The lowest dose or exposure to a chemical at which a specific effect is observed.

Time-weighted average concentration (TWA)
Refers to concentrations of airborne toxic materials which have been weighted for a certain time duration, usually 8 hours.

TLV (Threshold Limit Value)
A time-weighted average concentration under which most people can work consistently for 8 hours a day, day after day, with no harmful effects. A table of these values and accompanying precautions is published annually by the American Conference of Governmental Industrial Hygienists.

A relative property of a chemical agent and refers to a harmful effect on some biologic mechanism and the conditions under which this effect occurs.