concentration for a normal 8-hour working day (40 hours/week) to which all workers may be exposed without adverse effect.

(TLV-TWA, Threshold Limit Value-Time Weighted Average) The time weighted average airborne chemical concentration for a normal eight hour work day and a 40 hour work week to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.

An exposure limit averaged over eight hours that must not be exceeded during an employee's workday.

The yardstick used by the Occupational Safety and Health Administration (OSHA) to measure noise levels in the workplace. It is equal to a constant sound level lasting eight hours that would cause the same hearing damage as the variable noises that a worker is actually exposed to. (This hearing loss, of course, occurs over long-term exposures.) Same as LOSHA.

The average time, over a given work period (e.g., 8 hour work day), of a person's exposure to a chemical or an agent. The average is determined by sampling for the contaminant throughout the time period. Represented as TLV-TWA.

The TWA limit is the maximum average concentration to which an unprotected worker may be exposed over an eight hour working day. The TWA is computed as a simple arithmetic average of worker exposure projected over an eight hour day. Time weighted averages permit excursions above the TWA limit only as long as they: (1) Do not exceed the STEL or ceiling limit and (2) Excursions above the TWA must be compensated by equivalent excursions below the limit.

an average concentration over an 8-hour day for 5 working days.

Because the noise level is not constant over time, a method of computing the entire equivalent noise dose is required. This is the time weighted average.

The average value of a parameter (e.g., concentration of a chemical in air) that varies over time.

Method of measuring the maximum safe exposure of workers to hazardous materials or conditions.