The limit of concentration of an airborne agent to which humans can be exposed to without harmful effects. Substances fall under TLVs in three ways: 1) TLV-TWA–Time weighted average, based on an allowable exposure averaged over a normal 8-hour workday or 40-hour work- week; 2) TLV-STEL–Short-term exposure limit or maximum concentration for a brief specified period of time, depending on a specific chemical (TWA must still be met); and 3) TLV-C–Ceiling Exposure Limit or maximum exposure concentration not to be exceeded under any circumstances. (TWA must still be met.)