Time-Weighted Average Exposure

\[TWA=\frac{\sum(C_i t_i)}{\sum t_i}\]

Variables

TWAtime-weighted average exposure concentration

C1exposure concentration during period 1

t1duration of period 1

C2exposure concentration during period 2

t2duration of period 2

C3exposure concentration during period 3

t3duration of period 3

Description

What is this formula?

The Time-Weighted Average (TWA) Exposure calculates the average exposure concentration over a specified period while accounting for varying exposure levels and durations.

When to use it

Use this formula when exposure concentrations change throughout the day and a single representative exposure value is required.

Example

A worker experiences the following airborne solvent concentrations during an 8-hour shift:

Data:

C1 = 20 ppm for t1 = 2 h

C2 = 50 ppm for t2 = 3 h

C3 = 10 ppm for t3 = 3 h

Formula:

TWA = ((C1×t1)+(C2×t2)+(C3×t3))/(t1+t2+t3)

Substitution:

TWA = ((20×2)+(50×3)+(10×3))/(2+3+3)

Result:

TWA = 27.5 ppm

Applications

- Occupational hygiene

- Industrial toxicology

- Air quality monitoring

- Regulatory compliance

- Workplace exposure assessment

Note

The general TWA equation is based on a weighted average using all exposure periods. The fCalc implementation uses a three-period explicit form because implicit summations are not supported. Additional periods may be incorporated by extending the same weighted-average principle.