What’s a thorough examination and test?


Companies that have installed local exhaust ventilation systems have to undertake a “thorough examination and test” (TExT) at least once every 14 months. The objective of the TExT is to find any significant defects and to have them remedied to regain control. In most cases they will employ an external organisation to carry out these tests so that they can meet their legal obligations. There are a large number of companies offering this service – sadly not all of them do a proper job. Sometimes this is due to a lack of competence but it may also be because the test engineers are under pressure to test systems quickly to maximise the number of tests they can carry out during the day. Unfortunately that leads to corners being cut and too many testing companies carry out a minimal, rather than a thorough test.

One of our consultants was visiting a client recently to carry out ventilation testing on a system used to control fume generated during brazing operations in a plumbing shop. It consisted of 8 individual flexible arm captor hoods connected to a main duct from the fan (see photograph below). As I’ve discussed in a previous post, captor hoods are rarely effective at controlling contaminants and other types of hood are usually preferable. The ductwork design, with no attempt at balancing, means that it would be difficult to achieve the same velocity in each branch.  However, in this case, the design would probably be adequate providing management ensured that the users positioned the hoods are carefully over the plume of fume generated during brazing and the appropriate capture velocity could be achieved in each leg.

The system had previously been tested by another company and our consultant asked to see the report. He needed this so he could compare his results with those from the previous test so that he could determine whether there was any deterioration in the system’s performance. The report he was given (see below) was a good example – of an inadequate test report.


Guidance on what should be done during a TExT is provided in the Health and Safety Executive’s document HSG258 Controlling airborne contaminants at work: A guide to local exhaust ventilation. This advises that a TExT will normally involve three stages:

  1. A thorough visual examination to verify the LEV is in efficient working order, in good repair and in a clean condition.
  2. Measuring and examining the technical performance to check conformity with commissioning data. Even on a simple system this would normally include determining hood capture and face velocities and duct velocities and measuring static pressure at behind all hoods and across the air cleaner (where present) and fan.
  3. An assessment to check the control of worker exposure is adequate. At the very least, this will involve the use of smoke tubes or a dust lamp.

Of course it isn’t always possible to carry out all the tests recommended in HSG258 due to design, safety and practical considerations. Nevertheless a proper TExT, even on a simple system, will involve a number of different measurements and tests.

For the multi branch system, as in this case, I’d expect a TExT to involve, as a minimum, the following:

  • a thorough visual examination
  • a smoke test on each hood as a way of checking on how effective they were at capturing the fume generated by the brazing process
  • capture velocities measured on each hood at the point further from the hood where the contaminant could be generated
  • face velocities measured at each hood
  • static pressures measured behind each hood
  • measurement of the duct velocity in the main branch (it may not be necessary to measure duct velocities in each branch
  • the static pressure across the fan
  • checking the fan to ensure that it is rotating in the correct direction

The test carried out by the engineer from the previous company involved only a visual examination and measurement of duct velocities. According to the report, an assessment of control had been made from these results. Not only was the scope of the testing inadequate, the report states that the duct velocity had been measured using a vane anemometer. This is an inappropriate instrument for this type of test. I suspect the engineer had placed the anemometer at the entry to the duct inside the hood. Not only is this not the best way to measure duct velocity, It would be difficult to position the anemometer properly to measure the flow into the duct.

The “duct” velocities given in the report are all around 10 m/s. That would be adequate for conveying fume, but would the contaminants be captured effectively? No face or capture velocities are provided. If the hoods were used as receptors, and were positioned very carefully above the plume of contaminants, there was a chance that the system would achieve adequate control. But this would need to be verified by observations of the process and, ideally, a smoke test. Neither was done.  It’s probably more likely that the arms would be used a capture hoods. In that case it would be necessary to check that the capture velocity was adequate. With moveable hoods of this type the best approach would be to determine the distance from the hood face where the minimum acceptable capture velocity could be achieved and include that in the report so that the user could be informed. There is nothing in the test report to suggest any of these important measurements were done.

