I thought I would return to this subject before the summer holidays – a period when, unlike the rest of the country, we become incredibly busy. It remains a live issue on site, with some Health and Safety Executive (HSE) inspectors from a few areas of the country insisting with fervour that it’s mandatory to measure air flow on a negative pressure unit (NPU) before any licensed job.
That’s at odds with my assessment (following my recent strawpoll) that only 40% of licensed asbestos removal contractors (LARCs) use hand anemometers at all – and most of those not because they see any value, but because their local inspector insisted on it.
In my last post on this subject, I went back to the Approved Code of Practice (ACoP), and asked “Why are we doing this, and why is it important?”. In my view, only when you understand this can you make the judgement calls that the real world forces on you, when reality doesn’t quite fit what someone wrote 12 years ago, hundreds of miles away.
This time I will summarise some of those thoughts, show how data from the coal face calls the tests we do into question, and discuss what we really should be doing.
To go back to the very basics, we’re talking about the enclosures that operatives use to safely contain asbestos fibres and other traces of the material when working on a licenced removal job. The requirements for these are that they contain the hazard, and that they minimise the exposure for the operatives working within. We all agree that NPUs are critically important, as they underpin most of the controls that keep asbestos operatives and those immediately outside an enclosure safe.
NPUs – negative pressure units – do exactly that. They apply negative pressure, so that any damage to the enclosure results in clean air leaking in, rather than contaminated air leaking out. Also, clean air rushes in to replace the contaminated air they suck out and clean, which means that the operative is protected by a constant dilution of the contaminated atmosphere.
To underline their importance, paragraph 395 of the ACoP states:
Before starting work in the enclosure, a thorough visual inspection and smoke test must be conducted to check the enclosure’s integrity. The filtered air extraction equipment must be tested to ensure it is achieving negative pressure and the required air change rate.
Note the word ‘must’ that I underlined. Guidance very rarely uses such strong words – and when it does, it indicates that something is mandatory. But while it tells us what we should be testing for, it doesn’t tell us what the test itself should be.
Let’s look at some of the other wording. We are testing to ensure ‘negative pressure’, and that ‘required air changes’ are achieved. Required air changes is a rough measure of the dilution I mentioned above. Elsewhere in guidance it tells us that in the UK we should get at least eight air changes per hour (ACH) – that means simply you need sufficient NPU power to replace all of the enclosure’s air eight times every 60 minutes.
Negative air pressure means that the atmospheric pressure inside the enclosure should be lower than outside. In fact, it should be at least five pascals (5Pa) lower.
As I mentioned above, some HSE inspectors are insisting that NPUs MUST be tested using an anemometer or an onboard airflow meter. Their interpretation of the above guidance is that ‘test’ means to test how powerful the negative pressure unit is, and that a handheld anemometer should be used to do this.
You’ll have to read my earlier article for my reasons why I think these are inappropriate devices for the rough and tumble of site life. In this post I wanted to add some interesting data to illustrate my point.
The data in the table below was provided by a skilled (specifically trained) engineer on various sites, using an expensive calibrated anemometer – significantly more high-end than the ones typically used by LARCs. The equipment has been calibrated within three months of the test – and kept carefully off-site in between. The tests were repeated more than once in each setting and using different techniques to compare.
| NPU Ref | DOP Certified Airflow * | Onboard Airflow gauge + | Standard 5pt Anemometer Reading ** | Exhaust Anemometer Reading ++ | Spread (overall) DOP vs Lowest Site Reading | |
| Difference highest and lowest value | % | |||||
| 001 (1st Reading) | 5860 | 5301 | 4278 | 3922 | 1938 | 36.56% |
| 001 (2nd Reading) | 5860 | 5180 | 4758 | 4490 | 1370 | 26.45% |
| 001 (3rd Reading) | 5860 | 5332 | 4490 | 4745 | 1370 | 25.69% |
| 002 (1st Reading) | 2085 | 1796 | 1848 | 2047 | 289 | 16.09% |
| 002 (2nd Reading) | 2085 | 1821 | 1833 | 1992 | 264 | 14.50% |
| 002 (3rd Reading) | 2085 | 1927 | 1748 | 1935 | 337 | 17.49% |
| 003 (1st Reading) | 2450 | 2076 | 1095 | 1315 | 1355 | 55.31% |
Notes to the table:
As you can see there is a massive range of values. Note that readings – whether the on-board flow meter, the standard 5pt anemometer test, or the exhaust reading – swing enormously from moment to moment.
