A workplace safety expert, speaking on conditions of confidentiality, said oxygen normally makes up about 21% of the air we breathe. At 10% oxygen a person not wearing full breathing apparatus will be dead within three minutes, with unconsciousness starting at 14%.

In the holds of log carriers, oxygen levels have been shown to fall below 14% within 12 hours of the holds being closed. The highest cause of death on maritime vessels is by persons entering confined spaces without checking that the air is safe to breathe.

A study by Swedish occupational medicine researchers reported online and quoted by the NZ Herald says that when logs and wood are stored in confined spaces the surrounding air may rapidly become unsafe to breathe.

Microbiological activity in the wood may result in severe oxygen depletion and the production of carbon monoxide, carbon dioxide, hydrogen sulfide and hydrocarbon gases.

The study noted the occupational hazard for people entering unventilated stairways in ship's holds had not been properly appreciated in Sweden, despite five reported deaths and several injuries in Swedish ports between August 2005 and May 2007.

In New Zealand, ships’ crews are trained to understand the rules that apply to entering confined spaces. The man hole accesses to holds are also clearly marked with warnings.

Police yesterday named the men who died as Deongchil Oh, 56, of Korea, and Thi Ha Aung, 33, of Myanmar. It is not yet clear why they entered the hold without the required breathing apparatus. The ship had not been fumigated before they entered the hold.

The ship was due to leave for Tauranga at 11am yesterday but was still tied up this morning while International Transport Workers Federation (ITWF) inspector Grahame McLaren worked through the issues raised by crew. The ship missed its scheduled departure yesterday after crew refused to sail until safety issues were resolved relating to how logs are secured on deck.

Several investigations are underway, although police said their investigation had all but finished.

"Once it's determined there's an accident we just act on behalf of coroner and prepare the file," Sergeant Ken Andrews, of Northland Police told NZPA.

Sources: NZPA, NZ Herald and forest industry sources