Another 19 people were exposed to the gas, and were taken to Wanganui Hospital, but they were all discharged on Friday evening, a spokeswoman said.
Emergency services were called to Tasman Tanning in Castlecliff following a chemical spill.
Tannery workers, some of them were attended to by paramedics, were seen sitting or lying on the grass outside the factory.
Police, St John’s ambulance and the fire service attended, with multiple ambulances and the fire service’s hazardous materials unit at the scene. A decontamination unit was set up on Tod street.
Two people were ventilated after being exposed to the toxic gas.
Hydrogen sulphide is a colourless, poisonous, flammable gas. It has a strong smell, similar to rotten eggs. The hospital spokeswoman said the chemical affected lungs and breathing.
People living and working in the area told the Wanganui Chronicle the first they knew of the incident was a very strong smell of sulphur.
Tasman Tanning employs about 200 people and supplies leather to manufacturers around the world.
The family-operated business has been in operation since 1953 and make bovine upholstery and footwear leathers.
