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July 2, 2013

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Airborne hazardous substances – Trailing in the dust

Many professionals in the construction industry are waking up to the risks posed by airborne dust particles, but there’s no doubt that awareness of the issue among some remains worryingly limited. Tom Gunston considers the health effects brought about by silica-dust exposure and explores the potential control measures.

An understanding of the dangers posed by respirable silica dust and the steps that can be taken to reduce risks in the workplace are essential to overcome what could be the next big challenge for construction health and safety professionals. 

The notion that silica dust can cause serious health problems has taken some time to filter through to the construction industry. Some construction-equipment manufacturers were informed about the risks and developed dustless attachments for tools, but they didn’t sell. In short, no one seemed interested.

The HSE has been highlighting dust as an issue for many years. In some instances, its advice was embraced by the industry. Petrol cut-off saws, for example, were recognised as posing an issue, but one that could be solved through water suppression. However, until recently, some of the broader issues surrounding control of silica-dust exposure in construction remained far from the forefront of the industry’s thinking.

Over the last few years, measurement-device suppliers at the annual Safety & Health Expo have reported, anecdotally, an increased interest from contractors in measurement kit. Concurrently, refurbishment project managers concerned about dust exposure and nuisance are increasingly making inquiries through their supply chains.

Significantly, the HSE has now named silica dust as an issue to which Fee for Intervention (FFI)1 is applicable — warning those who break health and safety laws that they are liable for the recovery of the regulator’s costs for relevant enforcement action. So what does this all mean for the industry?

The dangers of dust

Hazardous dust can be loosely divided into ‘inhalable’ and ‘respirable’ dust.
Inhalable dust consists of larger particles, which are usually visible to the naked eye and can enter the mouth, nose and upper parts of the lungs.

Respirable dust consists of finer particles. Invisible to the naked eye, these can travel into the deepest part of the lungs, known as the gas-exchange region, where oxygen is absorbed into the blood and carbon dioxide is released from the body back into the air.
Concrete, brick, stone and similar materials can contain between 20 and 70-per-cent crystalline silica.2 Any ‘dusty’ process using these materials releases fine silica dust particles into the air.
Respirable crystalline silica (RCS) dust consists of tiny but very sharp crystalline dust particles. If breathed in, these particles can damage tissue in the gas-exchange region, causing scar tissue to develop. Exposure can lead to a number of serious health problems, including silicosis, chronic obstructive pulmonary disease (COPD) — which includes bronchitis and emphysema — and lung cancer.

The construction industry is the largest volume-consumer of silica-containing materials. Anyone working with these materials is likely to be exposed to RCS to some extent.

A recent review of literature by the Health and Safety Laboratory3 suggests there are roughly half a million construction workers at the risk of silica dust exposure. The same review suggests silica dust exposure leading to lung cancer is responsible for more than 800 deaths each year. This far exceeds the ten to 20 deaths each year attributed to silicosis alone, according to the Industrial Injuries and Disablement Benefit Scheme.4

Exposure limits
As a hazardous substance, RCS is covered by the Control of Substances Hazardous to Health Regulations 2002 (COSHH). The daily exposure limit for RCS is given as a Workplace Exposure Limit (WEL).

The current daily limit value is 0.1 milligrams of RCS dust per cubic metre of air. This is a very small amount — illustrated in the HSE ‘Time to Clear the Air’ leaflet5 as a pile of dust a fraction of the size of a penny.

However, even this tiny amount is not ‘safe’. The precise link between dust exposure and the instance of health effects is complicated and the subject of ongoing research, but essentially, more exposure means more risk. Exposures should be as low as practically possible, and not just kept below the WEL.

Measuring dust concentrations
Let’s consider a diamond blade chasing into concrete. Tests indicate that when standing in the dust plume generated by this activity, the user is exposed to very high levels of RCS.

Working in enclosed areas with breaking, or cutting equipment can also lead to high exposures. At the other end of the scale, working in the open, some distance from a dust-suppressed tool, will lead to lower exposures, although that does not mean the dust will not be a nuisance.6

In between these extreme cases, taking measurements is not always practical. Small differences in airflow, in working practices, or in the way the measurement system (usually a sampling pump) is positioned can give different results. Each measurement needs to be taken over many hours and it can take several days to retrieve the exposure measurements from the laboratory. Unlike manufacturing plants, construction sites do not usually have stable, indoor airflow environments and tasks that remain the same for weeks at a time.

