Mine Rescue Field Exam From the 2012 North Central Regional Mine Rescue Contest, Wilmington, IL Progress Indicator: Question 1 of 30 1.  The Threshold Limit Value (TLV) of a gas is: Choices: the average concentration of a gas a worker is allowed to be exposed to the maximum concentration of a gas a worker is allowed to be exposed to the average concentration of a gas a worker is allowed to be exposed to over an 8-hour working period the average concentration of a gas a worker is allowed to be exposed to over his normal work shift