Elimination or Substitution of Hazardous Substances
It may be possible to eliminate or substitute the substance by:
• Eliminating the process or type of work that requires the use of (or creates) the substance (e.g. outsourcing a paint-spraying operation).
• Changing the way that the work is done to avoid the need for the substance (e.g. screwing items together rather than gluing).
• Disposing of unused stock of substances that are no longer needed.
• Substituting the hazardous substance for one non- hazardous (e.g. switch from an irritant to a non- hazardous floor cleaner).
• Substituting a hazardous substance for one that has a lower hazard classification (e.g. exchange a solvent paint for a water-based paint).
• Changing the physical form of a substance (e.g. use pellets instead of powder).
Process Changes
It may be possible to change a process so that risks can be reduced, e.g.
• Brush painting rather than spraying reduces airborne mist and vapour.
• Vacuuming, rather than sweeping up, reduces dust levels.
• Damping of a substance during mixing or when clearing up reduces dust levels.
Reduced Time Exposure
Ill-health effects caused by hazardous substances are often related to the length of time of exposure and the dose (amount) of the contaminant. Reducing the time will reduce the dose (extending the time increases the dose). We should therefore look to minimise the time people work with hazardous substances, especially with those having acute effects. If a short-term exposure limit (15-minute TWA) exists for the substance, this must not be exceeded. We can achieve this by:
• Providing for regular breaks away from contact with the hazardous substance.
• Job rotation – where exposure of an individual is reduced by sharing the dose with other workers.
Enclosure and Segregation
Where it is not possible to reduce exposure, then we have to consider physical controls which enclose the hazard and segregate people from the process involving it.
Total enclosure of a process which generates dust or fumes will prevent the escape of airborne contaminants which could be inhaled by operators nearby. However, it may still be necessary to access equipment or material within that area, so the use of robotically-controlled, remote handling systems may be incorporated, allowing access without disturbing the integrity of the enclosure. Where isolation of the source is difficult, it may be more practical to enclose the workers to ensure that they remain segregated from the hazard (e.g. in a control booth).
Local Exhaust Ventilation (LEV)
This will contain and collect dusts, vapours and fumes where they are generated, and prevent them spreading
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further into the workplace. The contaminants will be filtered out and the clean air exhausted outside the workplace.
Topic Focus
A typical LEV system consists of:
• An intake hood that draws air containing the contaminant in at the point it is created. • Ductwork that carries the air from the intake
hood.
• A filter system that cleans the contaminant from the air to an acceptable level.
• A fan that provides the air movement through the system.
• An exhaust duct that discharges the clean air to atmosphere.
A typical LEV system extracting sawdust from a bench-mounted circular saw
Examples of LEV include:
• Glove boxes – total enclosures, often used in laboratories, accessed through flexible gloves and kept under negative pressure to prevent any release of contaminant.
• Fume cupboards – partial enclosures, again used in laboratories, accessed through a vertical sliding sash, with the enclosure kept under negative pressure so that the air flow is through the sash into the hood to prevent any release of contaminant.
• Captor hoods – hoods which can be positioned as near as possible to the hazard and capture contaminants by a negative air flow into the hood before they reach the operator, e.g. those used to extract woodworking dust or welding fume. • Receptor hoods – large structures designed to
capture contaminants which are being directed naturally into the hood, so that less air movement is needed to achieve uptake (e.g. a large intake hood above a bath of molten metal - the metal fume will be hot and rising up into the hood on convection
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Factors that Reduce Effectiveness
• Poorly positioned intake hoods. • Damaged or leaking ducts.
• Excessive amounts of contamination. • Ineffective fan due to slow speed or lack of
maintenance. • Blocked filters.
• Build-up of contaminants in the duct. • Sharp bends in the duct.
• Unauthorised additions into the system.
Inspection and Monitoring
LEV systems must be routinely inspected and maintained to ensure their continuing effectiveness.
• Regular Visual Inspections Check:
– The integrity of the system, and for signs of obvious damage and build-up of contaminant inside and outside the ductwork.
– Filters, to ensure they are not blocked (some have a collector can which can be emptied).
– The exhaust outlet is clear. • Planned Preventive Maintenance
May include: – Replacing filters. – Lubricating fan bearings. – Inspecting the fan motor. • Periodic Testing
– Ensure that air velocities through the system remain adequate.
– Can be done by visual inspection of the captive system using a smoke-stick, measuring air velocity at the intake and along the ductwork using an anemometer, and measuring static pressures with manometers and pressure gauges.
LEV provided as a control measure for hazardous substances should be thoroughly examined by a competent person every 14 months.
Dilution Ventilation
This operates by diluting the contaminant concentration in the general atmosphere to an acceptable level, by efficiently changing the air in the workplace over a given period of time, e.g. a number of complete changes every hour.
The air changes might be achieved:
• Passively – by providing low-level and/or high-level opening louvres.
• Actively – using powered fans.
This type of ventilation is intended to be effective in removing gas contaminants (sometimes fumes) and to keep overall concentration of any contaminants below the OEL.
Dilution ventilation is appropriate where: • The OEL of the hazardous substance is high. • The rate of formation of the gas or vapour is slow. • Operators are not in close contact with the
contamination generation point.
If a powered system is used, fans must be sited appropriately. If the contaminant is:
• Lighter than air, it will naturally rise up inside workrooms and be extracted at high level.
• Heavier than air, it will sink to the floor and low-level extraction will be more suitable.
Limitations of dilution ventilation systems:
• Not suitable for the control of substances with high toxicity.
• Do not cope well with sudden releases of large quantities of contaminant.
• Do not work well: – On dust.
– Where the contaminant is released from one point source.
• Dead areas may exist where high concentrations of the contaminant are allowed to accumulate.
Dead areas are those areas in a workplace which remain dormant so the air in them is not changed. This is usually due to the air-flow patterns produced by poor positioning of extraction fans and inlets for make-up air. Dead areas can move from one place to another as a result of changing the positions of fans and make- up air inlets, or by the intrusion of other air through windows and doors. Moving the position of machinery or workbenches can have similar effects.