The Definition of Time-Weighted Average (TWA)

Definition of Time-Weighted Average (TWA)

A calculation method used to determine a worker's average exposure to an airborne contaminant across a full work shift, typically 8 hours, accounting for fluctuations in concentration throughout the day rather than a single point-in-time measurement.

Example of Time-Weighted Average (TWA)

An air monitoring device worn by a worker in a paint shop records varying solvent vapor levels throughout the shift, and the resulting TWA is compared against the substance's permissible exposure limit to confirm compliance.

Frequently Asked Questions: Time-Weighted Average (TWA)

Why is TWA used instead of measuring peak exposure alone?

‍TWA reflects overall exposure risk across a full shift, since brief spikes in concentration may be less significant to long-term health than sustained exposure — though short-term limits are also tracked separately for acute risks.

How is TWA calculated?

‍TWA is calculated by summing the product of each measured concentration and the time spent at that concentration, then dividing by the total sampling period, usually 8 hours.

Does TWA account for exposures outside the standard 8-hour shift?

‍Standard TWA calculations are based on an 8-hour shift; for extended shifts, adjusted exposure limits or modified calculation methods are often applied to account for additional exposure time.