Thermal Comfort – A different sort of problem

Yesterday I travelled up to Glasgow where I’d been invited to give presentation on “Thermal Comfort” to a meeting of the Scottish Region of the British Occupational Hygiene Society (BOHS). Despite a few technical problems which delayed us starting (computers are great when they work!) the meeting went well and it was good to meet up with an enthusiastic group of people which included some new members of the Society.

Most occupational hygienists at some time or other will be faced with a situation where workers are complaining that the environment they’re working in is uncomfortable. Thermal conditions often contribute to this.

Thermal comfort is usually defined as “that condition of the mind that expresses satisfaction with the thermal environment” As this is about individual perception and preference, these problems are amongst the most difficult we have to address.

It’s most likely to be an issue in workplaces such as offices, but complaints or concerns can sometimes occur in manufacturing environments and other types of workplace. During the discussion, one of the audience told us about problems experienced by bus drivers – not surprising really when you consider that buses are effectively mobile workplaces with their own internal environment. Their large windows potentially presenting problems from radiant heat from the sun (or radiant heat loss on cold, sunless days) and there’s plenty of potential for draughts.

The key thing with “thermal comfort” is that it’s subjective – one person may complain about feeling warm while another may consider the same environment to be too cool – you can’t keep everyone happy all of the time. Studies have suggested that the very best you can do is achieve 95% satisfaction . So there will always be 1 person in 20 who is dissatisfied. Change things for him/or her, someone else will start to feel unhappy! In practice you’re doing well if more than 80% are happy!

There are two basic approaches we can take to evaluating thermal comfort

• a Qualitative approach which involves talking to and interviewing people in the workplace and looking around for obvious clues, using experience to make a subjective judgement
• a Quantitative approach where we carry out measurements. (However, interpreting the results from the measurements presents some difficulties)

I think that the best way to find out what someone thinks is to ask them! There are a number of questionnaires and checklists available from various sources, including the Health and Safety Executive and BOHS that can help with this qualitative approach. However, measurements can also be useful. They can help you to determine what factors are likely to be causing the problem. So, in practice, a combined approach is normally best.

Even if problems can be identified, it’s not always easy to find solutions. Particularly as improving things for one person is likely to upset someone else! Ideally, individuals should be given control over their own personal environment. But it’s not easy to achieve this in practice as changes that one person makes is likely to affect at least some of their colleagues.

The slides I used during the talk are available on Slideshare, including a version with my speaker’s notes.

Thermal comfort for bohs version for slideshare

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Workplace Exposure Limits

There are a number of reasons why occupational hygienists carry out workplace exposure monitoring, but in the majority of cases the primary objective will be to determine the degree of risk due to inhalation so that we can decide on whether exposure is adequately controlled. In such cases we need a benchmark against which the results can be compared – in other works an occupational exposure limit (OEL). If we don’t have a limit then it is almost impossible to decide on whether there’s a problem or not and the sampling exercise can then raise more questions than it answers as workers and managers will want to know what the results represent.

In the United Kingdom the Health and Safety Executive publish Workplace Exposure Limits (WELs) that can be used by employers to demonstrate whether exposure is adequately controlled. The WELs are published by the HSE in their document EH40 “Workplace Exposure Limits”.

A new edition of EH40 was released in December last year which replaced the previous version, first published in 2005. It includes the new limits that came into force on 18 December  for  a number of  substances including  mercury, phenol and sulphuric  acid mist. A hard copy can be purchased from HSE Books but it can also be downloaded free of charge from here.

At one time a new edition of EH40 was published every year as there were always a number of new limits introduced and changes to existing WELs. This was because the HSE used to devote significant resources to reviewing the effects of hazardous substances and setting exposure limits. However, a number of years ago they made a conscious decision to reduce the resources that they were devoting to setting WELs. This has meant that since the 2005 edition of EH40 was published there have been very few changes other than those required by the European Union. The changes included in the newly published version of EH40 all originated from the EU as European Indicative Occupational Exposure Values (IOELVs). It is mandatory for member states to implement these limits within a specified timescale and Member States’ domestic limits must be at least as stringent as the IOLEV.

