Dust Exposure

There are many common industrial processes which cause workers to be exposed to a wide range of toxic and harmful dusts. Although official statistics are hard to come by, John Cherrie of the Institute of Occupational Medicine has estimated that in Great Britain almost 10 million workers are exposed to dust at work.

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Handling of powders usually results in significant release of airborne dust, whether this involves the manual emptying of sacks, manual scooping of powders or mechanical transfer of powders and / or into open containers.

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​​Any tasks or processes that involve cutting or abrasion of solid materials will generate dust that will become airborne.

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Most of these processes are energetic which means that the dust created will form a cloud that can extend over a significant distance.

​Handling friable materials (i.e. substances that break up easily) can result in the formation of a dust cloud that can lead to significant exposure.

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​Common friable materials include :

  • Glass and rock wool
  • Refractory ceramic fibre
  • Cotton, flax and other vegetable fibres
  • Wool

​Disturbing dust settled on surfaces will normally result in at least some of it becoming airborne. ​Using compressed air and sweeping are particularly likely to lead to significant exposures and should be avoided .

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Sometimes the problem is obvious, and the sensible thing to do is to introduce controls. But in many cases an accurate assessment requires measurement of exposure. In these cases it’s important to have a “benchmark” against which the results can be compared, otherwise they’re meaningless. In Britain we have Workplace Exposure Limits – these are legal standards and where one has been set for a substance exposure must not exceed the limit. With carcinogens, mutagens and substances that can cause allergic asthma, the requirement is tighter – exposure must be reduced as far as practicable below the limit.

Although a significant number of  toxic dusts have been assigned Workplace Exposure Limits many of the dusts which can cause chronic obstructive pulmonary disease or other non-malignant respiratory disease have historically been viewed as ‘low-toxicity dusts’ and haven’t been assigned limits. In Britain, the guidelines for action to reduce airborne exposure to these dusts (10 mg/m3 for inhalable dust and 4 mg/m3  for respirable dust) are now known to be insufficiently protective. The need for a new exposure limit of 1 mg/m3  of respirable dust has been suggested by some authors and limits for dust exposure are currently under review in the EU. 

The Institute of Occupational Medicine, for example, has recommended that “until safe limits are put in place, employers should aim to keep exposure to respirable dust below 1 mg/m3 and inhalable dust below 5 mg/m³”.  The TUC has advised health and safety representatives that they “should try to ensure that employers follow a precautionary standard of 2.5 mg/m³ for inhalable dust … and 1 mg/m³ for respirable dust.” In Germany, the MAK commission has adopted a limit equivalent to 0.3A mg/m3 respirable, where A is the density of the substance in g/cm3. This is equivalent, for example, to 0.8 mg/m³ for many silicate minerals or 1.2 mg/m³ for titanium dioxide. All three bodies therefore regard 1 mg/m³ respirable as a more appropriate guideline than the 4 mg/m3 COSHH trigger.

A commentary in a recent edition of the Annals of Occupational Hygiene discusses the background in more detail.

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It’s a controversial issue, but my personal view is that working to 10 mg/m3 and 4 mg/m3 trigger values doesn’t provide adequate protection. Some employers might be concerned that working to lower levels would be costly, but in my experience it’s possible in most cases to reduce exposures to well below the widely used “triggers” by employing standard good practice approaches.

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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.