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October 31, 2005

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Dust and airborne contaminants: a health and safety guide

John Horsey provides an overview of the sources of airborne contaminants found in the workplace, the hazards they present to workers, the legislation in place to provide protection for staff , and the technology available to allow employers to comply with the law.

Every day, we all breathe in and out every few seconds. We assume, because we cannot see anything in front of our noses, that the air we breathe is pure and harmless – and for the most part it is – but air has the ability to hold a variety of substances, often invisible to the eye, which can cause health problems if inhaled.

There are four basic types of contaminant that can be carried by air in such a way that they can be inhaled:

1 Dust/particulates (solid particles)

Dust is produced by many industrial processes, including material removal, e.g. grinding, sanding, abrasive cutting and blasting, buffing, and polishing (particle sizes are typically 2 to 100 microns); materials handling processes, e.g. barrel-filling, sack tipping, conveyor transfer packaging (particle sizes are typically 2 to 100 microns); thermal processes, e.g. soldering, welding, smelting, casting (particle sizes are typically 0.5 to 10 microns); and laser processes, e.g. cutting, coding, engraving, welding.

Lasers are now used extensively throughout industry to mark identification codes, ‘best before’ dates, and to make deep engravings on a wide range of materials, such as wood, acrylics, and most other plastics. The emissions from laser processing of metals are usually either the condensed form of the metal itself, or its oxide. Stainless steels or metal alloys often emit chromium, cadmium and other toxic elements when lasered, and need more specialised attention. Laser marking or cutting processes, when applied to plastics, release a variety of toxic gases, including benzene, phosgene, etc. Particle sizes are typically 0.5 to 2 microns.

In each of these processes the concentration of particles in the air varies widely, as does the particle size analysis, i.e. for materials removal and handling processes a high proportion of the total weight of dust in the air will be large particles – 10 microns and above. With lasering processes most of the particles will be in the 1 to 2 micron range.

Dust particles greater than about 50 microns in size are not normally inhaled because the air speed created by the lungs in breathing normally is not enough to move them towards the mouth or nose.1 Particles between 10 and 50 microns can be breathed in but will be filtered out by the hairs and mucus in the nasal cavity (“breathe in through the nose and out through the mouth” is good old-fashioned advice probably given during the days of severe atmospheric pollution but just as appropriate now). Between 7 and 10 microns, particles will be inhaled into the lungs but will mostly be ejected by the ‘ciliary escalator’ – the small hair-like members in the surfaces of the respiratory tubes that pass dust particles from one to another until they can be ejected by coughing. Particles between 0.5 and 7 microns are the most dangerous because they can be deposited in the bronchioles and alveoli, where they can be absorbed. Particles smaller than 0.5 microns tend to behave more like gases and most will be exhaled.

2 Aerosol mists

These are liquid droplets formed either by mechanical processes, where water is broken down into a finely divided form, i.e. using spray guns, or by evaporation and then condensation back into liquid droplets. Droplet sizes are mostly very small – typically 0.3 to 1.0 microns. Processes include hot mixing tanks, paint spraying, inkjet printing, cleaning processes, etc.

3 Vapours

Substances in a gaseous state emitted from the surfaces of liquids, these behave in the same way as gases but will more readily condense into droplets. Processes are the same as for mists but also include the use of solvents for cleaning, as carriers for printing inks, paints, etc. and the use of adhesives. The beauty and cosmetic industry also makes use of organic solvents (acetone, MEK, etc.) and other volatile liquids (ammonia-based colours and tints) in large quantities as a result of the huge increase in nail treatments and decoration, and the massive expansion in hair-colouring and spray-tanning.

4 Gases

True gases, usually with very low boiling points, can be emitted as a result of all of the above processes in varying amounts. Along with vapours, these are responsible for the odours that characterise many processes.

The effects of breathing in dusts can vary from minor irritation, coughing, breathlessness, bronchitis, rhinitis and occupational asthma to extremely serious conditions such as heart disease and cancer, depending on the substance, its concentration, and the length of exposure time (see panel below).