The change in the HSE’s policy on setting WELs was, and still is, very controversial – at least as far as occupational hygienists are concerned. WELs are an important tool. One argument the HSE would make is that it is more effective to devote their scarce resources into developing advice on controls. I’ve some sympathy with this view but there are a wide range of different approaches that can be applied to any situation where exposure needs to be controlled, all involving different levels of expenditure. The best control approach will reduce exposure to an acceptable level without entailing more cost than necessary. So, to decide what is appropriate we need some measure of what is an acceptable level of exposure, which brings us back to the need for a benchmark – i.e. a credible exposure limit.

It could be argued that the onus on setting limits has been passed on to the producer of hazardous substances by the requirement to develop a “derived no effect level” (DNEL) under the EU Regulation on the Registration, Evaluation, Authorisation & restriction of CHemicals (REACH). However the procedure for setting DNELs normally results in a very low value well below any OELs that may have been assigned. Applying DNELs in the real world could result in an over cautious approach to control, so their suitability for use as OELs in practice is questionable.

There’s a database of OELs set by various EU member states, Canada (Québec), Japan, Switzerland, and the United States published by GESTIS, a joint project of the German Social Accident Insurance Institutions. The database, in English, is accessible online here.  They’ve also produced an app for the iPhone and iPad which can be downloaded via the iTunes App store.  A number of other databases on different aspects of hazardous substances, including one of sampling and analytical methods, are available via their website, so it’s a very useful resource for occupational hygienists.

John Cherrie of the Institute of Occupational Medicine in Edinburgh has recently used the GESTIS database to review limits for a number of substances. His findings that there can be a considerable variation in the OELs set by different countries and in many cases the UK had the highest OEL for a given substance, are very interesting.

How many samples?

As I’ve discussed in a previous post, there are many factors which lead to a wide variation in exposure for workers carrying out the same job. One implication of this is that it is very dangerous to draw conclusions from one or two samples. But how many samples need to be taken to ensure that exposure is adequately characterised? In practice, it’s difficult to draw up general rules which can be applied in every situation.

Over the last couple of years the British Occupational Hygiene Society in conjunction with their sister organisation in the Netherlands, Nederlandse Vereniging van Arbeidsdeskundigen, have been working on preparing some guidance on sampling strategy for occupational hygienists. This has now been published and can be viewed or downloaded from here. Recommendations are made on the number of samples that should be taken during a two stage approach to be used specifically for testing compliance with exposure limits.

1. Take 3 representative personal exposure measurements from random workers in the SEG. If all three exposures are <0.1xOEL, it can be assumed that the OEL is complied with.
2. Do a group compliance test. Take at least 6 more samples from the SEG, at least 2 per worker from workers picked at random. Then carry out a statistical analysis on all 9 results

I think that the science on which the guidance is based is sound, but I doubt that the approach set out is practicable for most organisations. The problem isn’t so much the number of samples (although the analytical costs could be a problem in some cases), but the time required to collect them. Following this guidance would probably involve four site visits and many companies wouldn’t be able to justify the cost involved.

So deciding on how many samples to collect can be one of the most difficult aspects of devising a sampling strategy. Although a better picture of exposure will be obtained if a large number of samples are collected,  a little common sense needs to be used, balancing the value of the results against the cost of the survey.

Selecting ear defenders

The use of personal protection should always be the last resort when looking for ways to control health and safety risks. Although this principle applies to ear defenders as much as any other type of PPE, it’s difficult to avoid their use in many industries where exposure to noise can be difficult to control, at least in the short term, by engineering measures.

