Four-hour time-weighted average tests

Following the long-awaited release of the revised HSG248 Asbestos: The Analysts’ Guide Vintec are going to try to explain what it all means for you.

First up is the much-misunderstood four-hour time-weighted average testing. Having spoken to several licensed asbestos removal contractors it sounds like the HSE are expecting these tests to be carried out far more frequently than they have in the past. The tests are a legal requirement and are by far the best way of monitoring employee exposure.

One common misconception is that you need to run a continuous four-hour air test to calculate a four-hour time-weighted average. In practice, there are very few occasions when this will be possible without completely overloading the filter and rendering it uncountable. It is perfectly acceptable to run multiple sequential air tests and calculate the overall result from these tests.

Four-hour time-weighted average testing must have a flow rate of 1-2 litres per minute and a minimum sample volume of 240 litres across all the combined tests.

Below is an example to demonstrate how the calculation works.

Example: Removal of AIB ceiling panels

This example covers the removal of a small area of AIB. The testing period starts when the AIB removal commences and also includes a period of fine cleaning. The testing starts at 9am and finishes at 1pm. Three consecutive tests of different duration were taken by the Analyst.
The below illustration shows the work activity, test durations and airborne fibre concentrations during the period of testing:

As you can see, the operatives entered the enclosure at 9am to start removing the AIB. This was completed at 11.15. They then conducted fine cleaning until 13.00.

The details of the three air tests results are as follows:

Test 1.

Sampling time: 09.00 to 10.00 (1.0 hours)
Result of test: 0.62 f/ml

Test 2.

Sampling time: 10.00 to 11.15 (1.25 hours)
Result of test: 0.78 f/ml

Test 3.

Sampling time: 11.15 to 13.00 (1.75 hours)
Results of test: 0.25 f/ml


Calculation of the four-hour time-weighted average

To calculate the four-hour time-weighted average:

(hours x f/ml) + (hours x f/ml) + (hours x f/ml)


When we add in the figures for the above tests we get:

(1.0 x 0.62) + (1.25 x 0.78) + (1.75 x 0.25)


The simplest way to work this out is to calculate the individual brackets first. This gives us:

0.62 + 0.98 + 0.44


Which gives us a four-hour timeweighted average exposure of 0.51 f/ml

Breaks and rest periods

The testing period doesn’t have to cover four hours of constant activity. Rest periods and lunch breaks can be included if the operatives leave the enclosure. To include this in the calculation you assume a zero exposure level for the duration of the break.

So if a one-hour lunch break was taken, this would be included in the calculation as: (1.0 * 0)

In a full calculation this would appear as:

(1.5 x 0.40) + (1.0 x 0) + (1.5 x 0.60)


0.60 + 0 + 0.90

Which, in this instance gives a four-hour time weighted average exposure of 0.38 f/ml

Of course, you don’t need to go through all the hassle of calculating your own four-hour time weighted averages. Our team of Analysts can take care of all this for you. For more information or to book an Analyst contact Vintec on 01923 661144.

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