In my view, the original report was inadequate. More time was needed to carry out a proper thorough examination and test. No doubt their company charged the client considerably less than we did – but, as with most things in life, you get what you pay for.

Even when an outside company is employed to carry out TExT the legal obligation to ensure that the test is thorough remains with the owner of the system. So they need to ensure that they’re using a competent company who will carry out the test properly in accordance with HSE’s guidelines.  Its important for employers to ensure that they employ a company who will do a proper job.

Testing walk-in spray booths

Spray painting inside a walk-in booth (photo credit: HSE)

We recently received a query from a client who carry out paint spraying of isocyanate based two pack paints in a large walk-in type spray booth. They wanted us to carry out sampling to help them to decide when it was safe to enter the booth without their workers wearing their air fed masks. This sounds like a good idea. In fact it’s essential for workers to know when the paint spray has been completely cleared from the booth. Isocyanates are very potent respiratory sensitisers causing asthma in susceptible individuals. Anyone sensitised to isocyanates will experience asthmatic symptoms even if they are only exposed to very low concentrations. However, air sampling isn’t the best way to determine the “clearance time”.

If it was possible to use a direct reading instrument that gives a reading of the instantaneous concentration it would be relatively easy to determine when the booth was cleared. Unfortunately there isn’t a direct reading instrument that can be used for isocyanates. Sampling has to be carried out using an integrating technique – that means that a sample is collected over a period of time, and, after it has been analysed in a laboratory, it is then possible to determine the average concentration present during the sampling period. To determine the  “clearance time” a series of samples would have to be taken one after another. This would be expensive (the analysis isn’t cheap) and the standard method for isocyanates is not particularly good. The samples are collected by bubbling air through a reagent dissolved in toluene and as a low detection limit would be required it’s difficult to take samples over a short period.

Consequently its better to use a different approach, releasing smoke into the enclosure and then timing how long it takes for it to be completely removed from the booth. This method is quicker, less messy and much less expensive. The test can also be used to check that there are no leaks from the booth – any smoke escaping should be clearly visible. The Health and Safety Executive explain how to carry out this test in their publication “Controlling isocyanate exposure in spray booths and spray rooms” which can be downloaded from their website here.

The following videos show a smoke test taking place in a typical booth.

Once the clearance time has been established it’s important to ensure that anyone who works in the booth is informed and a notice posted at the entrance to the booth, something like this:


The HSE would like to see automatic clearance time indicator fitted on walk-in spray booths, but in my experience few have them. This is something that should really be addressed by the manufacturers and suppliers of the equipment. A new booth should already have them fitted, and clearance times should be established during the commissioning of the equipment after it’s been installed.

Following on from the initial test checking the clearance time should also form part of the statutory thorough examination and test required for LEV systems under COSHH Regulation 9. All engineering controls deteriorate over time, so the time taken for the contaminants to clear is likely to increase, even with a well maintained booth.

What is “static pressure”?

We were running the BOHS module course P601 “Commissioning and Thorough Examination and Testing of Local Exhaust Ventilation Systems” last week. One of the concepts delegates often find difficult to get to grips with is “static pressure”, which is one of the main engineering measurements carried out during the testing of a local exhaust ventilation (LEV) system.

In essence, the static pressure in a point in the ventilation system is the atmospheric pressure inside the duct. As a fan increases the pressure inside the duct on the exhaust side, and as air moves from high to low pressure, air is expelled from the duct. This results in a reduction of pressure in the duct on the other side of the fan so that it is lower than atmospheric pressure, causing air to flow into the system (and, providing the hood is well designed, drawing the contaminants in with it).

Although it would be possible to measure the “atmospheric” pressure inside the duct, which would be the “true”, or absolute, static pressure, we don’t do that.  Typically the static pressures inside the system are only slightly lower than atmospheric pressure – in many cases the the difference between the inside and outside will be less than 1 KPa than an atmospheric pressure which at standard conditions is 101 kPa. Instead, we measure the difference between the absolute pressure at the point under consideration and the ambient atmospheric pressure outside.  This is relatively easy to achieve using a manometer either in conjunction with a pitot tube or by holding a tube at right angles to a hole in the duct, with the other end of the tube connected to a suitable manometer.

