ToolsAsbestosAir Sampling

4-Hour TWA Calculator

Calculate the 4-hour Time Weighted Average exposure for asbestos fibre concentrations across multiple sampling periods. CAR 2012 control limit check.

Last updated 25 February 2026

About this tool

Calculate the 4-hour Time Weighted Average (TWA) exposure from multiple air sampling periods during a work shift. Enter each sampling period with its measured fibre concentration and duration, and the tool calculates the overall TWA and checks it against the CAR 2012 control limit of 0.1 f/ml.

How to use

Add each sampling period with the measured fibre concentration in f/ml and the sampling duration in minutes. You can add as many periods as needed. The TWA is calculated over the standard 4-hour (240-minute) reference period. Any time not covered by sampling is assumed to have zero exposure.

Reference

Control of Asbestos Regulations 2012 (CAR 2012), Regulation 2. The control limit for asbestos is a concentration of asbestos in the atmosphere when measured in accordance with WHO methodology of 0.1 fibres per cm3 of air averaged over a continuous period of 4 hours. The short-term exposure limit is 0.6 fibres per cm3 averaged over 10 minutes.

Revision history

25 February 2026: Initial release.

Disclaimer

This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.

Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.

All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.

By using this tool, you accept full responsibility for how you interpret and apply the information provided.

Spotted an error? Let us know.

Sampling periods