A reaction between alkaline and acidic material caused the fumes which put a man in Whanganui Hospital on Tuesday afternoon.
Fire and Emergency New Zealand were called to a rural property at Whangaehu near Whanganui at about 12.30pm.
"We've got one person that's been overcome by fumes," Whanganui senior station officer Aaron Summerhays said.
The man was conscious when taken by ambulance and a FENZ spokesperson said he was unharmed.
"All we know is that he was in the shed around where the products were and got overcome by some fumes and managed to notify someone which tipped us off," Mr Summerhays said.
A cordon was put up at the gate of the Ruatangata Rd property as a hazmat unit was used to deal with the chemicals.
Fire crews from Whanganui, Marton and Ratana were called to the scene.