At least 38 labourers felt unconscious while cleaning Supra Dam in Bara area of Khyber Agency on Saturday.
Political administration said the annual repairing of the dam was underway and the labourers were cleaning the tunnel.
The oxygen level in the tunnel suddenly decreased and all 38 labourers felt unconscious gradually.
They were immediately transported to nearest hospital for first aid in critical condition.
However, now hospital staff avoid to give any statement while Police started its investigation.
