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    ON-TOOL CONTROLS ESSENTIAL TO MINIMISE SILICA EXPOSURE

    Research supports tough HSE enforcement line on control of respirable dust 

    Many construction activities such as grinding, finishing, polishing, mortar removal, sanding and cutting produce large quantities of dust including materials containing silica and gypsum in the inhalable and respirable size fractions and if uncontrolled can cause exposure exceeding UK occupational exposure limits.

    HSE has now published Research Report RR926 – On-tool controls to reduce exposure to respirable dusts in the construction industry – A review which reports findings that both on-tool local exhaust ventilation and water suppression is capable of effectiing exposure reductions of greater than 90%.

    Where the two on-tool methods were directly compared there was often no significant difference in control effectiveness.

    The objective of this project was to conduct a review of the literature on the subject of the effectiveness of on-tool controls and to summarise this information for HSE.  The Executive Summary is reproduced below.

     

    EXECUTIVE SUMMARY

    Objectives

    Many processes in the construction industry create large quantities of dust; often materials used in construction contain silica. If the dust emissions from these processes are not controlled they can cause exposures that exceed UK occupational exposure limits and consequently lead to occupational diseases such as cancer, silicosis, chronic obstructive pulmonary disease and asthma. A common way to control these hazards is to apply local exhaust ventilation (LEV) however, construction sites tend be temporary workplaces, which makes the application of traditional LEV difficult. One solution is to affix LEV to the tool being used or to use another mobile form of on-tool control such as water suppression.

    Much of the information held by HSE on on-tool controls requires updating and there has been much research carried out in the field in recent years. The objective of this project was to conduct a review of the literature on the subject of the effectiveness of on-tool controls and to summarise this information for HSE.

    Main Findings

    On-tool LEV is capable of reducing exposures created by processes such as; tuck-point grinding to remove mortar, surface grinding, finishing and polishing, block, slab, brick and tile cutting, floor and drywall sanding. In most cases exposure reductions of greater than 90 % were achieved, sometimes after modifications to the LEV hood. Water suppression was found to be an effective on-tool control for reducing exposure to respirable dusts. Where the two on-tool control methods were compared no significant differences were found.

    The volume flow rate of air for good on-tool control required is typically 50 m3h-1 as a minimum but ideally 80 – 130 m3h-1 is recommended. The choice of vacuum source is vital; typically industrial vacuum cleaners are used, which tend to recirculate air back into the workplace. It is important that they have a final filter with a filtration efficiency of at least 99 % to prevent reintroducing captured respirable dusts back into the workplace air. To this end vacuum cleaners with cyclone type pre-filters are desirable or a vacuum fitted with an automatic back flush system to maintain adequate volume flow rates. Where dusts containing crystalline silica are produced a minimum of a Class M vacuum cleaner with final filter efficiency greater than 99.9 % should be used. When using water suppression, the importance of the volume flow rate of water was not widely agreed upon. Although where it was considered a flow rate 0.5 lmin-1 was considered to be a minimum.

    Even with exposure reductions of 90 % and greater, on-tool controls never completely eliminated exposure and could not always reduce it to below occupational exposure limits, especially where materials contained silica. This may mean that the use of supplementary respiratory protective equipment (RPE) is required. It should be noted however that most of the studies reviewed measured task-based exposure and not whole shift exposure and that 8-hour time weighted average (TWA) exposures may be lower, especially where workers perform different tasks throughout the day.

    The use of on-tool controls was not without issue. Many workers commented that the addition of extraction hoses or the need to carry or move water tanks made the tools ergonomically difficult to use and adversely affected their productivity. Some field studies noted that as operators became more familiar with new tools the effectiveness of the controls improved. This shows that where new tools and controls are to be applied training will be an important part of achieving good control.

    An internet search was conducted to determine the types and availability of on-tool extraction devices. A wide range of power tools fitted with extraction and dust control devices were available for purchase or hire either direct from manufacturers or from retailers and hire companies. Most companies offering extracted tools also offered vacuum cleaners/extraction units; most were of unspecified dust class. Vacuum cleaner manufacturers tended to be those who specify the dust class. L and M class vacuum cleaners were widely available, H class vacuum cleaners were only available from a limited number of suppliers.

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