Assessment and control
Any task involving working on concrete, stone, or masonry by cutting, grit blasting, drilling, scabbling, breaking, or any similar process will cause silica dust. An enclosed area will increase the health risk to workers further, as will increasing the duration of exposure.7

If a task is generating silica dust then suitable control action will be expected. If it is possible to carry out works without using a method that creates excess dust, that should be the first option — but this is not always possible.

If a dusty task is necessary, the first line of defence is control at source, for which there are various solutions available.

Water suppression, a common method used on site and championed by the HSE’s ‘Time to clear the air’ campaign,5 is very effective for tools and processes that cannot be affected by moisture.
However, there is also a number of disadvantages of using water suppression — for example:
Lack of a water supply can be a problem, although portable water bottles can be used.
    Existing building elements, such as plasterboard or cabling, which may be present in refurbishment sites, may not respond well to water.
    Water and dust mixed together produces slurry, which can make surfaces slippery and can be tricky to keep to a minimum, especially on upper floors.
    Slurry can be difficult to collect and dispose of — if it is not collected effectively, then the dust is released back into the air when it dries out.
    Using electric power tools is generally not advisable around water.
On-tool extraction is typically dry-dust extraction using a vacuum hood attached to the tool, or the substrate.

Many dust hoods for drilling machines work perfectly well, but a number of products experience significant issues, including difficulties in clamping them to the machine, clogging up, wear and tear, or being too flimsy for use on a construction site.

Dust hoods are also a matter of personal preference and very task-dependent. For instance, battery hammer drills with on-board dust-capture units have a limited capacity for storing captured dust. Care is needed when emptying the dust cartridge, but there are no trailing leads or hoses, which improves ease of use on site.

Angle grinders, wall chasers, disc cutters, and any other saw used for stone cutting can produce very large quantities of very fine dust, which can be challenging to control. If water suppression is not practical then a form of on-tool dust extraction is likely to be necessary.

The hood must be well designed to control the quantity of dust generated by such tools and the speed at which it travels off the disc. Specific operating techniques may also be appropriate, and the hood must be plugged into a dust extractor that will work effectively with the hood. Dust may still escape if the hood cannot be kept close to the substrate.

Area ventilation is also useful for limiting the concentration of dust. Working in the open causes dust to disperse with the wind and helps reduce the risk to the machine operator, but it is difficult to collect the dust when it settles, plus airborne dust causes other problems, including nuisance and air pollution. The Mayor of London’s Office has published a useful best-practice guide on dust emissions,8 while similar guidance is available from many other sources.

Forced air ventilation, which is present in tunnelling projects, for example, helps prevent airborne dust concentrations building up in one area.

Containment and filtration can be very effective and are almost essential for ‘live’ refurbishment work, where part of the building is still in use. Containment alone — for instance, using ‘pop-up’ plastic screens — can present problems, as some dust escapes through doors and joints and, potentially, into the heating, ventilation and air-conditioning system if there are any areas of poor masking. A suitably powerful H-rated air cleaner with the outlet ducted outside the work area can solve this, as it creates a negative pressure in the work area, drawing clean air in and discouraging dusty air from leaking out.

Device ratings
Site vacs and dust extractors are rated L (low), M (medium) or H (high). The European Power Tools Association suggests that an M-rated vac is the minimum acceptable for use with RCS dust, while an H-rated vac provides even greater protection. HSE research has reached similar conclusions.9

Using a forward-light scatter camera (a very bright narrow-beam backlight) to detect any escaping dust, we have carried out tests on a number of M and H-class vacs used to control dust when drilling fixing holes. With a suitable hood, both types of vac captured the RCS dust satisfactorily. Design details, such as the ease of changing the bag without receiving a face full of dust, were also found to be important.

The tests found that vacs with an L rating, or no rating at all, may not work. The filters in these vacs can be too coarse to trap fine RCS dust. The vac will capture the larger inhalable particles, but the tiny RCS dust particles will work their way through the vac and be blown back into the air.