If ear defenders have to be used, it’s important that they are suitable and will provide adequate protection. Their performance is frequency dependant – like most noise control measures they are more effective against higher frequencies. So when selecting ear defenders for a particular application it’s important that this is taken into consideration. There are three methods that can be used to achieve this, which are described in Appendix 3 of the Health and Safety Executive’s publication  L108 Controlling noise at work.

I’ve recently uploaded a presentation to Slideshare which describes these methods and which illustrates them with some worked examples. It also briefly covers some of the other factors that need to be considered when selecting ear defenders.

Selection of ear defenders

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Risk Management Measures in the real world

At the end of October, I travelled over to Helsinki for a few days. I’d been invited to deliver a short course on “Risk management measures in a REACH context”  to personnel working for the European Chemicals Agency* (ECHA) who are responsible for evaluating the dossiers chemical manufacturers have to submit under the European REACH Regulation. Over 30 people attended the course, a much bigger number than I’d normally prefer. However, they were a really nice group of people who were keen to listen and contribute to the discussion making it an enjoyable experience for me, and, I hope, for the attendees. It was good to have the opportunity to put forward my perspective on how effective risks from hazardous substances are controlled in practice in the “real world of industry.

The principle objective of REACH is to protect human health and environment from chemical hazards, ensuring that risks from the use of chemicals are properly controlled. To achieve this, manufacturers have to undertake risk assessments for all “exposure scenarios” where their products are used and produce “extended safety data sheets” for substances, which must include appropriate “risk management measures”. We’re now starting to see these new style data sheets coming through to users.

During the training session, we looked at how exposures to chemicals vary and the practicalities of obtaining adequate data for the risk assessment process. However, the main discussion centred on the realities of how “risk management measures” are implemented in industry, based on my experiences helping companies to control the risks from using hazardous substances.

I think that there is a widespread impression that controls are much more effective than they are in practice. There are lots of reasons for this, which I’ve discussed in some previous posts. Problems can occur during all the key steps involved in the design and implementation of controls – see my Slideshare presentation and this post for some examples.

It’s a particular problem with local exhaust ventilation systems. In my experience they are rarely well designed and, in practice,  their influence on exposure is considerably less than the users (and designers/suppliers) believe. The “lower tier” exposure models commonly used to prepare the REACH risk assessments can assume that LEV is up to 90% effective. The system would have to be well designed and properly used and maintained for this to be the case and I think that it is rare for it to be achieved in practice. Consequently, exposure modelling with lower tier models can considerably overestimate the reduction in exposure achieved by LEV.

Manufacturers and importers of chemicals need to make judgements about the effectiveness of controls when carrying out their risk assessments and also when deciding on what risk management measures are needed. The danger of overestimating how good they are could compromise their risk assessments and result in risk management measures being specified that won’t adequately control exposure. It’s important, then, to have a realistic appreciation of the “real world” effectiveness of common controls.

Those extended data sheets I’ve seen so far seem to specify realistic controls for the exposure scenarios. However, they are phrased in very general terms. Again this is likely to be a particular problem with LEV. For many industrial organisers “LEV” means a captor hood – often the flexible “swinging arm” type. As I’ve discussed in a previous post, these are largely ineffective at controlling contaminants. But in many cases if a company follows a general recommendation to install LEV, this is what they’ll buy.  I think that if REACH is really to achieve it’s objective of improving control, then we need to ensure that the advice on risk management measures is as specific as practicable. So with LEV enough details needs to be provided to make sure that the design of the extraction hood is appropriate.

Inappropriate application of a captor hood

Another problem I’ve noticed with the new style safety data sheets I’ve seen is that where personal protective equipment is recommended the advise is too general. For example, recommending “wear suitable gloves”. This really isn’t any improvement on the older style sheets. Downstream users need more specific advice on what type of gloves are needed, particularly what they should be made of. The reality is that most users don’t have the expertise to select “suitable gloves” and in most cases the gloves used are made of an inappropriate material and are not used and managed properly.  This point is also relevant to other types of personal protective equipment. Again, I’d like to see more specific details provided.