What the law says

The Control of Substances Hazardous to Health Regulations (COSHH) provide details of the actual measures employers need to implement to comply with the Health and Safety at Work Act. Specifically related to airborne contaminants the Health and Safety Executive publication EH402 provides actual levels of contaminants, which can safely be allowed to exist in the workers’ breathing zones. These are presented in tables that reflect the level of risk, i.e. Table 1 lists Maximum Exposure Limits (MELs) for particularly harmful substances, while Table 2 is a list of Occupational Exposure Standards (OESs) for a wide range of substances. A similar approach to the COSHH Regulations has been adopted in most European countries, although the methods for applying the rules and policing them are often carried out in different ways.

Options for action

The COSHH Regulations list the actions to be taken, or at least to be considered, to control airborne emissions to within allowable limits. These can be summarised as follows:

1 Change the process to prevent the dust, fumes or gases from being generated in the first place.

2 Provide extraction of air from the process to remove airborne contaminants.

3 Provide personal protection equipment, i.e. face masks, airswept visors, etc.

While the first option sounds good in principle, in practice the seriousness of the contaminant issue often only becomes apparent after the process has been installed, so making changes to remove or reduce the dust/fume hazard is usually impractical. Providing PPE should be the last line of defence, so by far the most practical and effective method of controlling airborne emissions is via extraction systems.

The most important factor that influences the scale, cost and effectiveness of a fume extraction system is the volumetric flow of air (usually expressed in m3/hr) that needs to be drawn from the area close to the fume/dust source to prevent the contaminants from escaping into the workplace. Apart from the main objective of preventing the escape of fumes and dust, it is also important to engineer extract devices, if possible, to prevent contamination of equipment associated with the process, and, in some cases, the product.

The very best way to achieve all of these objectives is to try to enclose the process completely and introduce strategically placed inlet air vents to the enclosure such that air flows in a controlled way across the fume/ particle source towards the offtake.

The volumetric air flow is assessed using an enclosure as the capture device and then by adding together the areas of all the openings and multiplying the result by a factor that is the ‘in draught’ velocity. The air velocities into the enclosure when under extraction needed to prevent contaminants escaping are :

* Gently released gases: 0.3 m/s

* Vapours and gases released at a moderate velocity: 0.5 m/s

* Dust or droplets released at moderate velocity: 0.8 m/s

* Dust or droplets projected towards the enclosure opening: >1 m/s

What to do with the extract air

There are three basic options :

1 Discharge the air outside the building, i.e. to the atmosphere, untreated. This has the advantage of simplicity and lower long-term running costs, but there are many disadvantages, including:

* High installation costs;

* Possible contravention of the Environmental Protection Act, which limits the emissions of contaminating substances into the atmosphere;

* Problems with neighbours due to dust deposition (usually on cars), or complaints due to odours from emitted gas;

* Energy losses, where extracted air which has been conditioned now has to be replaced and reconditioned;

* Inflexibility – if the process has to be altered because expensive ductwork, etc. has to be dismantled and then re-installed resulting in considerable cost in downtime and installation costs;

* Compromise of ISO 14001 accreditation.

2 Discharge the air outside the building after filtering. This has the same disadvantages of energy loss, inflexibility, problems with ISO 14001 qualification, installation costs, etc. but does allow compliance with the EPA and usually avoids complaints from neighbours.

3 Filtration of the extract air and return to the workplace. This method of control, particularly for smaller processes where mobile filtration units can be used, avoids all of the above problems with discharge to atmosphere but requires two additional features to be successful:

a) a high degree of both particulate and gas filtration; and

b) automatic and continuous monitoring of the exhaust air to ensure early detection of any fugitive emissions of either gases or particulates.