Measuring static pressure in a duct using a pitot tube

This is really the differential static pressure and although we usually refer to the measurement as the “static pressure”, using this term is not strictly correct. However, this has become commonplace in ventilation testing. Use of the differential rather than absolute static pressure  has its advantages. The absolute pressure varies depending on ambient conditions, which change from day to day. Also the change in absolute pressure along a system is relatively small compared to atmospheric pressure and so can be difficult to quantify in practice (it is not easy to measure a small change and the change in the absolute pressure compared to atmospheric pressure is typically less than 1%).

The absolute static pressure before the fan (the suction side) is lower than atmospheric pressure, so the differential static pressure is negative. On the exhaust side of the fan, the absolute pressure is higher than atmospheric pressure, so the differential static pressure is positive.

Static pressure measurements are easy to make and can allow judgements to be made about the performance of the system and help to diagnose and  locate problems. We’ll come back to this in a future post.

Visualising Airflow


It can be difficult to make a qualitative evaluation of the effectiveness of LEV hoods because the contaminants are either invisible (in the case of most gases and vapours) or difficult to see (with fine dusts). Two main techniques can be used to overcome this problem:

  • smoke tests
  • dust lamps

Smoke released in the vicinity of a hood will move with the air in the same way as gases and vapours and can, therefore, give a good indication of how these contaminants behave and can help us judge how well they are being controlled by the LEV hood.


Using smoke to visualise air movement into a fume cupboard

Smoke testing is a good way of evaluating the effectiveness of LEV hoods intended to control gases, vapours and fume (fine particles) which will usually behave in a similar way to the smoke. However, dusts, due to their mass, may behave differently. Using smoke is still useful – if the smoke isn’t captured effectively there is little chance that the dust will be controlled – but other methods will probably need to be used for a proper assessment of control. In such cases using a dust lamp will normally be useful.

Very fine, respirable, particles cannot be seen with the naked eye, even where they are present at a high airborne concentration. When an intense parallel beam of light passes through the dust cloud, light scattering occurs that makes the particles visible to an observer looking along the beam towards the lamp. This effect was investigated by John Tyndall in the 19th century. Consequently it is it is often referred to as the Tyndall effect.

This method can help us to visualise the movement of the dust cloud, and is described in the HSE publication MDHS 82 The dust lamp – A simple tool for observing the presence of airborne particles. The following diagram, taken from the document, illustrates the principle.


Using a dust lamp to visualise fine dust (Source: HSE)

Note that it is forward scattered light that shows up the particles – that is where the light beam originates from the opposite side of the source to the observer. So the observer must stand on the opposite side of the dust cloud to the lamp, looking along the beam (taking care not to stare directly at the light source), as shown in the diagram.


Dust cloud revealed using a Tyndall beam (Source: HSE)

Often it’s enough to use these qualitative methods to decide whether an LEV system is controlling the contaminants.

Testing LEV systems when baseline data isn’t available


One of the main problems we encounter when carrying out a thorough examination and test of a local exhaust ventilation (LEV) system is the lack of “baseline data” – i.e. information on what the pressures and velocities should be when the system is operating in accordance with the design specification. Providing the system has been designed competently, if the measurements taken during the test match the baseline data, this tells us that it is working satisfactorily.

The main reason the information isn’t available is that no testing was carried out when the system was installed and commissioned. HSEs guidance in HSG258 is quite clear that a proper commissioning test should be undertaken to

  1. verify that the system is operating in accordance with the design specification
  2. ensure that it is controlling the contaminants effectively
  3. establish baseline data against which future test results can be compared

It would, perhaps, be wishful thinking to expect that, in future, all systems will be properly tested on installation. In our experience it is still not being done for many new systems we’re coming across. Hopefully the situation will improve over time.