For me a good model for user friendly control advice is the COSHH Essentials control sheets. These provide good, concise advice on control for common processes on a maximum of 2 sides of A4. Where LEV is recommended specific details on the hood design, including an outline diagram, is provided. These sheets aren’t perfect – their advice on personal protection is too vague, for example – but I think that overall they strike the right balance between brevity and the usefulness of the information.

A lot of work has to go into carrying out the risk assessments. It’s important that the output – i.e. the information on risk management measures – should be detailed enough to ensure that controls are properly designed and implemented. Unless this happens there’s a real danger that all the effort will be in vain and an opportunity to substantially improve control of hazardous substances at work will have been missed.

*ECHA’s role is to manage and coordinate the registration, evaluation, authorisation and restriction processes and to ensure consistency in the management of chemicals across the European Union.

COSHH Assessments – 7 key questions

 

In my last post I discussed the main problems that I often encounter with how COSHH assessments are carried out in practice. These were

• concentrating on the hazards rather than the risks
• neglecting to include process generated substances
• concentrating on inhalation exposures and neglecting other routes
• lack of emphasis on controls
• failure to consider measures needed to ensure continued effectiveness of controls

To be “suitable and sufficient” a COSHH assessment needs to address the risks associated with the use of hazardous substances and decide on what measures are needed to reduce them to an acceptable level. The best way to achieve this is to base the assessment on the work. COSHH assessments should be focused on the process or task rather than the substance.

Guidance on risk assessments typically outlines a number of key steps as illustrated in the following flow chart

 

 

Although this provides a good overview of the process it is a little vague. In particular, it doesn’t really set out what is involved in the crucial third step – evaluating significant risks.

Some time ago, I sat down and tried to work out exactly what I do when I carry out a COSHH assessment. I concluded that I ask myself a series of questions:

1. What substances are workers (and others) exposed to?
2. What harm can these substances cause?
3. Is exposure significant?
4. What is currently being done to control exposure?
5. Is that good enough?
6. What needs to be done to improve control?
7. What else needs to be done to ensure that adequate control is maintained? (e.g. testing controls, air monitoring, health surveillance, training etc.)

Let’s have a look at these in a little more detail

1. What substances are workers (and others) exposed to?

• Starting with the task or process work out what substances are present – including both those substances bought in and those generated by the process (the latter often present the most significant risks).
• Consider who could be exposed and how – i.e. by what routes (inhalation, skin, ingestion, injection)

2. What harm can these substances cause?

• For substances bought it should be possible to determine the hazards they can present to health by looking at the label on their containers and the safety data sheet that suppliers must provide.
• Information on process generated substances might be more difficult to locate, but the Health and Safety Executive publications and their website are often a good place to start.

For many people this is the end of the assessment, but if you stop here you have only identified hazards and haven’t addressed the risks. You need to continue to consider the other questions.

3. Is exposure significant?

This is probably the most difficult question to answer! Bear in mind that with hazardous substances the risk can be represented by the following equation :-

RISK = HAZARD X EXPOSURE

So the key is to try and quantify or estimate the degree of exposure. There are various ways this can be achieved, for example

• personal air sampling
• use of direct reading instruments
• semi-quantitative measurements (e.g. direct reading dust monitors)
• visualisation techniques (e.g. dust lamp or smoke tubes)
• observations

The methods used will depend on circumstances. Sometimes observation is enough where it is obvious that improved controls are needed.

Whatever methods are used, a judgement has to be made on whether exposure is “significant” or not. Essentially you need to ask yourself “does something need to be done to reduce exposure?”. If the answer is “yes” then exposure is significant. (This is something I’ll return to in a future post)

4. What is currently being done to control exposure?

This question may be combined with the previous one. In most cases a COSHH assessment will be undertaken for an existing process where there are likely to be controls in place. You’ll need to identify what they are.

