The Full Form of TWA is Time-weighted average.

A method of calculating a worker’s daily exposure to a hazardous substance (such as chemicals, dusts, fumes, mists, gases, or vapors) or agent (such as occupational noise), averaged to an 8-hour workday, taking into account the average levels of the substance or agent and the time spent in the area. A time-weighted average is equal to the sum of the portion of each time period (as a decimal, such as 0.25 hour) multiplied by the levels of the substance or agent during the time period divided by the hours in the workday (usually 8 hours). OSHA uses this guideline to determine permissible exposure limits (PELs) and it is essential in assessing a worker’s exposure and determining what protective measures should be taken. Also see permissible exposure limit.