However, we are still left with the problem of what to do when testing systems where no baseline data is available. One answer is that the testing company should recommission the system to generate baseline data for future surveys. In the ideal world we should do this, but the difficulty is that this would often increase the time needed to complete the work and, where the testing is being carried out by an outside organisation, lead to a higher cost, which the client might not be prepared to pay. Too many testing companies will obtain engineering test data and report it to the client without any evaluation of whether this indicates satisfactory performance and demonstrates adequate control. A company proposing to carry out a thorough test to recommission the system is put at a severe competitive disadvantage.

There isn’t a simple answer to this, but HSE have now provided some additional guidance on how to approach this problem on their website. They recognise that we need to be flexible, depending on circumstances. The nature of the system being a major consideration.

With simple systems with a single hood, the most important considerations are whether:

  • – the contaminant is being captured
  • – the contaminant does not settle out in the ductwork
  • – it is not being returned to the workplace

Establishing whether this is the case is not difficult for a competent occupational hygienist or engineer, using standard tests (smoke, dust lamp, velocity measurements, air sampling etc. as appropriate). Once this has been ensured other engineering measurements can be carried out on the system, such as static pressures. The system has then, effectively been recommissioned and baseline data established for future surveys.

It isn’t so easy with more complicated, multi branch systems. HSE’s advice in such cases is

you should still undertake a thorough examination and test, which will provide information on the current performance of the system……

However, it is accepted that such a report may not fully meet the advice in the (COSHH) ACOP, which recommends reference to intended operating performance, and you should indicate this in your report. You should also suggest that the user assesses whether the performance you have reported is providing adequate control*. Your examination should also identify any adjustments or repairs that you believe are needed……

Where you cannot make a professional judgement on the design performance standards of the LEV, or your assessment suggests that the exposure control may not be adequate, you should clearly indicate this in your report to the employer (client). The report should also clearly indicate where no information on intended performance was available.”

If all testing companies follow this advice, we will at least have a “level playing field”. However, many occupational hygienists may not feel comfortable leaving the client to decide whether the system is achieving adequate control.


* my emphasis

P601 – Competence in LEV testing

I received the following query a few days ago:

A large part of my working life during the last 14 years has been involved with the Thorough Examination and Test of Local Exhaust Ventilation (LEV) systems within the company I work for.

I have now moved on within the same company to a health and safety position, but over the last two years, I have been training another person to carry out the LEV testing under my direct supervision. This person I feel is now capable of carrying out the full inspection and tests on his own.

Being qualified myself to the BOHS P601 standard, can I signs off the reports done by my trainee as being correct (if I feel they are done to a satisfactory standard) or does he need to prove he is competent himself by attending a formal course such as the P601.

This was my answer:

“COSHH requires that anyone carrying out any work required by the Regulations must be competent. This applies to the testing of LEV systems.

Passing P601 is a way of demonstrating competence to test LEV systems to an employer, client or the regulatory authorities. HSE strongly encourage those testing LEV systems to have passed P601, but it is not a legal requirement as such (although I do know of one situation where an Inspector placed an Improvement Notice on a company that required their personnel to take P601).

If you are certain that your colleague is competent and you could demonstrate this by some means if challenged (particularly by the HSE) then, it is probably not necessary for him to have taken the course.  It is more important for consultancy/testing companies who provide a service to ensure their staff have the qualification as a means of reassuring the client that they know what they are doing. There are lots of companies offering LEV testing and not all of them are competent.

So, you need to make your own decision based on how confident you are that you can satisfy the HSE that your colleague is competent.

Of course, the above is only my personal opinion. A HSE Inspector could have a different view!”

The joys of LEV testing – Part 1

I’ve been out on site this week with Deb, getting our hands dirty doing some ventilation testing.

A number of the systems were quite large, with several hoods and branches. Most of the hoods couldn’t be reached when the process is operating due to safety considerations. This makes testing difficult. It’s only possible to measure face and capture velocities when it isn’t running and as the process itself will affect the airflow, measurements taken when it’s down are not representative. In such cases it’s often best to use static pressure behind he hoods (in a location that can be accessed safely) but these are only meaningful where there is data from an initial evaluation when the system is performing as intended and when it is known that control is being achieved. You also need to know what variance in pressure is acceptable (i.e. at what pressure the system stops being effective). Of course, most companies do not have such data!!