It was the second carbon monoxide poisoning incident in the tunnel in three days. Nineteen workers had been sickened Saturday.
"We are trying to understand what happened," said Eurotunnel, operator of the tunnel.
In the latest incident, the workers were changing rails on the train line between Calais, France, and Folkestone, England, when they became ill, Radio France Internationale reported.
Sixty-five people were evacuated from the tunnel for safety reasons, officials said.