An investigation into a chemical accident at Tasman Tanning's plant in Wanganui last week continues, but it is not known when the results will be known.
Two staff were flown to Wellington Hospital for treatment and another 19 assessed at Wanganui Hospital after being exposed to hydrogen sulphide last Friday afternoon.
The accident happened when two staff mistakenly mixed sulphuric acid into the wrong processor, creating a "cloud" of highly toxic gas.
Both men have since been discharged from hospital.
Yesterday a spokesman from the Ministry of Business, Innovation and Employment said inspectors were at the scene soon after the incident and the plant was back operating the next day.
However, the spokesman said the ministry could not give a timeline as to when the investigation would be completed.
"These investigations can take a long time to complete, sometimes up to six months or more," he said.
However, most investigations are completed sooner than that.
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The inspectors, formerly with the Department of Labour, work under terms of the Health and Safety in Employment Act.
That act demands that employers and people in control of workplaces must provide a safe working environment. In particular, they must identify equipment or workplace practices that are hazardous, and take steps to eliminate hazards, isolate people from hazards, or minimise the risk that someone could be harmed.
They are also required to train and supervise employees until they can work safely, involve employees in keeping the workplace safe and to report serious accidents or near misses to the authorities.
"The inspectors investigate when someone is killed or seriously harmed at a place of work. And they also investigate serious 'near misses' and incidents involving hazardous substances such as occurred at the tannery," the spokesman said.
The investigation's main aim is to identify why people were harmed, and what could have been done to prevent this. Knowing what went wrong can help employers and industries prevent similar injuries in future.
But where the investigations reveal breaches of the HSE Act, enforcement action may be taken.
The investigation usually involves talking to victims, witnesses and others with information relevant to the inquiry. Inspectors often conduct investigations at the scene.