ToolsAsbestosEnclosure Setup

NPU Air Changes Calculator

Calculate air changes per hour for an asbestos enclosure based on NPU extraction rate and enclosure dimensions. Check extraction adequacy.

Last updated 3 March 2026

About this tool

Calculate the number of air changes per hour achieved by a Negative Pressure Unit (NPU) in an asbestos removal enclosure. This is a quick sanity check to confirm whether your extraction capacity is adequate for the enclosure size.

How to use

Enter the enclosure dimensions (length, width, height in metres) and the NPU extraction rate in cubic metres per hour. The tool calculates the enclosure volume and the resulting air changes per hour, and indicates whether extraction is adequate.

Reference

HSG248 — Asbestos: The analysts' guide for sampling, analysis and clearance procedures (HSE, 2021). A minimum extraction rate of 1,000 cubic metres per hour is typically required. For larger enclosures, the number of air changes per hour should be checked to confirm adequate ventilation.

Revision history

3 March 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.

Enclosure length (m)

Enclosure width (m)

Enclosure height (m)

NPU extraction rate (m³/hr)

1,000 m³/hr minimum for small enclosures (<120 m³).