With a new process, the initial COSHH assessment should be carried out before the process starts. In that case you’ll need to identify what control options are available.

5. Is that good enough?

This may be asked in conjunction with questions 3 and 4. Once it’s been established what controls are available a judgement needs to be made on whether they are reducing exposure to a low enough level.

6. What needs to be done to improve control?

If existing controls aren’t good enough then, clearly, improvements will need to be made. A COSHH assessment should identify what measures are needed to control exposure.

Even where exposure is below exposure limits if there are ways of improving control they should be considered. For example, if personal protection is being used, even if it is adequate to reduce exposure below exposure limits, an attempt should be made to identify alternative controls.

Also if workers are exposed to carcinogens, mutagens or asthmagens, COSHH requires that exposure should be reduced to the lowest level reasonably practicable below any relevant limit. So in such cases it is particularly important to try to identify any additional controls.

7. What else needs to be done to ensure that adequate control is maintained?

This is probably the most neglected aspect of COSHH assessments even though the Regulations are quite explicit in requiring the assessment to consider what measures are needed to ensure compliance with all the regulations.

There are many examples in industry where expensive control measures are installed only for them to remain unused, used infrequently or used incorrectly thereby rendering them ineffective. To overcome these problems, effective management measures need to be put into place. COSHH Regulations 8 to 12 are about the things employers can and should do to ensure the controls they implement continue to work effectively. So once appropriate controls have been identified, the assessor needs to ask:

• what needs to be done to ensure that the controls are used properly
• what maintenance and testing is needed to ensure that engineering controls and personal protective equipment continue to operate effectively and what auditing should be carried out to ensure that the procedures and safe working methods are followed
• is exposure monitoring and health surveillance needed as additional checks that the controls are effective
• what information, instruction and training is required to ensure workers know why the controls are needed, how to use them correctly, procedures for reporting faults etc.

Common problems with COSHH assessments

The Control of Substances Hazardous to Health Regulations (COSHH) are undoubtedly the most important legal requirements for occupational hygienists working in Great Britain. They’ve been around for a long time now – the first version being enacted in 1988 – yet, in my experience, there are still many organisations that haven’t completely got to grips with them. One of the main problems is that many employers do not fully understand the Regulations and what they require and often misinterpret them.

In essence, COSHH is relatively straightforward. It simply sets out a framework for the management of risks created by using hazardous substances at work. I’ve often used the following diagram to summarise the main requirements of the Regulations

Employers whose operations involve the use of hazardous substances have to undertake a risk assessment in order to decide on what controls are appropriate. The other measures are then about they need to do to ensure that the controls remain effective (they’re the management controls that I’ve discussed in a previous post).

1. COSHH is about CONTROL

The first mistake employers often make is to forget the real objective of COSHH – i.e. controlling  the risks. They put a lot of work into the assessment, often generating a lot of paperwork, but don’t follow through to implement effective control regimes. The assessment is important – it is needed to establish priorities by deciding on which are the most significant risks, but it is primarily meant to be a tool to allow the employer to develop an action plan. It’s a means to an end not an end in itself.

2. Assess the RISK not the HAZARD

The second common problem is that many employers do not understand how to undertake the assessment.  It is quite common to focus on the substances used taking information from the safety data sheets provided by the suppliers and transferring it to a standard form , in effect, producing a summarised data sheet. This often involves a lot of time and resources, and although such summarised data sheets can be a useful information tool  they are not a risk assessment.

To see why this is the wrong approach it’s useful to take a look at the wording of COSHH regulation 6 which sets out the requirements for the assessment.

‘An employer shall not carry out work which is liable to expose any employees to any substance hazardous to health unless he has

a}made a suitable and sufficient assessment of the risks created by that work to the health of those employees and of the steps that need to be taken to meet the requirements of these Regulations

b) `implemented the steps referred to in sub-paragraph a.”

