Although the 50-year-old miner inhaled a lethal amount of the gas, a spokesman for the K+S mine near Wunstorf said the other men had been taken to various hospitals and were out of danger. Three have already been discharged, the spokesman said on Thursday morning .
The Lower Saxony state mining authority has begun an investigation into the accident, which happened late on Wednesday night. A spokeswoman for the authority said two officials were already at the scene.
Work has been suspended while fresh air is pumped through the tunnels and samples are tested. "At the moment it looks as if it was not a serious gas leak," the K+S spokesman said.
"This is the first time we've had a problem like this," he added. "It will have to be investigated." But he admitted that a miner had been killed last month by a falling rock. There was also an underground fire at the company's nearby Sigmundshall mine in 2009.
K+S operates four potash mines in Germany for the production of fertilizer and road salt. Around 800 workers are employed at the mine in question, which has been open since 1898.