Dust monitor

ALMOST ALL quarrying operations have the potential to create dust. Control measures should therefore be established to prevent the generation of levels that cause harm. These measures should be identified in the health and safety document, and measurements should be taken to monitor exposure and demonstrate the effectiveness of controls. 

Many minerals contain high levels of silica, so quarrying activities of these materials generate silica dust known as respirable crystalline silica (RCS) and particular care must be taken to control exposure. Guidance is available from the HSE; see document HS(G) 73 Respirable crystalline silica at quarries. Sandstone, gravel and flint typically contain over 70% crystalline silica, shale contains over 40% and granite can contain up to 30%. Inhaling RCS can lead to silicosis which is a serious and irreversible lung disease that can cause permanent disablement and early death. There is an increased risk of lung cancer in workers who have silicosis, and it can also be the cause of chronic obstructive pulmonary disease (COPD). 

The Control of Substances Hazardous to Health Regulations 2002 (COSHH) requires employers to ensure that exposure is prevented or, where this is not reasonably practicable, adequately controlled. The COSHH definition of a substance hazardous to health includes dust of any kind when present at a concentration in air equal to or greater than 10 mg/m³ 8-hour time-weighted average of inhalable dust, or 4 mg/m³ 8-hour TWA of respirable dust. This means that any dust will be subject to COSHH if people are exposed to dust above these levels. Some dusts have been assigned specific workplace exposure limits (WELs) and exposure to these must comply with the appropriate limits. For example, the WEL for RCS is 0.1 mg/m³ 8-hour TWA.

The Quarries Regulations 1999 cover all surface mineral workings, and include tips and stockpiles, as well as areas used for crushing, screening, washing, drying and bagging. Buildings and other structures are also included, as are common areas and prospecting sites. The Regulations were created to protect the health and safety of quarry staff, as well as others that may be affected by quarrying activities, such as those living, passing or working nearby, or visiting the site. 

The role of monitoring

In order to assess the risks posed by dust, it is necessary to undertake both workplace monitoring – inside buildings, vehicle cabs etc., as well as environmental monitoring in and around the quarry. The technology for doing so is similar but different instruments are available for every application. Ashtead supplies personal air sampling pumps when it is necessary to conduct compliance monitoring, or when the identification and measurement (in a laboratory) of a specific dust type, such as RCS, is required.

Once the dust risks at a quarry have been assessed, ongoing monitoring is more often conducted with direct reading instruments that employ optical techniques to measure the different particulate fractions. Portable battery-powered instruments such as the TSI SidePak and the DustTrak are ideal for this purpose and feature heavily in Ashtead’s fleet of instruments for both sale and rental.

Dust monitoring technology

The Science Museum in London features one of the early dust monitoring devices. It operated by passing sample air through a filter which was subsequently weighed manually so that the weight of dust in the volume of air passed through the filter could be calculated. This method was labour-intensive, costly and risked the problems associated with blocked filters and operator error. In addition, this method did not easily facilitate the discrimination of particle size. This is particularly important from a human health perspective because finer particles (PM_2.5 and smaller) are able to penetrate deep into the lungs and thereby cause more significant medical problems, especially if other pollutants are adsorbed on to the surfaces of such particles.

Direct reading devices have been developed to simplify monitoring and to enable the simultaneous measurement of multiple particle fractions. As a result, the managers of plant and machinery, that have the potential to create a dust hazard, are able to implement effective monitoring programmes to protect workers and others.

