Respirable crystalline silica exposures during asphalt pavement milling at eleven highway construction sites

ABSTRACT

Asphalt pavement milling machines use a rotating cutter drum to remove the deteriorated road surface for recycling. The removal of the road surface has the potential to release respirable crystalline silica, to which workers can be exposed. This article describes an evaluation of respirable crystalline silica exposures to the operator and ground worker from two different half-lane and larger asphalt pavement milling machines that had ventilation dust controls and water-sprays designed and installed by the manufacturers.

Manufacturer A completed milling for 11 days at 4 highway construction sites in Wisconsin, and Manufacturer B completed milling for 10 days at 7 highway construction sites in Indiana. To evaluate the dust controls, full-shift personal breathing zone air samples were collected from an operator and ground worker during the course of normal employee work activities of asphalt pavement milling at 11 different sites.

Forty-two personal breathing zone air samples were collected over 21 days (sampling on an operator and ground worker each day). All samples were below 50 µg/m³ for respirable crystalline silica, the National Institute for Occupational Safety and Health recommended exposure limit. The geometric mean personal breathing zone air sample was 6.2 µg/m³ for the operator and 6.1 µg/m³ for the ground worker for the Manufacturer A milling machine. The geometric mean personal breathing zone air sample was 4.2 µg/m³ for the operator and 9.0 µg/m³ for the ground worker for the Manufacturer B milling machine. In addition, upper 95% confidence limits for the mean exposure for each occupation were well below 50 µg/m³ for both studies. The silica content in the bulk asphalt material being milled ranged from 7–23% silica for roads milled by Manufacturer A and from 5–12% silica for roads milled by Manufacturer B.

The results indicate that engineering controls consisting of ventilation controls in combination with water-sprays are capable of controlling occupational exposures to respirable crystalline silica generated by asphalt pavement milling machines on highway construction sites.

KEYWORDS:

• Cold planers
• dust control
• engineering controls recycled asphalt pavement

Acknowledgments

The authors gratefully thank the members of the Silica/Asphalt Milling Machine Partnership, especially the milling machine manufacturers who developed engineering controls and provided their equipment for testing. The authors thank Gary Fore and Donald Elisburg for their leadership of the Partnership. The authors thank Partnership Chair Tony Bodway and his employer Payne and Dolan Inc., and Steve Henderson and his employer E&B Paving Inc., for their efforts on behalf of this study and for their assistance in arranging and conducting the field studies. The authors thank Peter Shaw and Edward Krieg, Jr., of NIOSH for their helpful comments. The authors also gratefully thank the National Asphalt Pavement Association, the International Union of Operating Engineers, the Laborers International Union of North America, and the Association of Equipment Manufacturers.

Disclaimer

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.