The incident happened as some 60 workers were changing rail tracks on the line between Calais, France, and Folkestone, UK.
A welder was taken ill in the early hours of Sunday and carbon monoxide poisoning was later diagnosed.
Tunnel traffic was unaffected.
Eighteen other workers have also been taken ill.
They were sent to local French hospitals to give them "the time needed to remove the carbon monoxide from their system", an official said, adding their condition was not a cause for concern.
The 41 other workers went home.
An investigation has been launched into the cause of the incident.
Nineteen workers have suffered carbon monoxide poisoning while working overnight in the Channel Tunnel, with one of them in a serious condition, French officials have said.