Filter tips

The equipment available for filtering contaminated airstreams can be summarised as follows:

1 Particulate filters

These can be subdivided into ‘wet’ and ‘dry’ systems. ‘Wet’ systems include self-induced spray wet collectors, venturi scrubbers, centrifugal units, spray chambers, and various combinations of these. They work on the basic principle that a liquid-scrubbing fluid, usually water but often dosed with additives for pH control and to break down surface tension, is broken down into a finely divided form and the resulting tiny droplets are brought into contact with the dust particles. The particles attach themselves to the droplets and the resulting larger droplets agglomerate before being removed from the airstream, usually by a centrifugal/ inertial process.

Benefits of ‘wet ‘ systems are:

* They enable filtration of very moist airstreams;

* They can be used in particularly explosive applications, e.g. titanium/magnesium grinding.

Disadvantages are:

* High initial capital cost;

* Low cleaning efficiency unless used with high differential pressures, which require high energy input costs; and

* Problems of water supply and effluent disposal.

‘Dry’ systems can be further sub-divided into two categories. ‘Self-cleaning’ filters consist usually of either fabric bag filters, envelope-shaped or cylindrical, pleated cylindrical cartridge filters, or electrostatic precipitators. In each case particles are collected on the filtration media surface, where they agglomerate to form a dust cake. This cake is then removed either by shaking the filter element (fabric filters only), cleaning with pulses of compressed air (fabric bags or pleated cartridges), or by mechanical rapping (electrostatic precipitators). The main benefit is that they only need to be changed every six to 24 months. Disadvantages, however, are:

* High cost of replacement filters and labour/downtime costs;

* High initial capital cost;

* Large physical size due to large surface area requirement to allow ‘online’ cleaning down;

* Explosion risks when used on ‘organic’ dusts (plastics, wood, resins etc.) – these risks require the use of extensive explosion protection measures and locating the unit outside the building;

* Secondary dust problems, where collected dust has to be bagged or handled for disposal.

The second category, ‘disposable filters’, usually consist of multipocket bags or pads (for low dust burdens) to remove coarse particles down to approximately 1 micron in size. Before air is returned to the workplace it is usual to use a HEPA (High Efficiency Particulate Air) filter. These remove typically 99.997 per cent of particles down to 0.3 microns in size. With each of the above air filters, the dust collects on the fibres of the filter fabric. When effectively blocked, the filter is removed for disposal. These have the benefits of:

* Low initial capital cost;

* Ease of filter change;

* Convenience and cleanliness of disposal; and

* Reduced explosion risks.

Relevant criteria, when choosing between self-cleaning and disposable filters, are the quantity of dust produced by the process, the nature of the dust, and operating and maintenance costs.

If the dust burden is such that disposable filters would need replacing too frequently, then the first choice would be the self-cleaning types of unit. If the particles of dust are ‘sticky’ in any way, e.g. because they have been condensed from a molten material such as plastic, or because they naturally carry an electrostatic charge, then disposable filters will have a distinct advantage. Low concentrations of organic dusts, which would create explosion hazards if pulsed or shaken from self-cleaning filters, can be collected in disposable filter elements.

It is important when comparing running costs to take all aspects of expense into account, i.e. the replacement costs of disposable filters can be outweighed by the high maintenance costs involved with self-cleaning filters.

2 Gas or vapour filters

Gases and vapours are removed from airstreams most commonly by one of two methods:

a) Gas scrubbing: This involves passing the gas or vapour through a chamber filled with ‘packings’ through which a scrubbing liquor is passed. The object is to provide a large number of sites where the liquid and gas can interact. This method is used particularly where soluble gases are being filtered (ammonia is a good example); also where acid gases are present, in which case an alkaline scrubbing liquor is used.

b) Chemisorption: The gases are passed through a bed of a solid substance that will chemically react with the gas. An example of this is alumina permanganate (porous pellets of aluminium oxide impregnated with potassium permanganate). Contaminant substances are oxidised and therefore effectively neutralised by the potassium permanganate.

c) Adsorption: The gases are passed through a bed of a porous substance, usually activated carbon. Molecules of the contaminant gases are attracted to carbon molecules on the large surface area by Van der Waals forces.