The TSI DustTrak DRX monitors are laser photometers that simultaneously measure five size-segregated mass fraction concentrations. The desktop and handheld monitors are continuous, real-time, 90°, light-scattering laser photometers that measure mass fraction concentrations corresponding to PM₁, PM_2.5, Respirable, PM₁₀, and Total PM fractions. They combine both particle cloud (total area of scattered light) and single particle detection to achieve mass fraction measurements. This size-segregated mass fraction measurement technique is superior to both a basic photometer and an optical particle counter (OPC), because it delivers the mass concentration of a photometer with the size resolution of an OPC. Typically, photometers can be used at high mass concentration, but they do not give any size information (unless used with size-selective inlet conditioners) and significantly underestimate large particle mass concentrations. OPC’s provide size and count information; however, they do not provide any mass concentration information and cannot be used in high mass concentration environments. The DustTrak laser photometers therefore offer an ideal solution for indoor dust monitoring, and the technology is available in a variety of formats including a personal (wearable) model, a portable battery-powered model and a desktop version for workspace monitoring.

Employing digital technology, these instruments are able to record measured values on an internal datalogger, such as 15 minute averages. In addition, they are also able to issue user-set alarms when dust concentrations approach dangerous levels. An optional external zeroing module is used when sampling over extended periods of time to minimise zero drift, and in humid environments users are able to condition the sample air stream with a heated inlet sample conditioner.

The DustTrak has two calibration factors: a photometric calibration factor (PCF) and a size calibration factor (SCF). The PCF accounts for the photometric response difference between A1 Test Dust and the aerosol under measurement, and the SCF accounts for the aerodynamic size difference. The standard calibration determines the SCF for the aerosol of interest; the process is easy and does not require comparison with gravimetric samples. The advanced calibration method yields high size-segregated mass concentration accuracy. This method involves two separate gravimetric measurements to obtain PCF and SCF in sequence.

The same dust monitoring technology is also employed by the TSI DustTrak Environmental (DTE), which has been developed specifically for applications such as dust monitoring at quarries. Fully compliant with stringent MCERTS performance requirements, the DTE employs a ‘cloud’ based data management system, which provides users with easy access to real-time data on dust levels, with the optional addition of other sensors. Alarm conditions can be set by users so that text and email alerts are issued when threshold levels arise. The DTE also monitors PMTotal, PM₁₀, PM_2.5 and PM_1.0 mass fractions simultaneously, which provides detailed information on the type of dust present, and means that alarms can be set for specific fractions.

Clearly, dust monitors can perform a vital role in helping to protect safety at working quarries. For example, a TSI DTE was recently hired from Ashtead Technology to perform monitoring prior to the commencement of quarrying operations, so that baseline dust levels could be established for comparison once the quarry is operational. Monitoring prior to operations is important, because airborne dust at a quarry is not necessarily derived from the quarry alone; local agricultural or industrial activities may also contribute to the particulate burden. This also highlights the advantages of 24/7 monitoring because dust pollution may be intermittent, so continuous monitors such as the DTE are able to identify peaks and thereby assist in the attribution of dust sources.

Ashtead Technology fitted the DTE mentioned above with a solar panel and rechargeable battery so that it could operate unattended for extended periods in a remote location. With web-based access to the data, site visits were minimised and costs lowered. This equipment was hired from Ashtead to avoid capital expenditure, and looking forward, the client is planning to add a Lufft wind monitor to the rental, because data on wind speed and direction helps with modelling and with the identification of dust pollution sources.

Summary

Ideally, quarry site monitoring should be undertaken prior to the commencement of operations to establish baseline levels for that site. Risk assessments can then be undertaken around the site and within buildings and vehicles/machinery. However, conditions can change significantly, so continuous monitoring is preferable. Changes in quarry practices and weather can affect environmental conditions, and workplace exposure can be affected by a wide range of factors such as broken filter bags, spillage, insufficient cleaning, filter blockage and dry (instead of wet) drilling or cutting.

With a variety of applications for dust monitoring, it is important that appropriate technology is employed, so the Ashtead Technology instrument fleet has been developed to meet almost every need, and technical advice is available to help consultants and quarry operators ensure that dust hazards are effectively monitored and managed. 

Josh Thomas is senior sales manager at Ashtead Technology. For more information, visit ww.ashtead-technology.com