Licensed asbestos assessors

Practical guidance to assist asbestos assessors to accurately report air monitoring.

Fibre concentration calculation guidance

The Guidance note on the membrane filter method for estimating airborne asbestos fibres 2nd Ed. [NOHSC:3003(2005)], Work Health and Safety Regulation 2011 and How to safely remove asbestos Code of Practice 2021 all refer to respirable airborne fibre concentrations expressed as fibres/mL. (fibres per millilitre of air).

All persons who conduct air monitoring for airborne asbestos fibres are required to use the below formula when calculating the dust concentration:

Where:

C = concentration (fibres/mL)
A = effective filter area (mm2)
a = projected eyepiece graticule area (mm2)
N = total number of fibres counted
n = number of graticule areas observed
r = flowrate of air through filter (mL/min)
t = single sample duration (minutes)

Alternatively, the following simplified formula can be used:

In order to perform this calculation, the person who conducts the air monitoring will need to seek the values for the effective filter area (A) and projected eyepiece graticule area (a) from the laboratory performing the fibre counting and/or suppling filter holders and cowls.

For further information on calculation and reporting airborne respirable fibre concentrations, refer to the Guidance note on the membrane filter method for estimating airborne asbestos fibres 2nd Ed. [NOHSC:3003(2005)].