Two people are being flown to Wellington Hospital in a critical condition after being exposed to hydrogen sulfide in a chemical spill at a tanning factory in Wanganui.
Another 19 people have been exposed to the gas, and were being assessed at Wanganui Hospital, a DHB spokeswoman said.
Emergency services were called to Tasman Tanning in Castlecliff about 5pm today following a chemical spill.
The spokeswoman said two people had been ventilated after being exposed to the toxic gas.
They were in a critical but stable condition and were due to fly to Wellington Hospital for further treatment tonight.
Hydrogen sulfide is a colourless, poisonous, flammable gas. It has a strong smell, similar to rotten eggs.
The 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.
One woman, who lived on Heads Rd not far from the factory, said the smell was "enough to make you dry retch''.
Gonville resident Brad said he could smell "something really bad'' at his house several kilometres away in Gonville Ave.
Vivek, from the Four Square shop across the road from the factory, said it was not until he heard a lot of sirens coming past that he realised something was wrong.
"Until then, I never heard anything. But I heard the sirens and came outside, and that's when I noticed the bad smell. It was very strong.''
The condition of the 19 other people taken to Wanganui Hospital was not known tonight.
Tasman Tanning employs about 200 people and supplies leather to manufacturers around the world.
The family-operated business has been in operation since 1953.