The key words that I’ve highlighted are “the risks created by the work”. The substances present hazards but, as the Regulations clearly state,  the risks are associated with the use of the substances.

I’ve heard many people state that they have carried out the assessments for all their substances. This normally indicates that they haven’t got it right. It’s not the substances that need to be assessed, but the work involving their use. For example, take a simple case where solvent is used for cleaning purposes. The risks, and the necessary controls,  associated with using a few millilitres applied with a cotton bud are clearly quite different than using several litres of hot solvent in a vapour degreasing bath. It is inappropriate to carry out a risk assessment of the substance – two assessments of two quite different processes are needed.

So the assessment should focus on the process. It should also concentrate on the risk not just the hazard. Risk is often defined as “the probability of harm occurring in practice”. For occupational hygienists it is best summed up by the following equation:

So the hazardous properties are important, but they are only half the story. The key to effective risk assessment, which s often neglected, is understanding the exposure of employees (and anyone else who could be affected by the work).

3. Identify ALL hazardous substances

Most COSHH assessments I’ve seen concentrate only on the substances that are bought in by the company. Yet there can be other substances present which need to be considered such as fumes, dusts and other airborne contaminants generated by the process or reaction products and intermediates in chemical manufacturing. In many cases it is these process generated substances that present the most significant risks, but they are often neglected.

Silica containing dust generated during cutting of paving slabs

4. Consider ALL routes of exposure

Many people carrying out assessments only focus on substances that can be inhaled. But there are other routes by which substances can come into contact with the body and be absorbed. All of these need to be considered.

Skin exposure by immersion (picture source: www.hse.gov.uk/skin/images/)

Skin exposure is a common problem in industry leading to direct effects on the skin such as irritation, chemical burns and dermatitis and some substances can be absorbed through intact skin (this is something I’ve addressed in a previous post). Ingestion of hazardous substances can sometimes occur – normally due to contamination of foodstuffs, often following skin exposure. Penetration through the skin can occur by injection where workers are handling needles (e.g. healthcare workers) or where the skin has been abraded or otherwise damaged.

5. Identify controls and other management measures

The other common mistake is that employers often do not properly consider the measures needed to control the risks. They forget that controlling risk is the objective of COSHH and that the Regulations specifically require that the assessment must set out “the steps that need to be taken to meet the requirements of …. (the)…Regulations””.  This means that they need to decide on what controls are needed and what other measures might be needed (i.e. on the use, maintenance and testing of controls, air monitoring, health surveillance and information, instruction and training)

The first step in solving a problem is to recognise that it exists, but we then need to decide how to resolve it. I’ve set out some of the common problems with COSHH that I come across; I’ll consider how to avoid them when carrying out a COSHH assessment in my next post.

Some implications of exposure variability

In our last post we saw how inhalation exposures to hazardous in the workplace are highly variable. The spread of results from an air sampling survey is usually quite wide and will usually conform to a skewed log-normal distribution.

This means that although there is a large spread of results the majority of exposures are at the lower end of the range with a relatively small number of high exposures. Nevertheless these higher results are valid and cannot be discarded.

This has a number of important implications for the design of air sampling surveys and the interpretation of the results. In particular for the number of samples that need to be taken to allow a clear picture of  the range of exposures to be obtained.

If only one or two samples are taken for a particular process or task then it is most likely that they will be from the lower end of the range where the bulk of the results are clustered. It is less likely that a result from the higher end of the range, which could be 3 or more times greater,  will be obtained. Care has to be taken, then when interpreting the results to decide whether exposure is adequately controlled. If the results from the small data set are close to the relevant occupational exposure limit, then it is likely that some workers on some occasions are likely to have exposures that will exceed the limit. Even if the results are substantially below the limit, this does not necessarily guarantee that exposure is adequately controlled.

