Dust control in manufacturing – An Overview of the hazard
Airborne contaminants occur in the gaseous form (gases and vapours) or as aerosols. In scientific terminology, an aerosol is defined as a system of particles suspended in a gaseous medium, usually air. Aerosols may exist in the form of airborne dusts, sprays, mists, smokes, and fumes.
A good starting point on dust control is to have some sort of understanding of what we are talking about when we talk about ‘Dust’. Dust is a collection of microscopic particles of material. Examples of the types of dust found in the work environment include:
- mineral dusts, such as those containing free crystalline silica (e.g., as quartz), coal and cement dusts;
- metallic dusts, such as lead, cadmium, nickel, and beryllium dusts;
- other chemical dusts, e.g., many bulk chemicals and pesticides:
- organic and vegetable dusts, such as flour, wood, cotton and tea dusts, pollens;
- biohazards, such as viable particles, moulds, and spores
Dusts are generated not only by work processes, but may also occur naturally, e.g., pollens, volcanic ashes, sandstorms, air pollution from traffic or agricultural dust. It is also worth noting that most indoor air quality surveys for offices will include particulate levels as part of a survey – although the expectations and standards used would be different than for most occupational exposure to dust surveys.
There are technical definitions around aerosols including particle size, but it is probably not too helpful to get overly technical. It is enough to understand that we are generally interested in the material that can be breathed into the nose and mouth (inhalable dust) and/or material that is fine enough to get into the deep lung (respirable dust). It is also important to understand that dust can be inherently toxic (e.g., mercuric oxide powder or stone dust containing silica) but that any dust in high enough quantities is damaging to the lungs.
How can dust be controlled?
Control of exposure to dust during work comes under The Control of Substances Hazardous to Health Regulations. In assessing exposure, reference may be made to specific Workplace Exposure Limits (WEL) in air (when we are looking at inhalation as the route of exposure). The specific standards which apply will depend on what the dust is made up of. But even if the dust is relatively non-toxic, the following standards always apply:-
Total inhalable fraction = 8-hr standard 10 mg/m3
Respirable fraction = 8-hr standard 4 mg/m3
Air sampling is used to assess exposures against these standards. The main purpose of conducting sampling is to:-
Compare exposures against a WEL.
Obtain data to complete a COSHH assessment.
Identify working practices or processes which contribute most to exposure and therefore where to focus control.
Demonstrate that control is being maintained.
To meet certain statutory obligations where sampling may be dictated.
How does COSHH fit into dust control?
As dust is a hazardous substance, the general COSHH principles apply i.e.. Assess risk, prevent exposure or where prevention not practicable reducing exposure to as low as reasonably practicable following the accepted hierarchy of controls (taking into account exposure standards), maintaining control, conducting training etc.
The Approved Code of Practice for the COSHH regulations makes reference to certain principles of control. Assuming that elimination and substitution have been explored, a critical element is the obligation to Design and operate processes and activities to minimise emission, release and spread of substances hazardous to health.
With dust control that means you could for example use bulk silos and screw feeds to transport product rather than less enclosed or manual feeds. Of course, a key focus will be on using properly designed and installed Local exhaust extraction (LEV) to control dust. LEV systems consist of an air–mover (usually a fan), an air–cleaner (usually a filter), ductwork, and inlet hoods e.g., an extracted and partially enclosed sack emptying station, a capture hood on a table saw or even using an attachment from properly filtered vacuum cleaner fitted to a portable tool to reduce dust release.
Other important considerations to reduce dust levels are the avoidance of dry sweeping and good housekeeping.
A case study in dust control and manufacturing
One particular client had some general concerns about dust in the workplace and we were commissioned to assess the extent of the issue and provide some data for their COSHH assessments.
Dust was being generated from:
- Environmental dust entering the site (large doors left open)
- Bead-blasting operations (both from the blasting media and the liberation of material from the components being cleaned)
- Powdered materials used in manufacturing.
- Excess dust left on supplied components that was disturbed during processing.
- Cardboard and paper in the warehousing areas or offices.
The skill of a consultant is to ask the right questions before a survey to plan out sampling to provide useful data as well as ask the right questions and observe practices during sampling to make sense of the data collected.
The sampling strategy was to initially use direct reading instrumentation. This had the advantage of proving some instant feedback on the extent of the issue but also was used to show what working practices and process were contributing most to airborne dust levels and how these levels varied over time and by distance from the source. The plan was to follow this up with personal exposure monitoring where required for compliance purposes.
Luckily for the client, almost all of the measured levels for airborne particulate were low. Some areas of improvement were noted, and recommendations made. It was concluded that full shift 8-hour Time Weighted Average (TWA) exposures were unlikely to exceed the UK eight-hour Workplace Exposure Limits (WEL) for respirable particulate of 4 mg/m3 or 10 mg/m3 for total inhalable particulate.