Protecting Machine Operators from Silica Dust: Enclosed Cabs

Construction workers who operate heavy equipment such as excavators, bulldozers, cranes, and backhoes frequently generate large quantities of respirable crystalline silica (RCS) dust. Exposure to even small amounts of RCS over time can cause silicosis, lung cancer, or chronic obstructive pulmonary disease (COPD), and other serious diseases.

A recent study published in the American Journal of Industrial Medicine found that 79% of the 100,000 workers exposed to RCS above the recommended exposure limit (REL), worked in the construction industry [Doney et al. 2020]. The NIOSH REL is designed to minimize adverse health effects from exposure to RCS from a full career of operating an enclosed cab for 10-hour workdays and/or 40-hour work weeks.

If enclosed cabs are not well designed or functioning properly, the operator may be exposed to harmful concentrations of RCS dust. NIOSH has studied the performance of enclosed cabs for protecting agriculture, construction, and mining industry workers from a variety of air contaminants over many years. NIOSH research found that filter efficiency and the use of a recirculation filter were the most important performance factors for enclosed cabs [NIOSH 2008]. An effective filtration system should remove at least 95% of respirable aerosols, such as dust, diesel particulate, and droplets. Enclosure integrity (sealed isolation from the outside environment) prevents wind-driven contaminants from penetrating the enclosed cab and minimizes air leakage around the filters. Based upon its extensive research, NIOSH [2018] recommends the following design considerations for enclosed cabs:

• Use a separate intake pressurizer fan and a high-efficiency respirable dust filter to supply air into the enclosure.
• Seal the enclosed cab from the outside environment to achieve effective contaminant reductions inside the enclosure and to achieve positive pressurization.
• Ensure that the system is recirculating filtered air inside of the enclosure, a practice which has been demonstrated to significantly improve the enclosure’s performance and reduce the time needed for the lowest steady state concentrations [NIOSH 2007a; Organiscak and Cecala 2008; NIOSH 2008b].
• Locate the cab filtration system inlet as far away from potential sources of contaminant generation.
• Avoid heaters, fans, or HVAC discharge vents located near the cab floor since this location can dislodge and aerosolize dust.
• Incorporate remote operator control of processes within the enclosure to reduce the need to open the enclosure to the outside.
• Use mechanical filter media which becomes more efficient with use through the creation of a dust filter cake [NIOSH 2012, Cecala et al. 2014].
• Monitor the performance of the enclosed cab system with a cab pressure instrument. A measurable loss in pressure over time can indicate significant problems, including a reduction in intake airflow into the enclosure or a loss of enclosure integrity.
• Size and locate filters to facilitate easy replacement and maintenance.
• Select interior materials that retain less dust and are easily cleaned to reduce dust generation inside of the enclosure.

In developing its silica standard for the construction industry, OSHA used the NIOSH research to develop its requirements for protecting workers in enclosed cabs from silica exposure. [29 CFR 1926.1153]. During demolition, grading or excavating tasks, the standard permits the use of enclosed cabs with high efficiency air filtration systems to protect heavy equipment operators (Table 1). In order to comply with the standard, enclosed cabs need to be properly designed by the manufacturer or retrofitted to include the following features:

• An intake air filter that is at least 95% efficient for particles in the 0.3-10.0 μm range (e.g., MERV-16).
• Free from settled dust.
• Door seals with properly working closing mechanisms.
• Gaskets and seals that are in good condition and working properly.
• Positive pressure with continuous delivery of fresh air.
• Heating and cooling capabilities

Well-designed enclosed cabs should provide a comfortable, safe and healthy environment for equipment operators. The enclosed cabs should reduce the operator’s exposure to airborne contaminants, like silica dust, that are generated outside the cab. Air filtration and pressurization are typically incorporated into the enclosure’s heating, ventilation, and air conditioning system [NIOSH 2018].

Unfortunately, in many cases, it may not be apparent to equipment operators if cabs in use today comply with the OSHA standard. Some cabs may be enclosed but not have an adequate pressurization and filtration system. Other cabs may have been originally designed properly but parts of the system were not adequately maintained. As a result, many equipment operators are being exposed to RCS. Since it is important that equipment manufacturers and owners communicate and work collaboratively to ensure compliance [CPWR 2019],   representatives from OSHA, NIOSH, and CPWR-The Center for Construction Research and Training (CPWR) met with a trade association representing the manufacturers to discuss the problem. CPWR also conducted a survey of 437 safety and health trainers with the International Union of Operating Engineers [CPWR 2019] about silica training, familiarity with the Occupational Safety and Health Administration (OSHA) silica standard, and cab filtration systems. Trainers raised concerns about inadequate cab maintenance, the expense of equipment required to control silica within the cab, and not understanding the standard. The survey identified a need for education about selection, use, and maintenance of cab filtration systems to control silica exposure.

