Threshold Limit Value; a term used by ACGIH to express the airborne concentration of a material to which nearly all persons can be exposed day after day without adverse effect. ACGIH expresses TLV's in three ways:

  • TLV-TWA: The allowable Time Weighted Average concentration for a normal 8-hour workday or 40-hour work week.
  • TLV-STEL: The Short-Term Exposure Limit, or maximum concentration for a continuous 15-minute exposure period (maximum of four such periods per day, with at least 60 minutes between exposure periods, and provided that the daily TLV-TWA is not exceeded).
  • TLV-CL: The Ceiling exposure limit - the concentration that should not be exceeded even instantaneously.
  • TLV's are reviewed and revised annually where necessary by the ACGIH.

Importance: If a TLV is exceeded, a potential health hazard exists and corrective action is necessary. Also see "Skin" relative to TLV's.