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Twenty people were overcome by ammonia fumes in Montague Gardens in Cape Town on Monday afternoon.
The city's disaster risk management centre said the incident happened when ammonia was released into the atmosphere in the Montague Gardens industrial area.
Spokesperson Wilfred Solomons-Johannes said a faulty safety valve appeared to have popped off a cylinder at the Igloo Ice plant.
He said the area was evacuated and 20 people were slightly overcome by the fumes.
The incident is being investigated and the plant will be required to report back on follow-up safety measures.