In response to the findings from its survey, CPWR published a Hazard Alert to inform workers and employers on the importance of protecting workers from silica dust when operating machinery from inside an enclosed cab. CPWR also published a fact sheet for companies that manufacture, sell or rent the equipment – Heavy Equipment with Enclosed Cabs: Do Your Customers Need to Comply with OSHA’s Silica or MSHA Dust Standards? to help them understand the requirements in the OSHA and MSHA standards so they can best advise their customers. The CPWR Hazard Alert recommend that enclosed cab equipment operators should complete the following three tasks before beginning work:

1. Inspect the system’s filters for damage or airflow bypass and notify the supervisor if the filters need to be cleaned or replaced.
2. Inspect the cab daily for holes, gaps, and cracks around doors, windows, joints, power line entries, and controls. Use silicone caulk or rubber gaskets to repair and seal these areas. Notify the supervisor if a door gasket or window seal needs to be replaced.
3. Check the pressure gauge daily to ensure it works. Monitor the pressure throughout the work shift to make sure positive air pressure is maintained in the cab and dusty air is kept out.

Well-designed and maintained enclosed cabs can reduce the operator’s exposure to airborne contaminants such as silica. If you operate heavy equipment in the construction industry, please share your experiences related to using enclosed cabs in the comment section below. What issues or barriers have you experienced?

Scott Earnest, PhD, PE, CSP, is the Associate Director for Construction Safety and Health.

CAPT Alan Echt, DrPH, CIH, Senior Industrial Hygienist in the NIOSH Office of Construction Safety and Health (retired).

Scott Breloff, Ph.D. is a Biomedical Research Engineer in the NOISH Office of Construction Safety and Health.

CDR Elizabeth Garza, MPH, CPH, is Assistant Coordinator for the Construction Sector in the NIOSH Office of Construction Safety and Health.

Christina Socias-Morales, DrPH, is a Research Epidemiologist in the NIOSH Office of Construction Safety and Health.

Jeanette Novakovich, MA, MS, PhD, is a Writer-Editor in the NIOSH Division of Science Integration.

References

Cecala AB, Organiscak JA, Noll JD, Rider JP [2014]. Key components for an effective filtration and pressurization system for mobile mining equipment. Min Eng 66(1):44–50.

Bartlet, G.; Sokas, R.; Betit, E. [2019]. Operating Engineers: Union Trainers’ Response to the Silica Standard. New Solutions: A Journal of Occupational and Environmental Health Policy. Nov 14, 2019.

Doney BC, Miller WE, Hale JM, Syamlal G. [2020]. Estimation of the number of workers exposed to respirable crystalline silica by industry: Analysis of OSHA compliance data (1979‐2015). Am J Ind Med.; 63:465–477.DOI: 10.1002/ajim.23109

Hazard Alert: Protect Workers from Silica Dust…Working in an Enclosed Cab CPWR Center Construction Research and Training [2020]. The Center for Construction and Research Training. Silver Spring, MD. https://www.cpwr.com/sites/default/files/publications/Enclosed_Cabs-Hazard-Alert.pdf

NIOSH [2007]. Recirculation filter is key to improving dust control in enclosed cabs. By Organiscak JA, Cecala AB. Pittsburgh, PA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2008–100.

NIOSH [2008]. Key design factors of enclosed cab dust filtration systems. By Organiscak JA, Cecala AB. Pittsburgh, PA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2009–103.

NIOSH [2012]. Dust control handbook for industrial minerals mining and processing. By Cecala AB, O’Brien AD, Schall J, Colinet JF, Fox WR, Franta RJ, Joy J, Reed WR, Reeser PW, Rounds JR, Schultz MJ. Pittsburgh, PA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2012–112.

NIOSH [2018]. Design, testing, and modeling of environmental enclosures for controlling worker exposure to airborne contaminants. By Organiscak JA, Cecala AB, and Hall RM. Pittsburgh PA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2018–123, IC 9531. https://doi.org/10.26616/NIOSHPUB2018123

Organiscak JA, Cecala AB [2008]. Laboratory investigation of enclosed cab filtration system performance factors. Min Eng 60(12):74–80.