Best Practice When Air Sampling for Silica- A Hidden Threat in Industrial Workplaces
Silica dust poses a significant risk in various industrial environments and can become a lethal hazard if not properly managed. Silica is a prevalent component in many organic materials found in industries like construction, mining, quarrying, and manufacturing, occurring naturally in the form of quartz.
In recent years, silica dust has garnered increased attention due to the growing awareness of its severe health impacts. Exposure to Respirable Crystalline Silica (RCS) can lead to serious, long-term health conditions, including silicosis, lung cancer, progressive massive fibrosis, and other irreversible diseases. RCS is classified as a carcinogen, and its effects can take years to manifest, leading to chronic illness and substantial health risks.
Hierarchy of Controls
Employers must adhere to the hierarchy of controls to effectively minimise workplace exposure to silica dust, prioritising methods from the most to the least effective:
- Elimination: This is the most effective measure, involving the complete removal of the hazard. Eliminating the use of silica-containing materials prevents exposure entirely. For instance, replacing sand used in abrasive blasting with a non-silica material such as metallic shot can significantly reduce risk.
- Substitution: If elimination is not possible, substitution offers a safer alternative. This strategy involves replacing the hazardous material with one that has a lower silica content. However, it is important to evaluate any new risks that the substitute material may introduce to ensure it is a safer choice overall.
- Engineering Controls: These controls aim to isolate workers from exposure by modifying equipment or the work environment. This might include installing protective barriers, employing local exhaust ventilation (LEV) systems, or redesigning equipment to reduce dust generation and spread.
- Administrative Controls: This category focuses on changing work practices to limit exposure. Examples include establishing exclusion zones, enforcing regular breaks, implementing shift rotations, and maintaining good housekeeping practices to reduce dust accumulation.
- Personal Protective Equipment (PPE): PPE serves as the last line of defence when other controls are not feasible or do not fully mitigate the risk. This includes items like gloves, masks, and overalls. For respiratory protection, Respiratory Protective Equipment (RPE) is critical but requires proper fit, training, and consistent use to be effective.
By systematically applying these control measures, employers can create a safer workplace environment, reducing the potential for exposure to harmful silica dust
Monitoring
Air sampling for crystalline silica is an essential component of workplace safety programmes, particularly in industries such as construction, mining, and manufacturing. Both the United States and the United Kingdom have established methods and guidelines for sampling and analysing crystalline silica to ensure compliance with occupational exposure limits (OELs).
Air sampling pumps are typically used for compliance purposes following well-established methods in both the U.S. and the UK. These pumps should comply with the latest versions of relevant standards, such as ISO 13137, which specifies requirements for air sampling pumps. Proper calibration is crucial for obtaining accurate measurements, with pumps calibrated for flow rate both before and after use to ensure the correct volume of air has been sampled.
Choosing the Right Pump and Sampler Selecting an appropriate air sampling pump is vital. The pump must adhere to specific standards and be equipped with the correct filter and sampling head to collect respirable crystalline silica. A common approach involves using a cyclone sampler to selectively capture the respirable fraction of dust, which is more likely to penetrate the lower respiratory tract and pose significant health risks. Various cyclone heads are available, each designed for different flow rates to ensure collection efficiency. The pump and sampling train must be capable of maintaining a constant flow rate throughout the sampling period, typically between 1.7 and 2.5 litres per minute for respirable silica sampling.
Collect a Representative Sample & Calibrate To ensure accurate and representative sampling, it is necessary to calibrate the pump with the sampling media in place before and after the sampling period. The pump should maintain a steady flow rate, with deviations not exceeding 5% of the target flow rate. Calibration ensures the air volume passing through the sampling media accurately reflects the worker's exposure over the sampling period. This process must include the entire sampling train, including the pump, tubing, and sampling head.
American and UK Methods for Sampling Crystalline Silica
Several standard methods are used in the U.S. and UK for sampling and analysing crystalline silica:
United States Methods
1. NIOSH Method 7500: This is a well-recognised method developed by the National Institute for Occupational Safety and Health (NIOSH). It uses X-ray diffraction (XRD) to determine the concentration of respirable crystalline silica, including quartz, cristobalite, and tridymite. Samples are collected on a filter using a cyclone sampler, and the filter is subsequently analysed in a laboratory.
2. OSHA ID-142: This is a method provided by the Occupational Safety and Health Administration (OSHA) that uses infrared spectroscopy (IR) to analyse the collected samples. The method is applicable to various forms of crystalline silica and involves gravimetric analysis followed by IR spectroscopy.
3. MSHA P-7: The Mine Safety and Health Administration (MSHA) has a method primarily used in the mining industry. It involves collecting respirable dust samples on filters using personal sampling devices and subsequent analysis using X-ray diffraction.
4. ASTM D7202: This method, developed by ASTM International, describes the use of infrared spectroscopy to determine respirable crystalline silica in the workplace atmosphere. It is applicable to quartz and cristobalite and provides an alternative to the NIOSH and OSHA methods.
United Kingdom Methods
1. MDHS 101: The "Methods for the Determination of Hazardous Substances" (MDHS) series includes MDHS 101, which is used for the determination of respirable crystalline silica. This method, published by the Health and Safety Executive (HSE), employs X-ray diffraction (XRD) to quantify the amount of crystalline silica on a filter. Samples are collected using personal samplers with a cyclone head to capture respirable particles.
2. MDHS 14/4: Although primarily designed for the general measurement of respirable dust, MDHS 14/4 can be adapted for crystalline silica sampling when combined with appropriate analytical techniques such as XRD or IR spectroscopy. This method outlines procedures for gravimetric analysis and sampling using a cyclone sampler.
3. EN 482: While not specific to crystalline silica, this European standard provides general requirements for the performance of procedures for measuring chemical agents in workplace atmospheres. It ensures methods, including those for silica, provide reliable and valid results.
4. ISO 16258-1: Although an international standard, it is widely used in the UK for workplace air monitoring of respirable crystalline silica. It provides guidelines for using direct-reading instruments and filter-based sampling methods to assess silica exposure.
Real-Time Monitoring
Gravimetric sampling, the type most commonly used with air sampling pumps, can take weeks to produce results, due to laboratory analysis required on the samples. Real-time dust monitors provide a solution and can be deployed to make assessments and adjustments to control measures before the gravimetric sample is taken.
Real-time instruments provide immediate information about exposure levels that can alert employees to exposure levels exceeding safe quantities. With real-time results, employers move through the assessment process faster and at a lower cost—through accurate reporting and instant validation of measures in place.
Real-time instruments will not tell you the proportion of silica within the total dust, but known types of material will have an estimated proportion of silica within them, which gravimetric sampling can do. The benefit of real-time monitoring is the ability to know when exposure is occurring and where the sources of exposure are, because of the real-time reading.
For silica, gravimetric analysis is still required for compliance, but incorporating real-time monitoring can speed up the process.
Conclusions
Choosing the correct air sampling pump and combining gravimetric monitoring with real-time solutions will maximise the ability to protect employees, ensure compliance and save lives. Protecting employees’ wellbeing is the legal and moral duty of employers. Consult with a qualified occupational hygienist where necessary; adhere to the pump user manual; and follow the steps outlined above to ensure employees’ safety is looked after and legislation is complied with.