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.


Is it covered by the syllabus?

The syllabus for a BOHS module is an important document. For course providers, it’s the only thing that tells us what we need to cover during the course. So in order to be credible it needs to be clear and unambiguous and nothing should appear on the exam which isn’t mentioned  in the syllabus.

Unfortunately the Faculty exam committee are not very good at setting syllabi.  That for M103 is a particularly poor example. It’s not realistic to cover everything  on the syllabus in the time available – at least not properly, and certainly not to the depth that some questions that can appear on the exam require.

We also get some problems with M103 where the syllabus makes a brief mention of a particular topic, but the exam goes on to ask detailed questions beyond the scope set out.  For example, with respect to fans, the wording of the syllabus is as follows:

“Fan types and their applications”

My interpretation of this is that we should provide a brief overview of the different types of fans and the situations where they may be used, possibly discussing their relative advantages and disadvantages. And I also think that this is probably all a Certificate level occupational hygienist needs to know about them.  However, we have had questions on the exam that require knowledge of

  • how noise is created by fans
  • fan and system curves – including the shape of the fan curve for different types of fans
  • how to use fan curves to select a fan
  • the fan laws, including calculations
  • the effect of installing fans in series and in parallel

None of these topics are mentioned in the syllabus and, to me, are not implied by the statement “Fan types and their applications”. The exam requires knowledge not required by the syllabus.

Not only that, I’d contend that a Certificate level hygienist does not need this knowledge – and if they did, well we’d need more than a day and a half to get someone starting with no knowledge of ventilation principles to the point where they can answer questions on such advanced topics.

Knowing that this type of question can appear on the exam, we do our best (that includes us, as the course provider, and the candidates).  Fortunately only a few questions on fans should be included on the exam and it is possible to pass even where the candidates only know what the syllabus requires – providing they have understood enough of the other topics covered by the course.

I hope that when the Faculty get round to reviewing this syllabus they’ll think about making sure the syllabus is credible and reasonable in what it requires. They need to ensure that it is possible for someone new to the topic to learn what the syllabus requires from scratch in one week. And, hopefully, they’ll make sure that the bank of exam questions properly reflects the content of the syllabus.

Introduction to Toxicology

This is a presentation I put up on Slideshare. Its relevant to BOHS Module M101 – “Effects of hazardous substances” – but also provides some useful background for some of the other modules, particularly M102 and M103 where the examiners seem to assume that candidates have some knowledge of toxicology (although it isn’t a pre-requisite for these courses).

Oral Examinations


Last week we ran a revision course for candidates intending to sit the oral examination for the BOHS Certificate. These take place every 3 months in March, June, September and December. The examination lasts for an hour (although it’s surprising how quickly thie time passes) and there will usually be a panel of three examiners who will ask a series of questions.

The exam can be an intimidating experience and its important to prepare properly. But many candidates are not sure what to expect.

I guess the first point to make is that the examiners are not out to fail you – they would like you to pass but they can only do that if you demonstrate your competence. They’re not out to trip you up or trick you. They will try to ask straightforward questions, and if there is an obvious answer that’s the one they are probably after. However, although they will try to frame the questions in a way you can understand, if you’re not sure what they are asking you, you can, and should, ask for clarification. They won’t bite your head off but will probably rephrase the question for you or provide some supplementary information to make it clearer. So don’t be afraid to ask.

The principle aims of the oral examination are to test  your ability to

  • recognise common health hazards and risks
  • describe, in a simple way, the toxic effects of hazardous substances and explain related terminology as you would to a worker or manager
  • decide on what sampling methods are appropriate to assess common chemical hazards and describe the key features of the equipment and sampling procedure
  • carry out sampling for common contaminants
  • carry out noise and vibration assessments
  • apply the hierarchy of control to typical industrial processes, describing appropriate controls
  • describe and apply the key principles of ventilation system design
  • assess the effectiveness of LEV systems
  • select personal protective equipment (RPE, gloves, ear defenders)

The examiners should not expect you to give detailed explanations of the “heavier” theory covered in the module exams. You may be asked about some basic principles and underpinning knowledge (such as legislation) but time is limited and the examiners will usually mainly concentrate on the more practical aspects of  occupational hygiene practice.