It is arguable that the DoP-certified test should be the most accurate, as it is done on a test rig, but this consistently gives a higher reading than either the onboard flow meter or either of the anemometer tests. In one case the DoP-certified value was stating a performance:
The hand anemometer readings swung between 5-11% when repeated.
Speaking as a scientist, any test that gives us this kind of range in our results is close to useless. But importantly, in any case this is a test of how powerful the NPU is, not what the ACoP says we must test for – negative pressure and air changes achieved.
Why is that relevant? Even if we decide that we can accept a swing of 20% and we should just pick an average, all this result would do is indicate ample airflow. But if there is air getting in somewhere else (a leak in the enclosure, big or small) you won’t have adequate negative pressure. Similarly, while we may be drawing out and cleaning lots of air, a poorly-designed enclosure will leave pockets of still air that’s not being cleaned at all.
As I said in my previous post, by testing a feature of good enclosure design, rather than the outcome we’re trying to ensure, we have gone down an unnecessary rabbit hole. And a warren that is costly, time consuming and potentially very misleading.
Let’s look again at what we are trying to achieve:
Impose 5Pa of negative pressure
Good airflow is essential for negative pressure, but in isolation it tells us little about whether we have achieved it, so how can we test for it?
There is a ready reckoner for 5Pa of negative pressure. If you balance the enclosure (air in and air out is roughly equal), and the flap deflection on your main air lock is around 250mm, you will have achieved approximately 5pa.
However, if the word ‘test’ in the ACoP requires us to be more scientific than this, you can actually measure negative pressure easily. Dwyer pressure gauges that accurately state what the pressure is are inexpensive, very simple, and robust. Simply tape one to the side of the enclosure and it will compare pressure inside and out. And what’s more this doesn’t leave you with a single reading taken at the time of the smoke test – it gives you constant monitoring.
No dead spots
Remember we need to ensure there are no dead-spots, and therefore no highly contaminated pockets of air. Simply measuring how impressive the airflow is at the NPU tells us nothing about how successfully we are cleaning and diluting the air. The only way you can actually test for sufficient air changes is by the ‘old fashioned’ smoke test. You can actively see the smoke moving, and can redesign the enclosure to make sure there are no dead spaces.
Even if the mandated eight air changes per hour sounds like one change every 7.5 minutes, we shouldn’t think that all smoke will be gone within that timeframe – dilution just doesn’t work like that. When you can no longer see any smoke, you’re getting an indication that you’re heading towards a complete change of the air.
Nearly all contractors use a minimum of 10 ACH, and often quite a bit more. Is it tremendously important that you are achieving 8.2, 9.7, 10.4 or 15.1 ACH per hour? I would argue that it’s not. In fact my position is firmly that nominal ACH is not a useful test of the effectiveness of our controls at all. It’s more important that you know there are no dead spots, and that smoke does indeed clear from all areas.
As I said in my previous post, we can likely achieve all that we want by having NPUs that are capable of drawing air to the tune of about 10 times the total enclosure volume. But the only way to test we’ve actually met the requirements in a balanced enclosure is:
As I hope I’ve explained, this is an area of regulation and guidance that is poorly understood and often (in my opinion) blindly and incorrectly enforced. The tests some HSE inspectors are insisting on indicate some things, but they don’t specifically tell us if we’re meeting the requirements of the guidance. And as is always the case, if we’re falling short of the guidance, we could be exposing operatives and building users to risk.
As with my earlier post, I’m hoping to stir up a discussion about the fundamentals of enclosure airflow. You might agree or disagree, but please get in touch to tell me why, so we can all learn and get better at what we do.
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