout leading to drowning.

Both men were well equipped and experienced snorkellers, one being a recreational dive instructor. One person was wearing a weightbelt and neither was with a buddy immediately to hand at the time of the incident.

The advice given to prospective recreational snorkellers in the Compressed Air Recreational Diving and Recreational Snorkelling Industry Code of Practice 2000, sections 3.2.2, 3.2.5 and 3.2.6 focuses primarily on other areas of risk to recreational snorkellers, those being:

This advice is typically provided by recreational snorkelling operators using a combination of media including briefings, demonstrations, written materials, illustrations and videos.

Recommendations

Workplaces conducting recreational snorkelling activities should provide the following advice to prospective snorkellers who intend to breath hold dive:

Lookouts, snorkelling supervisors and snorkelling guides should be aware of which snorkellers intending to breath hold dive and provide them with additional levels of supervision.

References

Further information

Phone: 1300 369 915
Website: http://www.whs.qld.gov.au/

Last updated July 28, 2005