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Industrial ventilation can help as a partial measure—but only if hoods, ducts, and airflow are all properly designed.
Many industries produce fine or "respirable" silica dust as part of their process—a potential health hazard to employees. The most common form of silica is quartz, one of the earth’s most common minerals and one that can pose a hazard when finely divided. Silica risks can be present in occupations that mine, produce or handle sand, clay, and rock, including concrete and asphalt production, stone-cutting and masonry, glass and paint processing, road construction and shipbuilding, and oil and gas production.1
Silica dust has been associated with silicosis, a respiratory disease that scars the lungs and reduces their ability to take in oxygen, as well as lung cancer, kidney and autoimmune diseases, and chronic obstructive pulmonary disease (COPD). Respirable silica has been classified as a carcinogen since 1996.2
The Occupational Health and Safety Administration (OSHA) has regulated silica exposure for many years, but it has recently updated its rules. The stricter regulations phased in for the construction industry in September 2017, expand to general industry and maritime activities on June 23, 2018, and begin governing hydraulic fracturing operations in 2021.3 For covered mining activities, the Mine Safety and Health Administration (MSHA) is in the process of drafting its own silica exposure regulations, which are anticipated to parallel OSHA’s.4
The OSHA rules impose updated requirements for exposure assessment, respiratory protection, medical surveillance, hazard communication and record keeping. They also cut the permissible exposure limit (PEL) in half, to no more than 50 micrograms per cubic meter (µg/m3). If the airborne concentration of silica exceeds 25 µg/m3 over eight hours, the employer is required to take actions to reduce potential exposure.3
OSHA has published a number of guides to help employers in various industries understand and comply with the new rules.3,5 The strategies listed for managing silica exposure include these common ones:
Local exhaust ventilation systems (dust collection) is named as one engineered control in the OSHA standards. Designed and maintained properly, a dust collection system can help reduce the amount of airborne dust as part of a larger mitigation strategy.
Defining an Effective System
Dust control goes beyond merely having a dust collector. It requires the management of three important "E's" at different points in the facility (Illustration 1):
To address all three zones, an effective dust collection system includes well-designed hoods and ducts that maintain recommended capture and conveying velocities; efficient dust collection technology; fans sized appropriately for the system airflow and pressure design requirements; and appropriate work practices to manage the dust and maintain the system. Here is more detail:
The design and placement of hoods at all points of dust generation is the most critical step in effective dust control. The performance of the hood in controlling nuisance dust emissions defines the maximum achievable performance of the dust control system as a whole. If the hood captures only 20% of the dust, the local exhaust ventilation system will only be 20% effective. The efficiency of the dust collector—one component—is limited to the portion of nuisance dust captured by the hoods and conveyed to the collector.
Maintaining recommended capture and conveying velocities for various dust is also critical. Heavy or toxic dusts require higher capture velocities to draw particulate into a hood, as well as higher conveying velocities to pull dust through the duct system to the dust collector. A qualified industrial ventilation designer should advise on the initial lay-out and on any remodeling of ventilation systems. Adding a hood or duct can throw off the delicate balance of air flow within the system.
A dust collector should effectively remove dust from the air volume it processes while working in a steady-state condition. With continuous cleaning collectors under stable operating conditions, an accumulated dust cake will be present on the filters. When this cake is dislodged during a cleaning cycle, the dust concentration around the filter will be much greater than the inlet dust load concentration. Because it is difficult to validate the actual accumulated dust load, relying on the theoretical performance efficiency of a new filter (for example, one rated MERV 14) may not be the best indicator of overall performance. Monitoring employee exposure and outlet emissions levels is a better performance indicator.
Best practices for dust collection design are contained in the Industrial Ventilation Design Manual published by the American Conference of Governmental Industrial Hygienists. A qualified industrial ventilation designer can develop a dust collection system according to these practices. Good design entails drawing sufficient control volume at each exhaust ventilation hood to contain and control the dust at each source. There’s a delicate balance to manage from a design standpoint, and if you try to tap into what’s already there with additional ducts, you can throw that balance off. It’s a good idea to have an industrial designer advise on any remodeling.
Process owners should engage the right experts to advise them on compliance with silica regulations. A knowledgeable employee health and safety (EHS) professional can help interpret exactly which regulations apply and how to best implement them, given the unique aspects of a specific facility and process. A trained industrial hygienist can help test and monitor exposure levels to verify the ventilation system is operating within the design parameters and advise on additional actions or work practices to reduce silica exposure. OSHA also offers an On-site Consultation program for small businesses.6
1 Frequently Asked Questions: Respirable Crystalline Silica Rule
2 OSHA Health Effects of Silica
3 Respirable Crystalline Silica Standard for General Industry and Maritime (29 CFR 1910.1053) and Respirable Crystalline Silica Standard for Construction (29 CFR 1926.1153)
4 30 CFR SS56-58; MSHA FAQ on Silica Exposure for Underground Miners
5 OSHA Publications on Crystalline Silica
6 OSHA On-Site Consultation Program