Control check

Since the criterion for whether the law is being complied with is the actual concentration of contaminants in the areas where operatives need to work, the only method of verifying that this is being achieved is to carry out a personal and background monitoring exercise.

This involves equipping relevant workers with personal sampling devices. These are small filter and air pump units, which simulate the quantity of air that a worker would breathe in a certain time period. The filter can be a particulate filter that is pre-weighed in the laboratory and then re-weighed after wearing, or it can be a tube of material that will react with gases known to be present and indicate the level of absorption. The concentrations of dust or gas can then be compared with the Occupational Exposure Limits listed in EH40.

In practice, when a dust or fume control system is first installed and deemed to be controlling the airborne contaminants adequately, either by the above sampling process, or, probably more commonly, by simple inspection (there are no visible signs of dust emissions around the process area or noticeable odours
, the system is signed off as ‘safe’ and a parameter selected that represents the performance of the system. This can be the static pressure measured just behind the extract enclosure but is usually – and preferably – the volumetric flow rate extracted from the process.

As required by the COSHH Regulations the system must be inspected and the representative parameter checked at regular intervals (usually once every 14 months).

It is also essential to determine that the filters are working correctly. Monitoring of exhaust air streams from filtration systems is, for many processes where the air is discharged to atmosphere, mandatory under the Environmental Protection Act but, while strongly advisable, monitoring of exhaust air being returned to the workplace is not!

Dust monitors for external air discharge can be of the opacity or light-scattering type, or they can use the tribo-electric principle, whereby the small transfer of electrical charge carried by most particles to a conductive probe is amplified and measured.

Dust monitors of the radioactive foil type can be used for internal emissions to the workplace. These are low-cost units, which work on a similar principle to smoke alarms.

Gas sensors of the oxygen depletion type can be effectively used to detect the presence of fugitive gas emissions that occur when adsorption or chemisorption filters have become spent.

Summary

* Dust and fume emissions in the workplace are likely to be harmful to workers if not controlled. This is particularly true of relatively new processes, such as laser marking, cutting, etc.

* The allowable levels that can breathed in are set out in EH40 and compliance is administered by the Health and Safety Executive.

* The preferred method for controlling dust and fumes, after elimination by changing the process, is extraction, this being optimised by good enclosure design.

* Recirculation of filtered air back into the workplace has many benefits but should only be considered if the quality of the air being recycled is monitored.

* Heavy dust loadings need to be dealt with using self-cleaning filters.

Light dust or fume burdens are better handled using disposable filters, which are also necessary if ‘sticky’ particles are present.

[Panel]

Possible effects of inhaling a variety of substances

Emission Possible effect on personnel

Cresol Damage to liver/kidneys, dermatitis, cancer

NO/NO2 Asphyxia

Phenol Damage to liver/kidneys, digestive disorders

Phosgene Pulmonary oedema

Benzene Known carcinogen – leukaemia

HCN Respiratory failure

Chromium Lung cancer, damage to liver and kidneys

Methanol Severe skin and eye irritant

Nickel Lung cancer, dermatitis

Styrene Poisonous, irritant of respiratory tract

See also Dr Chris Ide’s article on work-related lung disorders and respiratory conditions, Save your breath, in the March 2004 issue of SHP.

References and further information

1 Roach, RJ, Raymond, EA, Tyrer, JR and Sharp, BL: A technique for the indexed assessment of fumes generated by high-power laser processing

2 EH40 Occupational Exposure Limits, HSE Books, ISBN 0-7176-1315-1

An overview of occupational health and safety legislation across Europe is at www.europe.osha.eu.int/ good_practice/risks/ds/oel/members.stm

Industrial Ventilation: A Manual of Recommended Practice, ISBN 1 882417-22-4

 

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