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I’ve had a lot of very positive comments on April’s article about personal monitoring and the four-hour time-weighted average. I thought it might be a good idea to take another look – but this time take a deeper dive into the exposure geek pool. In this post I’m going to really focus on the challenges of the four-hour time-weighted average (4Hr TWA), and give a few more examples of how to do it.

What’s the point of the 4hr TWA?

It’s important to start with a quick refresher on the point of the 4hr TWA. Sam Lord of the Health and Safety Executive (HSE) recently summed it up neatly: the point of 4Hr TWA control limit testing is to really look at compliance, not just nod to it.

I discussed the wider context of the testing and explained the terminology in my previous post. The key point is that the four-hour control limit test is a duty-of-care test, intended to really examine whether we have complied with our duties as an employer.

To recap, the test specifications are:

Test Sample Rate (litres/min) Minimum Volume Minimum Graticules Resulting LoQ
4-hour Control Limit 1-2 240 100 0.04 f/ml


The control limit is 0.1 fibres per millilitre of air (f/ml) over four hours – measured inside the mask.

The newly (finally) published Analysts’ Guide specifies a simplified version of the World Health Organisation method – an attempt to increase the frequency at which these tests are being completed.

Despite referring to it as a ‘test’, it is better to think of the four-hour TWA as a calculation. It is intended to represent what happened to that person over a continuous four-hour period – it tells a story, if you like. The simplified rules are:

  • Sample rate must be 1-2 litres per minute
  • The total volume of air sampled must be 240 litres or more

There’s a very important point to make here: you don’t have to do a single four-hour air test to calculate a valid 4hr TWA. What you do need is a record of what happened over those four hours, with test results that support the ‘story’.

The HSE’s simplified approach to this is to require a minimum total air volume of 240 litres. If you do the maths, that means you need at least a two-hour (120-minute) test at two litres per minute.

However, now the analysts’ guide is out, it’s clear that how you get to those numbers is a bit more flexible. You could conduct three individual tests one after the other. Importantly, certain assumptions can also be drawn.

The calculation

Let’s go through some examples of how you do these calculations by hand. The calculations for all of these examples are taken directly from the new analysts’ guide. Please note that the HSE works in hours and fractions of hours, rather than minutes.

Also, before I start, I should point out that while the sums are straightforward, the calculations are yet another burden for asbestos professionals. Later on I’ll explain how Assure360 can do it all for you.

Example 1 – a series of tests to paint a picture

Consider this shift inside the enclosure:

  1. Removal of asbestos insulating board (AIB) tiles for 30 minutes (0.5hrs), with a result of 0.15f/ml
  2. Boring out of fixing holes for 45 minutes (0.75hrs), with a result of 0.1f/ml
  3. Fine cleaning of the enclosure for two hours and 45 minutes (2.75hrs), with a result of 0.06f/ml

Because we know the sample rate was at least one litre per minute, and the total test duration is four hours, we know that the total air volume exceeds the magic number of 240 litres. In this case we can tell the story of the whole four hours.

This is where the ‘time-weighted’ bit comes in – we multiply each of the measurements by the duration of that specific activity to calculate total exposure. Then we divide by the total duration to produce an effective average exposure rate for the entire duration of the tests:

Picture 1

Example 2 – what if the work lasts more than four hours?

An activity will often carry on beyond the tested period. In this case, the guidance says you can assume that the exposure continued at the same rate, provided the minimum flow rate and sample volume are met. Consider this activity:

  1. Removal of nailed-on AIB for six hours.
  2. Tested at two litres per minute, for three hours and 20 minutes, with a result of 1.2f/ml.

Here, the test duration and flow rate ensure the total sample volume is well over the minimum 240 litres. Accordingly, you can assume that the last 40 minutes of the four-hour window would be same as the result of the actual testing:

Picture 1

Example 3 – what if the work lasts less than four hours?

Similarly, where the work (and the air test) last for less than four hours, we can extrapolate. Consider this morning’s work:

  1. Removal of asbestos pipe lagging for two and a half hours.
  2. Tested at 1.6 litres per minute, for two and a half hours, with a result of 2.5f/ml.
  3. Operative exits the enclosure and no more exposure happens (e.g. lunch)

Again, as the sampling rate was 1.6 litres per minute for 150 minutes, this means that the total sample volume meets the 240-litre minimum. Accordingly, we can calculate a four-hour TWA:

Picture 3

Of course, if you’ve got the computing skills, you can create a spreadsheet that will do all of that for you. Alternatively it can be done by hand every time. But while the former is bad enough, the latter is quite soul destroying – and either could be prone to mistakes.

Making it simple

Happily we’re able to offer Assure360 customers an alternative. I’ve long sought to simplify personal monitoring for licenced asbestos removal contractors (LARCs), and I want to support the HSE’s renewed push to improve it. We’ve added a new TWA tool to the Assure360 platform, allowing the system to do the heavy lifting for you – and saving hours.

Last month our Paperless app was enhanced so that, along with the test result, it will collect three more data points:

  • Flow rate
  • Duration of the air test
  • RPE worn

Now that this data is being collected, the Assure360 database reports will, over the coming weeks, give you more and more power to do TWA calculations.

We’re already providing the original simple time-weighted result for every compliant test. We also calculate it for every situation where it is known that exposure continued for the full four hours. Next month we will be adding a system to pool multiple results together, to complete the suite of tools.

By automating calculation of the four-hour TWA, I hope that we can help ensure that more four-hour tests are carried out, and fewer mistakes and misunderstandings made. More than anything, we want to help the HSE improve the depth and quality of personal monitoring, and improve the safety of everyone who works in our industry.

Want to see first-hand how Assure360 Paperless simplifies the four-hour TWA? Why not get in touch to book a free demonstration?

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"When it comes to the licence assessment, Assure quickly enables you to extract the information which will help prove that you are complying with the HSE's licencing criteria."
Clinton Moore, Director, Sperion