Questions will often be based on workplace scenarios. These shouldn’t be too complicated but will not always be familiar to you. If you have never come across the situation before don’t panic. Think it through. If you work out what the hazards and risks are you should be able to decide on what assessment methods and control methods will be appropriate.  If you aren’t sure about something ask for clarification.



Simply being able to reel off facts isn’t proof of ability to perform a task. Yet the primary emphasis of BOHS modules is on rote learning  where trainees are required to learn masses of facts in order to pass their exams. In practice there is very little testing of the ability to apply the knowledge to solving problems. To me, this approach is rather old fashioned and doesn’t really reflect the needs of modern occupational hygiene practice.

An enormous amount of information available to us in the modern world – and it continues to expand exponentially. In our profession new substances continue to be developed (this is particularly true in the field of nanotechnology), information on toxic effects of familiar substances continues to increase (REACH is likely to ensure that this process accelerates) and control methods and good practice continue to be developed. On the other hand some facts that occupational hygienists have traditionally been expected to remember have become irrelevant with changes in industry and society.

It is not only unrealistic to expect a professional to absorb and remember a mass of facts, it is, in my opinion, poor practice. It is more important to know how and where to locate information than to memorise it – and then to be able to use it to analyse and solve problems. Of course, some facts need to be learned – but these need to be relevant to current circumstances.

The revamping of the BOHS’s module exams is an opportunity to revisit the syllabi and the exams. I’d like to see proper learning objectives established, based more on application of principles rather than memorising information, much of which is not that relevant to modern practice, and new questions set which can test these objectives.  The underpinning knowledge that needs to be memorised really needs to be reviewed, weeding out those facts and principles that are no longer that relevant.

At the moment it looks like this won’t be happening – the sample questions I’ve seem appear to be mainly reformatted versions of those currently in the multiple choice question bank. The new short answer questions should allow for more flexible marking, and negative marking is being eliminated, so overall the changes should be beneficial. However, I feel that a lot of work is still needed to bring the examination system into the 21st Century.

New format Module exams – Part B

Two weeks to go now to the launch of the new style BOHS module exams. Last week we received some details on the new format for Part B. Instead of one long question from a choice of two, candidates will be required to answer 5 out of 8 “micro-essay” questions. BOHS have now released some example questions which can be accessed here.

Given the time available for the exam hasn’t changed (2 hours 15 minutes total) with 45 minutes meant to be devoted to Part B, they are expecting no more than 10 minutes to be devoted to answering each of these “micro-essay” questions, so candidates answers will need to be concise, while ensuring that the key points are included. Experience will show how easy it is to achieve this.

BOHS Examination changes


This week we were running M103, “Control of hazardous substances” in Chester. As usual, we had a good group who were keen to learn, worked hard, contributed to discussions and seemed to get on well.  As usual it was a tough exam, but this will be the last course we will have run with the current style of examination. A few weeks ago BOHS announced that from November the 1st all module examinations will have a new format.

The exam will still be in two parts but from 1st November

  • Part A will consist of 40 short answer questions rather than 60 multiple choice questions.  There are pros and cons for candidates, but the biggest advantage is that there will no longer be any negative marking.
  • Part B will changed so that candidates will be required to answer 5 “micro-essay” questions from a choice of 8.
  • The pass mark will remain at 50%

Personally I think the change to part A will be beneficial. As there is no negative marking it will be advisable to try to answer all the questions. Currently some candidates hesitate on some questions where they are not 100% confident, even though they may know the answer.  And it will only be necessary to get 20 out of 40 right.  Another benefit is that as each question is worth 4 marks, some answers may gain partial marks. This will be particularly helpful with calculation questions where marks can be gained providing an appropriate method is used even if the final answer is incorrect due to a slip in the calculation (working will need to be shown to ensure this happens). Currently a small slip results in a negative mark for an incorrect answer even where the candidate understands the principles involved.

There are still some aspects of the new system that still need to be clarified by the Faculty, in particular:

  • What exactly the “micro essays” will consist of. We haven’t been provided with any clear guidance on this or example questions yet
  • Which type of examination will be given to candidates re-sitting exams taken before 1st November
  • Whether course providers will be given access to the question papers or feedback on the examinations.

We’ll keep you posted on this as soon as we receive the information from BOHS, so watch this space!