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Engineering Controls Reduce Crystalline Silica Exposures During Asphalt Pavement Milling in Highway Construction

Release Date: May 19, 2016

Cold-milling machines, which have toothed, rotating cutters, are used to grind, remove, and recycle payment for highway construction. Workers operating the equipment, as well as workers assisting in the vicinity of the operations, may be exposed to dust containing crystalline silica. Crystalline silica can cause silicosis, a debilitating and potentially fatal lung disease, as well as other adverse health effects. Approximately 251,000 U.S. workers are employed in highway, street, and bridge construction and are at risk of exposure to respirable crystalline silica.

Only a few studies have been conducted to measure exposures to workers during milling, but they didn’t contain enough detail to allow researchers to evaluate whether exposures were due to a lack of engineering controls, or poor maintenance of controls that were already in place. To fill in the data gaps, D. R. Hammond, S. A. Shulman, and Alan S. Echt, researchers from the National Institute for Occupational Safety and Health (NIOSH), sampled workers for crystalline silica while using two cold milling machines made by two different manufacturers and equipped with water spray and local exhaust ventilation engineering controls. The results of the study will be published in the July 2016 issue of Journal of Occupational and Environmental Hygiene (JOEH).

Both milling manufacturers designed their dust controls using recommendations contained in NIOSH Publication No. 2015–105, Best Practice Engineering Control Guidelines to Control Worker Exposure to Respirable Crystalline Silica during Asphalt Pavement Milling, a publication which was the result of work coordinated by the Silica/Asphalt Milling Machine Partnership, an alliance of pavement milling machine manufacturers, industry, labor, and government stakeholders, such as NIOSH and the Occupational Safety and Health Administration (OSHA).

In the study, which represented the final phase of field testing for the engineering controls, full shift samples were collected on operators using the two machines, as well as ground workers, who walk along adjusting controls and directing traffic, a few feet away from the cutting point. Workers were sampled over 21 days, at 11 different work sites, with a total of 42 samples collected. Samples of the material being milled were analyzed for the crystalline silica content as well. Milling machine ventilation involved enclosures and/or ducts around the rotating drum and conveyers, and water sprays were employed on conveyers and drums to control dust, in addition to cooling the cutting surface. The samples represented a variety of conditions and surfaces – day and night shifts, highways, streets, parking lots, different milling depths, etc.

Sample results were compared to the NIOSH Recommended Exposure Limit (REL) for respirable crystalline silica of 50 micrograms per cubic meter of air, since at the time of the study the new OSHA rule on crystalline silica had not yet been released. However, the new OSHA permissible exposure limit (PEL) is the same as the NIOSH REL used in the study. These occupational exposure limits have been adopted based on the risks of developing silicosis, lung cancer, and other respiratory and adverse health effects from exposure to respirable crystalline silica.

Results of the study include:

The study showed that installation of local exhaust ventilation and water spray on cold milling machines were engineering controls that were successful in reducing crystalline silica exposures to below the NIOSH REL. As noted by the authors:

Following this testing, the manufacturers of both asphalt pavement milling machines made plans to include the ventilation as a standard feature on all new half-lane and larger asphalt pavement milling machines.

The full article, “Respirable crystalline silica exposures during asphalt pavement milling at eleven highway construction sites,” is available for a fee by searching the JOEH website at http://www.tandfonline.com/toc/uoeh20/current. Members of the American Industrial Hygiene Association can access the full text of the article at no cost.

Field Guide for Controlling Silica Dust Exposure on Asphalt Pavement Milling Machines, the result of the Silica/Asphalt Milling Machine Partnership collaboration, can be accessed at http://www.silica-safe.org/training-and-other-resources/manuals-and-guides/asset/Field-Guide-for-Controlling-Silica-Dust-Exposure-on-Asphalt-Pavement-Milling-Machines.pdf.

NIOSH Publication No. 2015–105, Best Practice Engineering Control Guidelines to Control Worker Exposure to Respirable Crystalline Silica during Asphalt Pavement Milling can be accessed at http://www.cdc.gov/niosh/docs/2015-105/pdfs/2015-105.pdf.

COPYRIGHT ©2016, ISO Services, Inc.

The information contained in this publication was obtained from sources believed to be reliable. ISO Services, Inc., its companies and employees make no guarantee of results and assume no liability in connection with either the information herein contained or the safety suggestions herein made. Moreover, it cannot be assumed that every acceptable safety procedure is contained herein or that abnormal or unusual circumstances may not warrant or require further or additional procedure.


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