The number of patients being treated at an industrial site in Levin has risen to 22.
Hato Hone St John ambulance, police and fire crews were called to Hamaria Rd about 6:30pm after a chemical incident.
Firefighters set up decontamination gear at Alliance Group's meat processing plant after reports of a gas leak.
It was initially reported 19 people were being treated.
Five ambulances, three rapid response units, two operations managers and a St John Major Incident Support Team were called to the scene.
A fire crew from Otaki was also called in to help Levin firefighters and a specialist fire unit arrived from Palmerston North.
A spokesperson for Alliance Group confirmed processing at the plant had suspended after a chemical incident at the site led to a "gas reaction".
St John said while it was not yet know whether any of the patients was badly injured, no one has been taken to hospital and no further ambulances had been requested.
Police have cordoned off the area.
