MAC (Maximum Allowable Concentration) is the common abbreviation for a legal exposure standard. It is defined as the allowable time-weighted average concentration for an 8-hour work day and a 40-hour work week in which a worker may be repeatedly exposed without adverse health effects. Related value in the USA is TLV (Threshold Limit Value), this is not a legal standard but a guideline published by the American Conference of Governmental Industrial Hygienist (ACGIH). Both values are measured in the breathing zone of the welder.