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August 23, 2022

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dusts and silica

Protecting rail workers from respirable dusts and silica

Many industrial processes generate varying levels of dust, which may come from machining parts or from working with raw materials. The rail industry is no exception to this rule, with both workers and the general public at risk from the health effects associated with inhalation and exposure. The experts at SOCOTEC look into what these risks are and how rail organisations can effectively control them.

What dusts pose a level of risk?

Protecting Rail Workers from Respirable Dusts and SilicaInhalable dust is defined as any dust with a particle size less than 100mm. It approximates to the fraction of airborne material that enters into the nose and mouth during breathing, and is therefore available for deposition in the respiratory tract. Larger particles are trapped in the nose and throat or in the branching structures of the airways, where the body’s own protective mechanisms rid the body of the dust. Respirable dust is the fraction of dust below 10mm, which is likely to reach the deepest region of the lungs. At these sizes, the dust becomes almost invisible to the naked eye. Different dusts have varying effects on health and can ultimately lead to diseases such as chronic bronchitis and occupational asthma.

A dust that poses a major concern is respirable crystalline silica, often referred to as RCS. RCS crystals are approximately 5mm, or 1/60th of the size of a grain of table salt. Although it arises in dusty environments, respirable crystalline silica dust is invisibly fine. It is breathed in through the nose and mouth and can stay in the lungs for many years. It can cause irreversible lung damage before any symptoms develop, with the possibility of illnesses continuing to worsen even after exposure stops.

Where do we get silica from?

Silica is present in almost all rocks and stones. The amount of silica is dependent on the type of stone, and can vary from 2% in limestone and 30% in granite to over 70% present in sandstone. Cement can contain anything between 25 and 70% RCS, depending on the mixture used. The amount of RCS generated depends on the type of stone and what is being done with it.

Cutting concrete using a disc cutter without dust suppression can generate high levels of respirable dusts, potentially containing up to 70% RCS. Handling sands in a way that generates fine dust such as dropping from a height, or blowing the sand with compressed air, can also generate high levels of dust containing over 70% RCS. The use of ballast in the railway industry is a potential source of dust. Relaying works involving the dropping of ballast from wagons, or moving ballast stocks with mechanical shovels, can result in high levels of dust.

Health effects of RCS

Dusts entering the lungs are attacked by the body’s defence mechanisms. When dusts travel deep into the air sacs of the lungs where oxygen is transferred to the blood system, macrophages try to attack the silica particles. These macrophages are unable to break down the silica and die, carrying the dust particles into the walls of the lungs to form scar tissue.

Silicosis is a major disease risk from RCS dust that causes small, hard nodules of scar tissue to develop in the lungs. Silicosis usually takes some years to develop, although there is also an acute form that occurs at very high exposure levels and can kill within months. While the main symptoms are coughing and difficulty breathing, workers with silicosis face an increased risk of tuberculosis and lung cancer, and may develop kidney disease, arthritis and related diseases. Exposure to respirable crystalline silica may also cause chronic obstructive pulmonary disease. Currently, 800 people in the UK die from silicosis per year – that is an average of 15 deaths per week. All of these deaths are preventable with the correct controls being applied.

Legislation and exposure limits

The COSHH (Control of Substances Hazardous to Health) definition of a substance hazardous to health includes dust of any kind when present at a concentration in the air equal to or greater than 10 mg.m3 (eight-hour time weighted average) of inhalable dust and 4 mg.m3 (eight-hour time weighted average) of respirable dust.

Exposure to any dust is known to cause or worsen chronic obstructive pulmonary disease (COPD). The IOM have suggested that exposure should be controlled to 5 mg.m3 (eight-hour time weighted average) of inhalable dust and 1 mg.m3 (eight-hour time weighted average) of respirable dust. Some dusts, including RCS, have been assigned specific Workplace Exposure Limits (WELs), and exposure to these must comply with the appropriate limit. The limit for RCS is set at 0.1 mg/m3 averaged over an eight-hour shift (approximately 1/40,000th of a teaspoon of dust per day). In addition, RCS is classed as a carcinogen, meaning exposure must be reduced as far as possible.

There is a requirement under the COSHH regulations to adequately control exposure to any material classified as harmful to health, including RCS. A range of control measures are available and should be implemented without depending on the use of Respiratory Protective Equipment (RPE) as the primary method of control. RPE can be used to further reduce exposure after all other measures are put in place.

