Four Pan Pac Forest Products workers required medical treatment yesterday following a gas emission from a kiln at the Whirinaki site.
The kiln was the same one that had been shut down in April 2015 because of issues with emissions safety and health concerns.
Pan Pac general manager Doug Ducker said there had been an operational failure on the kiln, and the staff members had been subjected to gases emitted from the kiln which were normally burned. He said there had been a limited period of exposure and staff "registered with effects" were treated.
Three people were taken to the Napier Medical Centre and one to Hawke's Bay Hospital.
A hospital spokesperson said the person had since been discharged.