Studying towards the NEBOSH Diploma in OH&S Unit B exam next January? Then this might be right up your strasse.
Machine made mineral fibre (MMMF) refers to a range of inorganic fibrous materials that have been manufactured and used as insulating / reinforcing materials. Historically these materials were called man-made mineral fibres.
MMMFs include rock and glass wool used for sound proofing and fire protection, ceramic fibres used for furnace lagging and glass fibre used in glass-reinforced plastic products. Some of these materials have specific properties that put them into their own category known as refractory ceramic fibres (RCF).
Asbestos is not an MMMF because it is a naturally occurring mineral mined from the earth. All MMMFs are synthetic by definition.
COSHH and EH40
MMMFs are a hazardous substance as defined by the COSHH regulations and are subject to a workplace exposure limit (WEL). In fact MMMFs are subject to two WELs. One which is expressed in terms of the number of fibres per millilitre of air (f/ml) and the other which is expressed in terms of the milligrams of MMMF per cubic metre of air (mg/m3).
The WELs can be found in EH40 (here) and are both long term exposure limits (8-hour TWA exposures).
They are 5 mg/ m3 and 2 f/ml.
[Note that the HSE guidance note OCE5: Insulation link above is out of date as it states the WEL as 1 f/ml. And note that RCFs have separate WELs]
The Potential for Confusion
The fact that there are two WELs stated in different units might easily give rise to some confusion. For example, should you want to calculate the average concentration of MMMF from information given to you as part of a Diploma Unit B exam question. Because the examiner could, quite legitimately, give you information relevant to both units and ask you to sort out the mess. And of course by “the examiner could” what I mean is “the examiner has”. That is exactly what they have done in a previous exam question.
Sorry – I have no intention of sharing the specific question with you.
I know; such a tease.
But I will use an example that I made earlier (in true Blue Peter style) for your delight and edification:
Imagine we have some results from the analysis of two personal dosimetry samples taken during two typical work shifts.
Analysis 1 gives an MMMF fibre count of 26,800 fibres collected over an 8 hour period.
Analysis 2 gives an MMMF weight of 15 mg collected over a 6 hour period.
[And no, before you ask, these are not the actual numbers used in the exam question. Learning answers by rote won’t get you where you need to be at Level 6].
Both samples were collected by the relevant MDHS sampling methods using an air pump drawing air at a fixed rate of 0.5 litres per minute (l/min) for the fibre count sample and 2.0 l/min for the gravimetric (weighed) sample.
So how do we calculate the average concentration of MMMF? That’s the question.
The answer is to work out what units we need to state the concentration in and then to work out how to convert the figures given to that correct unit of concentration.
If we want to compare the average concentration to the WEL that is stated in f/ml then clearly we have to use the fibre count analysis. We can’t use the gravimetric analysis information because we would end up with incompatible units.
The fibre count WEL is stated in units of f/ml.
We know the total number of fibres collected during the sampling period. But we don’t know the total number of millilitres of air that was sampled – we have to work it out from the pump flow rate and the sampling time.
The pump flow rate for the fibre sampling as 0.5 litres per minute. Note the unit: l/min. We need to know the total volume of air that went through the sampling train over the entire sampling period in millilitres.
There are 60 minutes in an hour. So 0.5 l/min x 60 = 30 l/hr
The sample was collected over an 8 hour period. So 30 l/hr x 8 = 240 l collected during that sample period.
And there are 1000 ml in 1 litre. So 240 l x 1000 = 240,000 ml air
The final step is to calculate the average concentration of fibres per ml collected.
Which is 26,800 f / 240,000 ml = 0.1117 f/ml
To calculate the gravimetric figure we need to use the mg/ m3 unit and therefore we must use the gravimetric analysis information.
The pump flow rate for the gravimetric sampling as 2.0 litres per minute. As before we need to know the total volume of air that went through the sampling train over the entire sampling period. But this time we need that stated in cubic metres.
There are 60 minutes in an hour. So 2.0 l/min x 60 = 120 l/hr
The sample was collected over a 6 hour period. So 120 l/hr x 6 = 720 l collected during the sample period.
And there are 1000 l in 1 m3. So 720 l / 1000 = 0.72 m3 air
The final step is to calculate the average concentration in milligrams per m3 collected
Which is 15 mg / 0.72 m3 = 20.8 mg/m3
Calculating the 8 Hour TWA Exposure from these Figures?
You will remember that the LTELs stated in Eh40 use an 8 hour reference period. They are 8-hour TWA limits. Therefore to compare the result obtained by sampling to these limits we have to calculate the actual 8-hour TWA exposures.
There is an equation for doing this which you might have come across:
Actual 8-hour TWA exposures = C1T1 + C2T2 + C3T3..etc.
Where C1 is the concentration of the contaminant in air for exposure period 1 and T1 is the exposure time in hours for exposure period 1 and so on…
If we want to work out the actual 8-hour TWA exposures to MMMF given above then we have to use this equation. But we also need to know what the exposure to MMMF was during all other work activities on any given work shift. Because these additional fractional exposures need to be included in the equation to give the 8-hour TWA.
Let’s assume that there are no other MMMF exposures during either of the work shifts that were sampled in the scenario above.
For sample 1 the maths is very easy:
C1 = 0.1117 f/ml
T1 = 8 hours
So the equation gives 0.1117 x 8 = 0.1117 f/ml
[i.e. the 8s cancel each other out]
For sample 2 the maths is not the same:
C1 = 20.8 mg/ m3
T1 = 6 hours
So the equation gives 20.8 x 6 = 15.6 mg/ m3
A very easy mistake with this type of question would be to ignore the actual question and treat it as if it were a typical 8-hour TWA calculation question (as have been asked several times over the years). I.e. rather than actually working out the MMMF concentration (as requested) just shoe-horn the numbers that are presented into the standard C1T1 ../ 8 equation. Despite the fact that the data and units are incompatible.
Guess what? Doesn’t work. If I remember correctly the Examiner’s Report on this question said that many people had done just that.
This is why it’s useful to understand underlying principles. Inevitably there are bits of the Diploma that are a test of memory and not much else. But there are other topics where a good understanding of what, how and why is helpful. Because if we understand that then we will be in a better position to solve problems that are presented to us in slightly different ways.
With thanks to John and Tanya.
Dr Jim Phelpstead BSc, PhD, CMIOSH