One way to avoid this problem is to take enough samples so that a reasonable estimate of the range of exposures can be obtained. This is easier than it sounds. How many samples are enough? Unfortunately, guidance is sketchy.

The British Health and Safety Executive, in Appendix 2 of their guidance document HS(G) 173 “Monitoring strategies for toxic substances”, gives the following advice on the number of samples required is given:

• for worst case sampling, 1 in 5 workers should be monitored, unless a smaller number can be justified;
• for representative measurements, if there are fewer than 10 employees, only 5 need to be included, but although caution is advised for "more complex situations involving more then ten employees", no further advice is given on how to proceed.

However, many occupational hygienists would consider that even in simple cases, considerably more than 1 in 5 workers need to be included.

The American Industrial Hygiene Association, in their A Strategy for Assessing and Managing Occupational Exposures, suggest that 6 to 10 measurements are normally enough to obtain a reasonable estimate of worker exposures from each group.  It will usually be practicable to collect this number of samples during a one day survey. However, there are many situations where only a small number of workers, fewer than this, carry out a given task. It may require repeated sampling on  several days to obtain 6 samples

It is worth bearing in mind that, a better picture of exposure will be obtained if a larger proportion of workers are covered by the survey.  A little common sense needs to be used, balancing the value of the results against the cost of the survey. Sampling is expensive and there is always pressure from clients to keep costs down.

The  British Occupational Hygiene Society and the Nederlandse Vereniging van Arbeidsdeskundigen, have recently published a draft document on Testing Compliance with OELs for Airborne Substances (available here) which addresses these issues. It’s not an easy read but there will be a workshop on the proposals during the first day of the forthcoming BOHS Autumn Scientific Conference taking place in Leeds on 14th and 15th  November. I’m sure there will be other opportunities to discuss it in the future. I’ll be returning to it in a future post.

Variability in exposure measurements

Whenever an occupational hygienist carries out an air sampling survey, either for a group of workers carrying out the same tasks in a similar way, or for the same individual worker carrying out the same job on different days, it’s inevitable that a range of results will be obtained.  However, although most people would probably accept that it would be unreasonable to expect all the results to be the same, in my experience many people, including health and safety professionals don’t fully appreciate just how much variation there is in worker exposures.

To illustrate this, let’s look at an example.

The following results were obtained from a survey for sulphuric acid mist during anodising of metal components. This is an electrolytic process where sulphuric acid to create an protective or decorative oxide layer on the surface of  the metal. The process is called “anodising” because the component forms the anode electrode of an electrical circuit.

As you can see, there is quite a wide spread, ranging from 0.04 mg/m³ to 0.20 mg/m³ . The highest result is 5 times higher than the lowest. Many people find this surprising, but it’s not untypical for sampling results. You might also have noticed that the results are not evenly distributed across the range. This can be seen more easily if we plot them on a histogram

Here’s another set of results from the same facility from a survey carried out at a later date

And plotted as a frequency histogram:

Again we can see a similar pattern but with a larger spread – the highest result is seventeen times higher than the lowest.

The pattern evident in these two examples is not untypical for sampling results. What we can see is that

1. there is a relatively wide spread of results
2. the majority are at the lower end of the range
3. there are a small number of results that considerably higher than the majority

Statistically, we typically find that the sample results for a group of “similarly exposed” workers conform to a log-normal distribution. This is a skewed distribution with the majority of results bunched together,  but with a small, though significant, number of very high results.

There is often a tendency to consider these low frequency high results as untypical and the temptation is to exclude them, particularly if they exceed the relevant exposure limit. Sometimes that may be valid – if a good reason can be found for rejecting them. But in many cases the high results are simply part of the natural variation in exposures and can’t be ignored.

There are other important implications too. Particularly for interpreting the results from surveys where only a few samples were taken. It is more likely that the results will be from the lower end of the distribution as the higher results have a low frequency.

