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Fire and Emergency New Zealand East Otago assistant area commander Craig Geddes said crews were called at 3.30pm on Sunday after a man at a Mountfort St property accidentally smashed a small vial containing chloropicrin.
The chemical had been used in the past as a soil fumigant and rabbit poison, but had historically been deployed in chemical warfare, he said.
Mr Geddes said Outram Volunteer Fire Brigade Chief Fire Officer Stu Casey made the decision to escalate the response based on the advice of chemistry experts. Dunedin's hazardous material (Hazmat) command truck was called in, along with pumps from St Kilda and Dunedin.
``He did a great job,'' Mr Geddes said of Mr Casey.
Fire crews donned level-4 gas suits, the highest level of protection available. They then set about isolating, identifying, recovering and disposing of the poisonous chemical.
However, there was no immediate danger to the public due to the small amount of the chemical spilled and the response was largely precautionary, he said.
The property owner, who asked to remain anonymous, said he had found the vials of chloropicrin at his holiday home in Central Otago and took them home to Outram.
The property owner had accidentally smashed one of the vials while moving things around in his shed on Sunday afternoon.
The man had been seen by Dunedin Hospital accident and emergency department staff but had not suffered any ill-effects.
One of the chemistry experts consulted by Fenz was University of Otago professor Lyall Hanton.
Prof Hanton said it was the first time he had come across chloropicrin at in incident in his role on the region's hazardous substance technical liaison committee, a group with representatives from government agencies, fire and police.
``I only get called in when it's a bit exotic.''
Because the vials contained only about 5-10ml of the chemical in a relatively low concentration, there was no immediate danger to the property owner, but Fenz had done the right thing in escalating the response to include a Hazmat command unit, he said.
``If you were faced with a World War 1 chemical you'd want to take some precautions.''
Prof Hanton said he had entered the shed after Fenz had completed its clean-up, and he could still smell a whiff of the chemical, even after the tiny volume spilled had been dealt with.
Yesterday, Prof Hanton visited the Dunedin Central Fire Station where the chemical had been stored after being recovered from the Outram property, where he had assisted with its ``remediation'' (neutralisation and destruction).
When used in chemical warfare, chloropicrin has characteristics similar to tear gas and causes vomiting.
It would force soldiers to remove their gas masks, leaving them exposed to other, more deadly chemical agents, Prof Hanton said.
The chemical had been used by both sides during World War 1, but was first used by German soldiers against Italy on the Western Front in the spring of 1917 , according to a 1936 report by Colonel Adelno Gibson of the United States Army chemical warfare service.
What is chloropicrin?
Structural formula: CCl3 NO2 .
Discovered: 1848 by Scottish chemist John Stenhouse.
Appearance: Colourless to faintly yellow-orange liquid.
Use: Soil fumigation (broadspectrum herbicide, insecticide, fungicide and antimicrobial), chemical warfare (with properties similar to tear gas).
Absorption: Inhalation, ingestion, skin contact.
Effects: In high concentrations skin exposure can cause blisters and difficultly breathing. Inhalation causes vomiting, coughing and choking, and can lead to death.
If found: Call 111 immediately.