The reason why RPE is the last resort is because it does not remove the hazard (only controls it). In addition, RPE will only protect the worker if it is worn correctly, the worker is face fit tested and is clean-shaven. The efficiency of tight-fitting RPE can reduce to 10% of the expected protection with only one days’ beard growth. Controls that reduce or remove the respirable dusts containing RCS will protect everyone in the area of work by preventing them from becoming airborne.

Sources of RCS dusts in the railway sector

There are a number of potential sources of RCS within the railway industry. The obvious is works involving ballast, particularly when new ballast is being dropped on site for track works. The fine dusts that are present within the ballast will become airborne when the ballast is dropped from rail wagons, or when moving the ballast using mobile plant.

Works involving used ballast are likely to generate less dust, as many of the fines present in the ballast will have washed away over a period of time. However, packing and tamping works can still generate dust containing silica and needs to be controlled.

Many of the powered traction units in use on the railway have sandboxes fitted. This allows a small amount of sand to be delivered to the traction wheels to improve grip in poor conditions, such as wet rails or railheads contaminated with leaf mould. When filling and testing the sanding equipment in depots, there is the potential for exposure to sand dust that can contain high levels of the invisibly fine RCS dusts.

A further potential source of RCS is repairs to station platforms or repairs to troughing where concrete materials are being drilled or cut using disc cutters or other power tools. The use of power tools can generate very high levels of dust, potentially affecting not only the power tool operator but also anyone else in the immediate vicinity. In addition, high levels of fine RCS can become spread over a wide area, potentially becoming airborne due to wind or when clearing up after the job is completed.

Consideration also needs to be given to dust exposure when mixing dry ingredients for concrete. There is the potential for exposure during the tipping of sacks and the initial mixing before water is added.

How to minimise exposure to dust:

The COSHH regulations list a hierarchy of controls in order to minimise the risk of exposure to hazardous materials. These are noted below:

  • Eliminate the use of hazardous martials
  • Substitute the material with a less hazardous material
  • Use engineering controls such as enclosures to prevent release of dusts
  • Provide extraction at source to capture hazardous dusts before they become airborne
  • Use in a manner which minimises dust release
  • Use of PPE/RPE.

Wet materials do not generate dust. One of the best options is to either pre-wet the materials to prevent dust release, or use fine water sprays to dampen the dust and minimise the amount becoming airborne. For ballast works, often the weather will naturally wet the material. However, over the summer months, there is a greater risk of materials being dry and potentially producing more dust.

Wetting is also an option when using disc cutters for platform repairs. Water dust suppression can virtually eliminate the dust generated when cutting stone or concrete using powered disc cutters. When cleaning up spillages or residues, all dusts should be removed using wet methods or by vacuuming. The use of brushes or blowing away dusts using compressed air simply stirs up the dust and results in more dust becoming airborne, potentially increasing exposure.

When drilling concrete, all tools should be fitted with on tool extraction. Often referred to as local exhaust ventilation (LEV), it captures and removes the dust before it becomes airborne. Where possible, all operators should position themselves upwind of ballast drops or other dust generating exercises, and avoid being in the area if not required. Operators directly involved will still need further protection, but others can avoid exposure. When pouring sands or mixing cement, pour the ingredients slowly into water to minimise the generation of dust. Adding sand to water reduces the amount of dust generated. When mixing cement, start mixing slowly to minimise the amount of dust produced.

If all else fails, or if other controls are not sufficient to control exposure, RPE should be adopted as a last resort. In order to be effective, operators depending on RPE should be face fit tested to ensure that it fits well enough to protect against exposure. RPE should be kept clean and be inspected to ensure that it has suffered no damage before use. If using tight fitting RPE, all operators need to be clean-shaven. Operators who are not clean-shaven should be provided with positive pressure RPE that does not depend on a tight seal around the nose and mouth to provide sufficient protection.

Lastly, carry out monitoring to identify and quantify potential exposures, allow valid risk assessments to be completed for all tasks, select appropriate controls, and ultimately protect the health of all workers by eliminating exposure. Training is also required to ensure all workers are aware of the risks and know how to control the risks to an acceptable standard.

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1 year ago

The units for particle size appears to be incorrect