Understanding variation is important for occupational hygienists as it has major implications for how we design our surveys  and interpret the results.

Controlling silica exposure during fettling of castings

In foundries, once  the casting is removed from the mould it is usually necessary to remove excess metal and remedy defects. This process is usually referred to as “fettling” or “finishing”.

Fettling normally involves the use of powered hand tools such as chippers and grinders. The operation presents a number of significant risks. Using power tools exposes the workers to high noise levels and hand-arm vibration. There is also a risk from exposure to the dust generated by the process. The dust will largely consist of metal particles, but this is usually of low toxicity. The main concern occurs where sand is used for the moulds in which the metal is cast. This is crystalline silica. Some particles of sand from the mould adhere to the metal and grinding during fettling can lead to the release of fine dust including particles of respirable crystalline silica. “Respirable”  particles are smaller than 10 microns in diameter and can reach the deepest regions of the lung. Regular, repeated exposure to respirable crystalline silica can lead to silicosis, a serious, debilitating lung disease.

Risk of silicosis – Source: HSL

Respirable crystalline silica has a very low Workplace Exposure Limit of 0.1 mg/m3 as respirable dust. In fact, as the above chart shows, long term exposures to concentrations much lower than this can lead to some workers developing silicosis. There is also evidence that prolonged workplace exposure to crystalline silica can lead to an increased risk of lung cancer, although this is only likely to occur in those workers who have already developed silicosis. Given the nature of the risk exposures need to be reduced as low as practicable.

So, it’s important to ensure that the dust generated during fettling is properly controlled, particularly when there is a risk of exposure to silica. In most cases, the most practical and effective way of doing this is to install well designed local exhaust ventilation. The Health and Safety Executive have developed a number of sheets providing practical advice on how to control dust and fume generated during foundry processes, including fettling. For small castings they recommend the installation of an extracted booth. The work is carried out inside the booth which then contains the dust generated allowing it to be removed effectively by the extraction.

Source: HSE COSHH Essentials for foundries. FD7 “Fettling small castings”

Recently, one of our consultants was carrying out a survey in a foundry. They had installed this type of booth for their fettling operations. However, as can be seen in the following photograph,  The booth was not being used in the way intended – the worker was carrying out the work outside the booth.

The dust generated was not contained and, consequently, the extraction would not be as effective as it should be. The worker will have a higher exposure than if he carried out the fettling inside the booth.

In a previous post I outlined the key steps needed to control health hazards in the workplace. The first steps are recognising that a risk exists and then making sure that appropriate, effective controls are specified, designed and implemented. In this case the risk from dust exposure was recognised and a local exhaust ventilation system with an appropriate hood design was installed. The problem is that it is not being used properly, considerably reducing its effectiveness.

Once controls have been implemented they need to be properly managed to ensure that during use they continue to do the job they were designed to do. This requires training, supervision, maintenance, testing, audit and review. It’s a management responsibility to ensure that controls are properly used so more vigilant supervision seems to be required.

In this example, there could be a number of possible reasons why the booth was not being used correctly. Perhaps management and the workforce don’t fully understand the health risks and so don’t appreciate the importance of using the controls properly. Perhaps the workers haven’t been properly trained on how to use the booth. However, there could be a problem with the design of the booth. It is possible that carrying out the work inside the enclosure presents the operator with some practical difficulties. Perhaps the fine work required is difficult to complete properly if the casting is inside the booth or the booth dimensions, particularly the height, could cause the worker to adopt an awkward posture which causes discomfort and could lead to musculoskeletal problems. Solving one problem often creates another. Ergonomics is often neglected when designing engineering controls for chemical hazards. Ideally workers need to be consulted and involved in the specification and design of the controls and its good practice to build and test a prototype before finalising the design. Proper commissioning of the controls should also check for usability.

Further investigation would be required to get to the root cause of this problem. However, the case illustrates the importance of proper management of the design, implementation and use of controls.