There are also issues relating to multi-purpose wet and dry vacs, even if M or H-rated. Silica dust particles are very fine and if the vac is not set up properly, they will get through to the main filter.
If the vac is then used wet, the damp dust can ‘cake’ on the filter. Any built-in filter-cleaning mechanism is intended to clear dust, not caked mud, so the vac will be running at reduced capacity. Recovering a filter from this condition can be extremely difficult, to the extent that it may require replacing entirely.

Wet and dry vacs may be supplied with the filter bags, membranes, or other items required for wet use and dry use, and the user needs to know how to configure each of these properly.

Many site vacs seem to be used wet or dry, as needed, with no changes made to the configuration, and sometimes with no bag fitted — with the lower part of the vac used as a dustbin and upended into a skip when full. Even if a bag is used, a paper bag will not survive wet use. Some vacs require a bag for dry use but not for wet; other vacs require a plastic bag for fine dust and a paper bag for conventional waste.

Dedicated dust extractors are designed to be very good at capturing fine dusts but are typically for dry-use only. It may be advisable to use a rated extractor for dust and a cheaper wet and dry unit for ‘generic’, non-dust-related site work. This approach can also help with the management of the equipment. Any dust extractor, whether a dedicated dry-only unit, or an M-rated wet and dry site vac will be classed as local exhaust ventilation (LEV) if used for hazardous dusts, such as silica.

Finally, any dust-capture system must work as a system. Changing the vac or the hood, or sometimes even the hose, can cause a system to function poorly, or not at all. ‘Mix and match’ arrangements are also made more difficult by the lack of standardisation in vac attachments. This situation is gradually improving, and manufacturers or suppliers should be able to provide attachments to get from one hose size to another, without resorting to gaffer tape.

Respiratory protective equipment

Respiratory protection is the last line of defence against dust exposure. Every effort must be made to capture the dust at source but RPE can still prove invaluable in dealing with any residual dust that might escape. Even if the dust capture, or suppression is very effective, a dust mask with an assigned protection factor of at least 20 (FFP3 or P3) is advisable. Once a suitable mask is identified, it must be fit-tested. The Fit2Fit website provides guidance on fit-testing and fit-test providers.10
The good news is that demand for higher-rated dust masks appears to have risen over the last few years.

The future
The construction industry has made some very effective progress over the last few years in relation to the health effects of dust. Awareness is increasing and dust-control measures are in place on many sites. Water suppression is widely used and demand for M and H-rated vacs, fit-testing and other dust-control equipment has seen a marked increase in recent years.

However, a lot of equipment in use on sites is probably not up to the job, and there are some contractors and operators that do not seem to be taking dust seriously. There is still some way to go yet.    

References
1     HSE (2013): The basic health and safety mistakes crippling British industry —
    www.hse.gov.uk/fee-for-intervention/basic-safety-mistakes
2     HSE (2013): Control of exposure to silica dust — a guide for employees —  www.hse.gov.uk/pubns/indg463.pdf
3     Brown, T (2009): In-depth review. Silica exposure, smoking, silicosis and lung cancer — complex interactions. Health and Safety Laboratory, Buxton. Published in Occupational Medicine 2009;59, pp89—95
4     Statistics: Coal worker’s Pneumoconiosis and Silicosis — www.hse.gov.uk/statistics/
causdis/pneumoconiosis (accessed May 2013)
5     HSE (2008): Time to clear the air! Protect your lungs when using cut-¬off saws — www.hse.gov.uk/pubns/misc830.pdf
6     HSL (2011): Levels of respirable dust and respirable crystalline silica at construction sites, HSE Research Report 878
7     Construction Dust FAQs — www.hse.gov.uk/construction/faq-dust.htm
8     The Greater London Authority and London Councils (2006): The control of dust and emissions from construction and demolition, MoL/Nov06/MR D&P/MT/GLA915
9     HSL (2012): On-tool controls to reduce exposure to respirable dusts in the construction industry — a review, HSE Research Report 926
10 www.fit2fit.org


Tom Gunston manages the product test laboratory for VJ Technology.

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