Workers quite seriously affected were taken to hospitals in Calais and Lille in northern France. 65 people were immediately evacuated on Monday from the Channel Tunnel for safety reasons.
About 60 people, including workers commisionned by Eurotunnel as well as Eurotunnel employees were changing rails on the line between Calais in France and Folkestone in England when the incident occured.
An investigation has been launched. "We are trying to understand what happened" said Eurotunnel.
The company said that about 20,000 sensors are arranged in the tunnel, specially to monitor carbon monoxide levels.